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Proposed Rule2026-19584

Flight Operations: Pilot requirements; Use of oxygen

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Published

September 24, 2026

Issuing agencies

Transportation DepartmentFederal Aviation Administration

Abstract

FAA proposes to raise the altitudes at which a pilot is required to don an oxygen mask for commuter and on demand operations, as directed by the FAA Reauthorization Act of 2024, and revise the pilot oxygen mask requirements applicable to general aviation operations in pressurized aircraft. The proposed amendments would allow the operation of airplanes at higher altitudes without requiring at least one pilot at the controls to wear and use an oxygen mask. If adopted, these proposed changes would reduce regulatory and economic burdens on operators by changing certain requirements pertaining to pilots' use of oxygen masks.

Full Text

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<title>Federal Register, Volume 91 Issue 184 (Thursday, September 24, 2026)</title>
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[Federal Register Volume 91, Number 184 (Thursday, September 24, 2026)]
[Proposed Rules]
[Pages 60530-60546]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2026-19584]

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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Parts 91 and 135

[Docket No. FAA-2026-12145; Notice No. 26-17]
RIN 2120-AM24

Flight Operations: Pilot requirements; Use of oxygen

AGENCY: Federal Aviation Administration (FAA), U.S. Department of
Transportation (DOT).

ACTION: Notice of proposed rulemaking (NPRM).

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SUMMARY: FAA proposes to raise the altitudes at which a pilot is
required to don an oxygen mask for commuter and on demand operations,
as directed by the FAA Reauthorization Act of 2024, and revise the
pilot oxygen mask requirements applicable to general aviation
operations in pressurized aircraft. The proposed amendments would allow
the operation of airplanes at higher altitudes without requiring at
least one pilot at the controls to wear and use an oxygen mask. If
adopted, these proposed changes would reduce regulatory and economic
burdens on operators by changing certain requirements pertaining to
pilots' use of oxygen masks.

DATES: Send comments on or before November 23, 2026.

ADDRESSES: Send comments identified by docket number FAA-2026-12145
using any of the following methods:
<bullet> Federal eRulemaking Portal: Go to <a href="https://www.regulations.gov">https://www.regulations.gov</a> and follow the online instructions for sending your
comments electronically.
<bullet> Mail: Send comments to Docket Operations, U.S. Department
of Transportation (DOT), 1200 New Jersey Avenue SE, Room W58-213, West
Building 5th Floor, Washington, DC 20590-0001.
<bullet> Hand Delivery or Courier: Take comments to Docket
Operations in Room W58-213 of the West Building 5th Floor at 1200 New
Jersey Avenue SE, Washington, DC 20590 between 9 a.m. and 5 p.m.,
Monday through Friday, except Federal holidays.
<bullet> Fax: Fax comments to Docket Operations at (202) 493-2251.
Docket: Background documents or comments received may be read at
<a href="https://www.regulations.gov">https://www.regulations.gov</a> at any time. Follow the online instructions
for accessing the docket or go to the Docket Operations in Room W58-213
of the West Building 5th Floor at 1200 New Jersey Avenue SE,
Washington, DC

[[Page 60531]]

20590 between 9 a.m. and 5 p.m., Monday through Friday, except Federal
holidays.

FOR FURTHER INFORMATION CONTACT:
Part 135 information: Chris Morris, Air Transportation Division,
AFS-200, Federal Aviation Administration, Room 834, 800 Independence
Avenue SW, Washington, DC 20591; telephone (202) 267-8166; email <a href="/cdn-cgi/l/email-protection#fec7d3bfb8add3ccceced3bd918c8c9b8d8e91909a9b909d9bbe989f9fd0999188">[email&#160;protected]</a>.
Part 91 information: Jamelle Poppe, General Aviation and Commercial
Division, AFS-800, Federal Aviation Administration, 800 Independence
Avenue SW, Washington, DC 20591; (202) 267-1100; email <a href="/cdn-cgi/l/email-protection#d4edf9959287f9ece4e4f997bba6a6b1a7a4bbbab0b1bab7b194b2b5b5fab3bba2">[email&#160;protected]</a>.

SUPPLEMENTARY INFORMATION:

List of Abbreviations and Acronyms Frequently Used in This Document

AC: Advisory Circular
AD: Airworthiness Directive
AFM: Airplane Flight Manual
CAA: Civil Aviation Authority
FL: Flight Level
hPa: Hectopascals
ICAO: International Civil Aviation Organization
MSL: Mean Sea Level
NPRM: Notice of Proposed Rulemaking
NTSB: National Transportation Safety Board
PO<INF>2</INF>: Partial Pressure of Oxygen
SARPs: Standards and Recommended Practices
S<INF>P</INF>O<INF>2</INF>: Blood Oxygen Saturation Level
TUC: Time of Useful Consciousness
USAF: United States Air Force
USN: United States Navy

Table of Contents

I. Executive Summary
II. Authority for This Rulemaking
III. Background
A. History
B. International Discrepancies in Mask Requirements
C. Safety Risk Analysis and NTSB Findings & Recommendations
D. United States Military Oxygen Mask Requirements and
Pressurization Events
E. Related Regulatory Actions
IV. Proposal
A. Revising Part 135 Oxygen Mask Requirements With More Than One
Pilot on the Flightdeck
B. Revising Part 135 Mask Requirement to Flight Level 350 if One
Pilot Is on the Flightdeck
C. Revising Part 91 Oxygen Mask Requirements
D. Terminology Changes
V. Regulatory Notices and Analyses
A. Regulatory Impact Analysis (RIA)
B. Regulatory Flexibility Act
C. International Trade Impact Assessment
D. Unfunded Mandates Assessment
E. Paperwork Reduction Act
F. International Compatibility
G. Environmental Analysis
VI. Executive Order Determinations
A. Executive Order 13132, Federalism
B. Executive Order 13175, Consultation and Coordination With
Indian Tribal Governments
C. Executive Order 13211, Regulations That Significantly Affect
Energy Supply, Distribution, or Use
D. Executive Order 13609, Promoting International Regulatory
Cooperation
E. Executive Order 14192, Unleashing Prosperity Through
Deregulation
VII. Additional Information
A. Comments Invited
B. Confidential Business Information
C. Electronic Access and Filing
D. Small Business Regulatory Enforcement Fairness Act

I. Executive Summary

As directed by section 834 of the FAA Reauthorization Act of 2024
(``the Act''), FAA is issuing this notice of proposed rulemaking (NPRM)
to propose the amendment of 14 CFR 135.89(b)(3) to increase the flight
level \1\ at which at least one pilot at the controls must wear an
oxygen mask in a pressurized aircraft from Flight Level (FL) 350 to
FL410. As required by section 834 of the Act, FAA is proposing to
revise Sec. 135.89(b)(4) to increase the flight level at which a
single pilot at the controls on the flightdeck must wear an oxygen mask
in a pressurized aircraft, from current FL250 to FL350. This amendment
would change the current altitude at which a pilot must wear an oxygen
mask when the pilot is the only pilot at the controls and would clarify
that Sec. 135.89(b)(4) applies to operations when only a single pilot
is assigned to the flight. Section 834 also directed FAA to consider
applicable safety data, risks, and investigations and recommendations
of the National Transportation Safety Board (NTSB) in its NPRM. Based
on that evaluation, FAA is proposing to amend Sec. 135.89(b)(4) to
increase the flight level at which a single pilot on the flightdeck
must wear an oxygen mask from current FL250 to FL350.
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\1\ Flight level is defined as a level of constant atmospheric
pressure related to a reference datum of 29.92 inches of mercury.
Flight levels are stated in three digits that represent hundreds of
feet. For example, FL250 represents a barometric altimeter
indication of 25,000 feet; FL255 indicates 25,500 feet. 14 CFR 1.1.
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In addition to satisfying the statutory mandate to publish an NPRM
proposing amendments to Sec. 135.89(b)(3) and (4), FAA is proposing to
amend part 91 supplemental oxygen requirements in this NPRM. Although
assessing the risk associated with the proposed part 135 amendment, FAA
found that at higher altitudes, pressurization events in modern
business aircraft are rare. Given that operators may fly these aircraft
in accordance with the requirements of either part 135 or part 91, FAA
proposes revising the part 91 supplemental oxygen mask regulation to
allow a pilot to operate at any altitude without donning a mask when a
quick-donning mask is available to the pilot. The proposed change would
align part 91 regulations with International Civil Aviation
Organization (ICAO) Annex 6, Part II Standards and Recommended
Practices (SARPs) more closely. In addition, the proposed changes to
part 91 would uphold the Safety Continuum \2\ by tolerating more risk
in general aviation operations than in commercial operations.
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\2\ See The Safety Continuum--A Doctrine for Application Sept.
2014, <a href="https://downloads.regulations.gov/FAA-2015-1621-0018/attachment_1.pdf">https://downloads.regulations.gov/FAA-2015-1621-0018/attachment_1.pdf</a>, and Section III. 4. of this preamble for
additional discussion about safety continuum application to parts 91
and 135 pilot oxygen mask requirements.
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Currently, Sec. 91.211(b)(1)(ii) requires one pilot at the
controls of a civil aircraft of U.S. registry with a pressurized cabin
to wear an oxygen mask that is secured and sealed at all times when the
aircraft is operated above FL410, and also requires that if the
aircraft is operated above FL350 with only one pilot at the controls,
that pilot must wear an oxygen mask that is secured and sealed. The
proposal would amend Sec. 91.211(b)(1) to state that, if quick-donning
masks \3\ are available to the pilot(s), then operations are allowed at
any altitude for which the aircraft is certified without a pilot at the
controls wearing an oxygen mask. In an aircraft without quick-donning
masks, however, at flight altitudes above FL350, one pilot at the
controls would be required to wear an oxygen mask that is secured and
sealed.
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\3\ 14 CFR 25.1447 (c)(2)(i) defines a quick-donning mask as one
that can be placed on the face from its ready position, properly
secured, sealed, and supplying oxygen upon demand, with one hand,
within five seconds and without disturbing eyeglasses or causing
delay in proceeding with emergency duties and allows, while in
place, the performance of normal communication functions.
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FAA anticipates the effect from these changes would be reduced
oxygen usage by aircraft flying between the current and proposed flight
level thresholds, generating savings for operators needing less
frequent oxygen tank servicing, which FAA estimates can cost between
$100 to $400 per fill-up. Without exact data on the potential reduction
in oxygen usage and subsequent need for service to estimate cost
savings, FAA instead estimates that for every percentage point decrease
of the roughly 4.7 million parts 91 and 135 turbojet and multiengine
turboprop operations that no longer require servicing due to this
rulemaking, applicable aircraft operators would save between $4.7

[[Page 60532]]

million to $18.6 million in oxygen servicing fees annually.

II. Authority for This Rulemaking

FAA's authority to issue rules on aviation safety is found in Title
49 of the United States Code (U.S.C.) Subtitle I, section 106,
describes the authority of the FAA Administrator. Subtitle VII,
Aviation Programs, describes in more detail the scope of FAA's
authority. FAA is issuing this NPRM under the authority described in 49
U.S.C. 106(f), which establishes the authority of the Administrator to
promulgate regulations and rules, and 49 U.S.C. 44701(a)(5), which
requires the Administrator to promote safe flight of civil aircraft in
air commerce by prescribing regulations and setting minimum standards
for cybersecurity and other practices, methods, and procedures
necessary for safety in air commerce and national security. This NPRM
is within the scope of that authority.
In addition, section 834 of the FAA Reauthorization Act of 2024
(Pub. L. 118-63) (the Act) directs the Administrator to issue an NPRM
concerning whether to revise Sec. 135.89(b)(3) and (4) to apply only
to aircraft operating at altitudes above FL410.

III. Background

A. History

1. Regulatory History
In 1961, FAA published a final rule \4\ that required one pilot of
a two-pilot crew engaged in commercial operations to wear and use an
oxygen mask above FL250.\5\ However, if each crewmember on the flight
deck was provided with a quick-donning oxygen mask, the one pilot did
not need to wear and use an oxygen mask until the airplane was operated
above FL350. FAA maintained these requirements in the 1964 rule
establishing Sec. 135.83, Requirements for use of oxygen.\6\ In the
1964 final rule, FAA noted that many comments on the NPRM expressed
that the minimum altitudes at which the use of oxygen was required
should be increased. FAA disagreed, stating that increasing the minimum
altitudes required for pilot supplemental oxygen would compromise
safety. In 1978, Sec. 135.83 was recodified as Sec. 135.89 and
remains unchanged.\7\
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\4\ Oxygen Mask Requirements and Altitude Training for Flight
Crewmembers Assigned to Duty on Turbine-Powered Airplanes Operated
Above 25,000 Feet final rule, 26 FR 1055 (Feb. 3, 1961).
\5\ The 1961 rule used both mean sea level (MSL) and FL when
referring to the altitude at which pilots must use oxygen masks.
This NPRM uses FL rather than MSL when referencing altitudes to
match current regulatory notations.
\6\ Miscellaneous Amendments to parts 1, 42, 47, 61, 91, and 135
final rule, 29 FR 2988 (Mar. 5, 1964).
\7\ Regulatory Review Program; Air Taxi Operators and Commercial
Operators final rule, 43 FR 46742 (Oct. 10, 1978).
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In 1967, FAA published an NPRM proposing requirements pertaining to
the use of oxygen equipment in all aircraft operating under part 91.\8\
Specifically, FAA proposed Sec. 91.32, which, in pertinent part, would
require a pilot operating a civil aircraft with a pressurized cabin to
wear an oxygen mask, secured and sealed, at flight altitudes above
FL350. Based on the comments received, the 1970 final rule instead
required that on any aircraft having more than one pilot at the
controls, it was not necessary that one pilot breathe supplemental
oxygen at all times at flight levels at or below FL410 if each flight
crewmember at the controls had a quick-donning type oxygen mask. If
only one pilot was at the controls while operating above FL350, that
pilot was required to wear an oxygen mask.\9\ In 1989, part 91 was
recodified and Sec. 91.32 became Sec. 91.211, but the content of the
regulation remained the same and is unchanged to this day.\10\
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\8\ Installation and Operating Requirements for Oxygen Equipment
and Supply NPRM, 32 FR 10602 (July 19, 1967).
\9\ Installation and Operating Requirements for Oxygen Equipment
and Supply final rule, 35 FR 6385 (Apr. 21, 1970).
\10\ Revision of General Operating and Flight Rules final rule,
54 FR 34284 (Aug. 18, 1989).
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2. Withdrawn Notice of Proposed Rulemaking 82-11
In 1982, FAA issued an NPRM, Pilot Oxygen Mask Requirement, (``the
1982 NPRM'') \11\ proposing to amend the pilot oxygen mask requirements
in parts 91, 121, and 135. The NPRM proposed not requiring one pilot to
wear and use an oxygen mask on a pressurized aircraft with a total
pressure volume of at least 20,000 cubic feet operating under parts 91
and 121 up to and including FL450, or the maximum certified altitude of
the airplane, whichever was lower. The NPRM noted FAA had issued
exemptions from Sec. 135.89(b)(3) to permit operation of pressurized
airplanes up to and including FL410 under part 135 without requiring
one pilot at the controls to wear and use an oxygen mask.\12\ The NPRM
suggested the FL350 limitation in Sec. 135.89(b) was unnecessarily
restrictive and therefore proposed to remove the differences between
part 91 and part 135 pilot oxygen mask requirements.
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\11\ Pilot Oxygen Mask Requirement NPRM, 47 FR 35146 (Aug. 12,
1982).
\12\ See 47 FR 35146.
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The 1982 NPRM received 16 public comments. Three commenters
disagreed with the proposal. Several companies, labor organizations,
and industry associations supported the proposal. However, the
Aerospace Industries Association of America, an unnamed aeronautical
consultant, and the United States Air Force (USAF) raised questions
about the severe physiological consequences resulting from high
altitude decompression and the potential for flightcrew incapacitation.
In 1986, FAA withdrew the 1982 NPRM, citing concerns regarding
insufficient data related to rapid depressurization events.\13\
Specifically, there was a lack of information on pilot reaction times,
the interval before initiation of emergency descent procedures, the
rate of increase in cabin altitude, the maximum cabin altitude reached,
and the time required to descend to an altitude where supplemental
oxygen was no longer necessary for the occupants. The withdrawal notice
indicated that this lack of comprehensive data precluded a rational
assessment of regulatory alternatives to the existing requirement for
at least one pilot to wear and use an oxygen mask at altitudes above
FL410.
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\13\ Pilot Oxygen Mask Requirements Withdrawal of Notice of
Proposed Rulemaking, 51 FR 9432, 9433 (Mar. 18, 1986).
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FAA noted in the withdrawal of the NPRM that USAF opposed changes
in the 1982 NPRM that would have relaxed the civil pilot oxygen
requirements. Their objection was focused on USAF requirements for an
oxygen mask to be worn by the pilot and available to all other
crewmembers when flying pressurized aircraft above FL410 as a
mitigation to the physiological effects caused by exposure to high
altitude. At the time, most pressurized aircraft in USAF inventory,
such as the VC-140 and CT-29,\14\ were designed in the 1950s and lacked
the robust design standards of modern business jets in operation today.
FAA has learned that since at least 2010, USAF has permitted operations
above FL410 without either pilot wearing an oxygen mask in certain
aircraft such as the C-37, which is based on the Gulfstream V.\15\
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\14\ Air Force Magazine, May 1982, pages 161 and 162 <a href="https://www.airandspaceforces.com/app/uploads/2024/09/AFmag_1982_05.pdf">https://www.airandspaceforces.com/app/uploads/2024/09/AFmag_1982_05.pdf</a>.
\15\ Air Force Instruction 11-2VIP, Vol. 3. (Feb. 12, 2010),
Supplement, Flight Operations (May 19, 2011).
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Since that time, significant advancements have been made in data
collection, processing, and interpretation. In addition,

[[Page 60533]]

improvements in aircraft design and manufacturing have resulted in more
aircraft operating above FL410, thereby increasing opportunities to
gather data on pressurization events at higher altitudes. Accordingly,
FAA believes that sufficient data now exists to estimate the risks
associated with the proposed changes to the regulations accurately.
3. Petitions for Exemption
Since 1979, there have been 46 petitions for exemption from Sec.
135.89(b)(3) or (b)(4), or both. The exemption requests asked to permit
one or both pilots at the controls to operate pressurized aircraft
without utilizing supplemental oxygen above the levels required by the
regulation. To support these exemption requests, petitioners included
decision features of their aircraft. These included mitigators such as
type certification resulting in a highly improbable decompression
failure rate at altitude, automatic Emergency Descent Mode, dual
pressure bulkheads, cabin pressure warning systems, and quick-donning
masks. The petitioners noted that providing the relief the petitioner
requested would save them money on oxygen servicing and enable more
frequent or longer flights at high altitudes where greater fuel economy
is achieved. FAA routinely issued grants or partial grants of exemption
until 1986. Since 1986, following the withdrawal of the 1982 NPRM, FAA
has denied 15 petitions for exemption from Sec. 135.89.\16\ Since the
mid-1990s, six part 91 operators have petitioned for exemption from
Sec. 91.211(b)(1)(ii), which requires one pilot to wear and use an
oxygen mask any time the aircraft is above FL410.\17\ FAA denied all of
these petitions for both Sec. 91.211 and Sec. 135.89 because the
petitioners did not present safety mitigations unique to their
operations. The mitigations the petitioners proposed--such as having
certain aircraft design features and performance during
depressurization--would apply to anyone operating that make and model
of aircraft rather than just that particular operator. In its denials,
FAA typically stated ``[s]hould the FAA determine that it is
appropriate to allow the relief requested, it will take appropriate
action to change the rules rather than issue an exemption so that all
persons may benefit from such a change.''
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\16\ FAA most recently denied Springfield Aircraft Charter and
Sales, Inc.'s petition for exemption from Sec. 135.89. In the
denial, FAA stated that if it found it could grant relief from the
current regulation, it would initiate a rulemaking rather than issue
grants of exemption. FAA Denial of Exemption No. 11613, Docket No.
FAA-2014-0315 (May 18, 2015).
\17\ Exemption Nos. 21408, 11994, 8499, 8479, 6141, and 6817.
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In the years following the exemption denials for both parts 91 and
135, FAA gathered and analyzed millions of hours of flight data at high
altitude. This information has significantly enhanced understanding of
the effects and likelihood of decompression, crewmember best practices,
improved design features, and common causes of decompression.\18\
Despite these advancements, current regulations have not been updated
to reflect this new information and improved practices. This improved
understanding informed this proposed rulemaking to update the oxygen
mask regulations.
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\18\ See section III.C. of this preamble.
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4. Safety Continuum Application to Parts 91 and 135 Pilot Oxygen Mask
Requirements
Since 2012, FAA has utilized the Safety Continuum to balance the
needs of aircraft manufacturers, aircraft owners, and aircraft
operators with the need for public safety. The Safety Continuum guides
FAA in determining the proper level of safety for the type of
operation. Under 49 U.S.C. 44701(d), FAA must consider both the duty of
an air carrier to provide service with the highest possible degree of
safety and the differences between air transportation and other air
commerce when prescribing regulations and minimum standards. In
accordance with the statute, the Safety Continuum provides that there
is the need for a higher level of safety as the type of aircraft and
operations become more complex and as occupants become further removed
from understanding and managing risks.
One way FAA implements the Safety Continuum is by setting different
aircraft design standards based on the intended use for those aircraft.
The Safety Continuum also addresses differences in operational risks by
promulgating different operating regulations such as part 91 and part
135. When considering regulatory changes, FAA employs the Safety
Continuum to determine the appropriate regulatory level of safety for
the intended operations. This balanced approach allows FAA to meet or
exceed safety objectives while imposing appropriate requirements on the
aviation industry and the public.\19\
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\19\ See The Safety Continuum--A Doctrine for Application (Sept.
2014), <a href="https://downloads.regulations.gov/FAA-2015-1621-0018/attachment_1.pdf">https://downloads.regulations.gov/FAA-2015-1621-0018/attachment_1.pdf</a>.
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Part 91 generally applies to general aviation operators, while part
135 \20\ generally applies to holders of an air carrier or operating
certificate issued under part 119 \21\ who conduct commuter or on-
demand operations or both. The two parts must account for, among other
things, risk assumed by the pilot and flightcrew and risk assumed by
passengers. Part 135 requires a higher level of safety due to the
statutory requirement for an air carrier to provide service with the
highest possible degree of safety in the public interest.\22\
Therefore, FAA has imposed more stringent regulatory requirements under
part 135 than part 91because there is a higher expectation of safety
when carrying passengers or cargo for compensation. For example, FAA
requires more extensive training programs with more frequent competency
\23\ and proficiency checks \24\ under part 135 than it does under part
91. With a loosening of the oxygen mask requirements for part 135, part
135 requirements would now be more flexible and lenient than the
requirements of part 91. This result would be inconsistent with FAA's
safety continuum which would call for more stringent requirements for
part 135 operations that involve the transportation of passengers and
cargo for compensation. Therefore, FAA is proposing to revise part 91
supplemental oxygen mask usage requirements.
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\20\ 14 CFR 135.1.
\21\ 14 CFR 119.1; 14 CFR 119.21(a).
\22\ See 49 U.S.C. 44701(d).
\23\ 14 CFR 135.293.
\24\ 14 CFR 135.297.
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B. International Discrepancies in Mask Requirements

FAA oxygen mask regulations are currently more stringent than ICAO
standards. This places a greater burden on part 91 and part 135
domestic operators as compared to their international peers, requiring
domestic operators to perform additional maintenance and incur
additional expenses due to increased oxygen usage.
ICAO is a specialized agency of the United Nations established in
1944 by the Convention on International Civil Aviation (Chicago
Convention). The Chicago Convention established rules of airspace,
aircraft registration, and safety, among other things, and remains in
effect to this day. In accordance with Article 37 of the Chicago
Convention, ICAO member states, including the United States, agree to
collaborate to achieve the highest practicable degree of uniformity in
aircraft and personnel regulations, and in all matters where it will
facilitate and improve air navigation. To achieve this, ICAO

[[Page 60534]]

adopts Standards and Recommended Practices (SARPs) for a variety of
aviation issues.
SARPs are published in 19 Annexes to the Chicago Convention. FAA's
policy is to meet its obligations under the Chicago Convention by
conforming to ICAO SARPs to the maximum extent practicable.\25\ Member
States' Civil Aviation Authorities (CAAs) each integrate, to the
maximum extent practicable,\26\ the ICAO SARPs into their national
legal frameworks and practices and are responsible for regulatory
oversight.
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\25\ FAA Order 1240.11, Assessing Compliance with ICAO Standards
and Recommended Practices (SARPs) and Implementing their Provisions
(Nov. 6, 2007), available at: <a href="https://www.faa.gov/regulations_policies/orders_notices/index.cfm/go/document.information/documentID/17693">https://www.faa.gov/regulations_policies/orders_notices/index.cfm/go/document.information/documentID/17693</a>.
\26\ Although Article 37 of the Chicago Convention requires each
contracting state to collaborate to achieve the highest practicable
degree of uniformity in aircraft and personnel regulations with ICAO
SARPs, Article 38 of the Chicago Convention provides member states
the option to file a difference from the SARPs that notifies the
ICAO Council of the differences between the member states'
regulations and ICAO SARPs.
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Annex 6, Part I contains SARPs applicable to the operation of
airplanes by operators authorized to conduct international commercial
air transport operations. If adopted by FAA, these SARPs would be
applicable to part 135 operations in the United States. Specifically,
Sec. 4.4.5.2 of Annex 6, Part I, requires that all flight crewmembers
of pressurized airplanes at flight duty stations operating above an
altitude where the atmospheric pressure is less than 376 Hectopascals
(hPa) (FL250) shall have available a quick-donning type of oxygen mask
which readily supplies oxygen on demand.
For part 91 operations, Annex 6, Part II establishes SARPs
applicable to the operation of general aviation airplanes.\27\
Attachment 2.A of Annex 6, Part II supplements Sec. 2.2.3.8, and
places the same requirements that would apply to part 135 operators by
requiring quick-donning oxygen masks be accessible when operating above
an altitude where the atmospheric pressure is less than 376 hPA
(FL250).
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\27\ ICAO Annex 6, Part II Applicability states ``The Standards
and Recommended Practices of Annex 6, Part II, are applicable to
international general aviation operations with aeroplanes.''
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Although ICAO requires quick-donning masks be available above
FL250, there is no requirement for flight crewmembers to wear oxygen
masks at any specified altitude in either commercial air transport or
general aviation operations as currently exist in the United States.

Table 1--Comparison of ICAO SARPs and Current FAA Rules
------------------------------------------------------------------------
ICAO SARP Current FAA rule
------------------------------------------------------------------------
Annex 6, Part I, 4.4.5.2: All flight 14 CFR 135.89(b)(3) Two pilots
crew members of pressurized airplanes at the controls: Whenever a
operating above an altitude where the pressurized aircraft is
atmospheric pressure is less than 376 operated at altitudes above
hPa (FL250) shall have available at 35,000 feet MSL, at least one
the flight duty station a quick- pilot at the controls shall
donning type of oxygen mask which will wear, secured and sealed, an
readily supply oxygen upon demand. oxygen mask required by
paragraph (b)(2)(i).
14 CFR 135.89(b)(4) One pilot
at the controls: If one pilot
leaves a pilot duty station of
an aircraft when operating at
altitudes above 25,000 feet
MSL, the remaining pilot at
the controls shall put on and
use an approved oxygen mask
until the other pilot returns
to the pilot duty station of
the aircraft.
Annex 6, Part II, 3.4.4.2.2: All flight 14 CFR 91.211(b)(1)(ii) Two
crew members of pressurized airplanes pilots at the controls: No
operating above an altitude where the person may operate a civil
atmospheric pressure is less than 376 aircraft of U.S. registry with
hPa (FL250) shall have available at a pressurized cabin at flight
the flight duty station a quick- altitudes above FL350 unless
donning type of oxygen mask which will one pilot at the controls of
readily supply oxygen upon demand. the airplane is wearing and
using an oxygen mask that is
secured and sealed and that
either supplies oxygen at all
times or automatically
supplies oxygen whenever the
cabin pressure altitude of the
airplane exceeds 14,000 feet
(MSL), except that the one
pilot need not wear and use an
oxygen mask while at or below
FL410 if there are two pilots
at the controls and each pilot
has a quick-donning type of
oxygen mask that can be placed
on the face with one hand from
the ready position within five
seconds, supplying oxygen and
properly secured and sealed.
14 CFR 91.211(b)(2) One pilot
at the controls:
Notwithstanding paragraph
(b)(1)(ii), if for any reason
at any time it is necessary
for one pilot to leave the
controls of the aircraft when
operating at flight altitudes
above FL350, the remaining
pilot at the controls shall
put on and use an oxygen mask
until the other pilot has
returned to that crewmember's
station.
------------------------------------------------------------------------

C. Safety Risk Analysis and NTSB Findings & Recommendations

1. Introduction
Section 834 of the Act mandated that, in issuing this proposal, FAA
consider applicable safety data and risks, including pertinent
incidents and accidents as well as the investigations and
recommendations of the NTSB. FAA considered the physiological risks of
higher altitude flights, the severity of those risks, data on
decompression events, and the likely causes of decompression.
Modern advancements in aircraft design have reduced the likelihood
of explosive decompression events, as discussed in III.D. of this NPRM.
In addition, over the last fifty years FAA has gathered more data on
decompression and the likely causes of decompression events from
information sources that are both publicly available or information
protected from public disclosure under 14 CFR 193, as discussed in
III.C.2 of this NPRM. This, in conjunction with the ongoing industry
interest in reducing the burden imposed by current oxygen mask
requirements as expressed in the exemption requests noted in section
III.A.3 of this preamble, discussions between FAA and industry, and the
mandates of section 834, has led FAA to believe that raising the
minimum altitude at which a pilot must don an oxygen mask will reduce
regulatory burden on part 91 and part 135

[[Page 60535]]

operators while maintaining an appropriate level of safety during high-
altitude flight. These challenges include reduced oxygen availability
in emergencies, impaired pilot communication, hygiene concerns, disease
risk, and increased fatigue.
In addition to the changes to part 135 proposed by section 834 of
the Act, FAA wanted to assess the risk associated with a possible
change to part 91. Specifically, FAA was considering an amendment to
Sec. 91.211(b)(1)(ii), proposing that if two pilots are at the
controls of an airplane that has been designed so that the occupants
will not be exposed to cabin pressure altitude that exceeds 40,000 feet
for any duration after decompression from any failure condition not
shown to be extremely improbable, and each pilot has a quick-donning
oxygen mask available, then one pilot need not wear and use an oxygen
mask while at or below the aircraft's maximum certified altitude.
However, based on the data and analysis explained in section IV.C.
of this preamble, FAA decided to propose a change to Sec. 91.211(b)(1)
to allow operations at any altitude provided quick donning masks are
available to the pilots.
2. Likelihood Analysis of Decompression Events and NTSB Findings and
Recommendations
In the FAA Safety Risk Management review process, risk is
determined by considering both the likelihood of an event occurring and
the severity of its potential consequences. These factors are
incorporated into a safety risk matrix to determine the risk level for
each severity-likelihood combination based on the operation's safety
expectations.
To determine the risk associated with the proposed amendments, FAA
assessed the likelihood of a decompression event occurring above FL250.
FAA used data from the 2023 General Aviation Survey to estimate the
number of flight hours flown above FL250 by part 91 and part 135
operators.\28\ Considering significant factors over the past decade,
such as the impact of the COVID-19 pandemic and advancements in modern
aircraft capabilities, FAA adopted a conservative approach.
Consequently, FAA estimated that between January 2015 and May 2025 part
91 and part 135 operators collectively logged over 20 million flight
hours above FL250.
---------------------------------------------------------------------------

\28\ The 2023 total landings data for parts 91 and 135 turbojets
and multiengine turboprops is found in table 2.3 of the 2023 General
Aviation and Part 135 Activity Surveys, <a href="https://www.faa.gov/data_research/aviation_data_statistics/general_aviation/cy2023">https://www.faa.gov/data_research/aviation_data_statistics/general_aviation/cy2023</a>.
---------------------------------------------------------------------------

FAA then reviewed reported pressurization events that occurred
during that 10-year period. Out of the over 20 million flight hours
flown above FL250 by part 91 and part 135 operators, FAA found
approximately 200 reports of decompression events. Of those reports, 75
percent occurred from FL260 up to and including FL410, while 25 percent
took place above FL410. FAA categorized each event as ``explosive,''
``rapid,'' or ``gradual.'' \29\ Among the 200 reported events, three
were possible rapid decompressions and one was an explosive
decompression. Of these four events, two rapid decompressions resulted
in accidents (discussed below), while the remaining rapid decompression
and the explosive decompression both landed safely without further
incident.
---------------------------------------------------------------------------

\29\ For purposes of the assessment, an explosive
depressurization is defined as a loss of cabin pressure resulting in
ambient pressure being reached within three seconds or less. A rapid
depressurization is one in which ambient pressure is reached in more
than three seconds, but less than 60 seconds and a gradual
depressurization occurred when it took more than 60 seconds for
cabin pressure to reach ambient pressure.
---------------------------------------------------------------------------

Only one accident resulted in fatalities: an event involving a
Cessna 560 on June 4, 2023, operated under part 91 by a single
pilot.\30\ NTSB concluded that the probable cause of this accident was
pilot incapacitation due to loss of cabin pressure for undetermined
reasons and that the owner/operator's decision to operate the airplane
without supplemental oxygen onboard contributed to the outcome. FAA
does not consider the accident relevant to the risk analysis because it
was determined that the aircraft was not flown in an airworthy \31\
condition. Furthermore, the pilot lost consciousness before reaching an
altitude at which current regulations require the use of an oxygen
mask.
---------------------------------------------------------------------------

\30\ NTSB Aviation Investigation Final Report ERA23FA256 (May
13, 2025), available at: <a href="https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateFinalReport/192300/pdf">https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateFinalReport/192300/pdf</a>.
\31\ 14 CFR 3.5(a)--Airworthy means the aircraft conforms to its
type design and is in a condition for safe operation.
---------------------------------------------------------------------------

In addition to the accident noted above, on May 23, 2016, a Cessna
Citation 501SP experienced a complete decompression at FL430 due to
multiple failures in the pressurization system.\32\ Upon recognizing
the depressurization, the pilot delayed the donning of his oxygen mask.
Had he immediately donned his oxygen mask after noticing the
depressurization, it is likely that the event would not have resulted
in damage to the airplane.\33\ There were no fatalities or serious
injuries resulting from this accident; however, the pilot and two
passengers sustained minor injuries. Consequently, NTSB did not issue
any safety recommendations in response to this accident.
---------------------------------------------------------------------------

\32\ NTSB Aviation Accident Final Report CEN16LA197 (Jul. 5,
2018), available at: <a href="https://www.faa.gov/sites/faa.gov/files/2023-12/CEN16LA197.pdf">https://www.faa.gov/sites/faa.gov/files/2023-12/CEN16LA197.pdf</a>.
\33\ See Safety Risk Assessment for Use of Pilot Oxygen Masks in
CFR parts 91 and 135 Operations, Section 3.
---------------------------------------------------------------------------

In summary, due to the rarity of decompressions occurring above
FL250, combined with advances in aircraft design and safety standards,
FAA finds that increasing the minimum altitude at which oxygen masks
must be worn continuously will not adversely affect safety. The two
accidents that occurred during the review period and within the
operational parameters relevant to this NPRM occurred due to failure to
follow required procedures and regulations. FAA concludes that the
proposed rule would not have changed the outcome of either event. In
all other cases, the aircraft that experienced a decompression event
landed successfully without further incident, and in several cases, the
pilots admitted they were not wearing their oxygen masks prior to the
occurrence.
In addition to the two events discussed above, another accident FAA
considered during its risk analysis was the crash of a Learjet 35 that
occurred on October 25, 1999.\34\ The accident resulted in the deaths
of all six people on board, including golfer Payne Stewart. Though the
accident did not occur within the 10-year review period, its notoriety
and subsequent safety recommendations warranted inclusion in this
analysis. The Learjet 35 lost cabin pressure for undetermined reasons.
Air traffic control received the last communication from the aircraft
while it was climbing through 23,200 feet. Subsequently, the aircraft
continued climbing to more than 46,000 feet, flew on autopilot for
nearly four hours, deviated from its intended course, and ultimately
ran out of fuel and crashed in South Dakota. The NTSB determined that
the probable cause of the accident was pilot incapacitation due to
failure to receive supplemental oxygen.
---------------------------------------------------------------------------

\34\ NTSB Aviation Accident Final Report DCA00MA005 (Nov. 28,
2000).
---------------------------------------------------------------------------

Following the Learjet 35 accident, the NTSB issued 11 safety
recommendations to FAA.\35\ These

[[Page 60536]]

included: installing automatic emergency pressurization systems on
aircraft to maintain safe cabin pressure if the primary system fails;
the importance of thorough maintenance practices including improved
documentation and follow-up on recurring pressurization issues; and
improving pilot training to better recognize and respond to the early
signs of hypoxia during cabin pressurization events. Of the 11 safety
recommendations, NTSB closed 10 as acceptable, while one was found
unacceptable. Two safety recommendations were due to an underlying
issue that greatly exacerbated the problem created by depressurization.
---------------------------------------------------------------------------

\35\ NTSB Safety Recommendation A-00-109 through -119 (Dec. 20,
000), available at: <a href="https://www.google.com/goto?url=CAESagHuR6pNOk8H6P0_7pcGzmAcaJrM_-MJZur6OXcLNiq4XWJOi6_NOfwO5cr_XbThCedTBr06X-uoHilEwTxK8mN_4ZNjOY9PtW20oqAe0VWeQoEcJg1wdzO5bJN2Y21qSupRLMqB5Bb30pIE=">https://www.google.com/goto?url=CAESagHuR6pNOk8H6P0_7pcGzmAcaJrM_-MJZur6OXcLNiq4XWJOi6_NOfwO5cr_XbThCedTBr06X-uoHilEwTxK8mN_4ZNjOY9PtW20oqAe0VWeQoEcJg1wdzO5bJN2Y21qSupRLMqB5Bb30pIE=</a>
---------------------------------------------------------------------------

Safety Recommendation A-00-113 proposed that FAA mandate clear and
explicit emergency procedures for all pressurized aircraft approved to
fly above FL250. These procedures should include instructions for
flightcrews to don oxygen masks as the first and immediate action upon
the onset of a cabin altitude warning, followed by steps to identify,
to manage, and to resolve the condition that triggered the warning.
This recommendation sought to ensure that pilots prioritize donning
oxygen masks in the event of a pressurization problem.
FAA responded by reviewing Airplane Flight Manuals (AFMs) for all
part 25 and part 23 aircraft certified to fly above FL250. The Learjet
35/36 Airplane Flight Manual (AFM) did not contain an emergency
procedure requiring the flightcrew to don oxygen masks immediately
after the cabin altitude warning activated. The AFM contained an
abnormal procedures checklist allowing the flightcrew to troubleshoot
the pressurization system before donning oxygen masks. FAA noted during
a 1999 Special Certification Review that the flightcrew may become
incapacitated due to this delayed practice. On June 8, 2000, FAA issued
a final rule ``Airworthiness Directives; Learjet Model 35, 35A, 36, and
36A Series Airplanes,'' proposing to require revising the AFM to add
emergency procedures instructing the flightcrew to first don oxygen
masks prior to troubleshooting activities.\36\ FAA issued the AD on
November 6, 2000 and became effective on January 4, 2001.\37\
---------------------------------------------------------------------------

\36\ See 65 FR 36391.
\37\ Airworthiness Directives; Learjet Model 35, 35A, 36, and
36A Series Airplanes, 65 FR 71239.
---------------------------------------------------------------------------

In addition, Airworthiness Directives (ADs) were issued to mandate
all AFMs for part 25 and part 23 aircraft certified to fly above FL250
be revised to include donning oxygen masks as the first step following
a cabin altitude warning. The NTSB closed Safety Recommendation A-00-
113 as acceptable on January 28, 2004, recognizing the positive impact
of these changes on aviation safety. By requiring checklists for
Learjet airplanes and all other airplanes certified to fly above FL250
to include an immediate and explicit procedure for the flightcrew to
don oxygen masks before troubleshooting the pressurization system, FAA
mitigated risk associated with high altitude operations and the rule
change being proposed today.
Similarly, Safety Recommendation A-00-112 also focused on the
importance of flightcrew procedures in the event of a pressurization
issue. The recommendation addressed the need for immediate action items
to be explicitly included in checklists. It recommended FAA change FAA
Order 8400.10, ``Air Transportation Operations Inspectors Handbook,''
\38\ which prior to the accident stated that immediate action items
``may be stated as policies rather than checklist items when
appropriate.'' The order had provided an example of flightcrews donning
oxygen masks in the event of a loss of cabin pressure. It stated, ``in
this example the loss of cabin pressure checklist would contain
subsequent items based on the assumption that the flight crew is on
oxygen . . . .'' NTSB believed that FAA should remove this specific
example and review the appropriateness of allowing policies to
substitute for checklist items. On June 10, 2003, FAA made a change to
Order 8400.10 that NTSB found acceptable, resulting in widespread
improvements in checklist design and enhancing flight safety at high
altitudes.
---------------------------------------------------------------------------

\38\ FAA Order No. 8400.10 (Aug. 23, 1988), canceled by FAA
Order No. 8900.1A ``Flight Standards Information Management
System,'' (Sept. 13, 2007) available at <a href="https://drs.faa.gov/browse/excelExternalWindow/869F1EB61A16C5ED852571AA005BE67A.0001">https://drs.faa.gov/browse/excelExternalWindow/869F1EB61A16C5ED852571AA005BE67A.0001</a>.
---------------------------------------------------------------------------

The unacceptable action was Safety Recommendation A-00-109, which
recommended FAA update guidance on high altitude operations to reflect
more accurately the time of useful consciousness (TUC) and rate of
pilot performance degradation following a depressurization, and to
highlight the effect of hypoxia on a pilot's ability to perform complex
tasks in a changing environment. FAA responded to the recommendation by
twice revising AC 61-107, ``Operations of Aircraft at Altitudes Above
25,000 Feet MSL and/or Mach numbers (Mmo) Greater than .75;'' however,
the NTSB found both revisions inadequate and closed the recommendation
as ``unacceptable'' on September 18, 2013.\39\
---------------------------------------------------------------------------

\39\ AC 61-107B was revised to its current version (AC 61-107B
Change 1) on September 9, 2015, after continued discussions between
the NTSB's Chief Medical Officer and FAA's Office of Aerospace
Medicine. The principal changes involved additional information
about the reasons for individual variability in hypoxia tolerance,
the considerable range in TUC at a given flight altitude, and a
caution statement related to the TUC times presented in Figure 2-3
of the advisory circular. NTSB Safety Recommendation A-00-109
remains classified as ``Closed--Unacceptable Action.''
---------------------------------------------------------------------------

Even though the recommendation was officially closed, two years
later, FAA revised AC 61-107 again, adding a CAUTION note to Figure 2-
3, ``Times of Useful Consciousness Versus Altitude'' table. The note
currently warns that many factors can affect a pilot's TUC, and pilots
should use the lowest (i.e., shortest) TUC value as their limit at a
given flight altitude. Therefore, the table now indicates that at
43,000 feet or above, during a gradual decompression, TUC is less than
9 seconds, and after a rapid decompression there is only a very short
``nominal'' time to don an oxygen mask. By updating this guidance, FAA
has helped to ensure that pilots are more aware of the risks associated
with depressurization at altitude, understand the importance of
immediately donning oxygen masks, and are better equipped to handle
such situations.
Similar to the above discussion, FAA's review has shown that, due
the numerous actions taken by FAA in response to this accident combined
with the other advances discussed elsewhere in this NPRM, this proposed
rule would not adversely affect the safety issues identified by NTSB.
3. Safety Risk: High Altitude Physiology
Though high altitude flight poses several physiological hazards,
for the development of this proposed rule, FAA analyzed and focused
upon the safety risk of hypoxia and the associated TUC. The proposed
changes to parts 135 and 91 would require pilots to use oxygen masks at
certain altitudes where time to don oxygen masks is measured in single-
digit seconds before hypoxia causes unconsciousness. As for risks posed
by other threats such as hyperventilation, barotrauma, and altitude
decompression sickness, this proposal does not impact those risks and
therefore they are not discussed further.
Hypoxia, discussed in detail below, is the most common threat
following an aircraft decompression event. FAA

[[Page 60537]]

recognizes that hypoxia resulting from decompression is a catastrophic,
time-sensitive physiological hazard for which preventive measures have
traditionally provided robust risk mitigation. The proposed rule does
not impact the underlying physiological risks associated with hypoxia
and the inherent limits on pilot response time once impairment begins.
FAA therefore emphasizes the continued importance of preventive
strategies to mitigate safety risk, including oxygen mask use and
operator procedures, training, and risk awareness to address hypoxia-
related hazards. Although the proposed amendments to existing
regulations would permit operations at certain altitudes without a
pilot continuously wearing an oxygen mask, thereby limiting the
effectiveness of oxygen mask usage as a preventive strategy, the
changes do not prohibit operators from requiring stricter oxygen masks
at any altitude. Operators and air carriers, upon considering the risks
to and effects on human physiology, may choose to require oxygen masks
be donned at lower altitudes than the minimum requirements proposed in
this NPRM.
i. Hypoxia/Hypoxic (Altitude) Hypoxia
Hypoxia is a state of oxygen deficiency in the blood, tissues, and
cells sufficient to cause an impairment of body functions.\40\ Though
any tissue will die if deprived of oxygen long enough, the greatest
concern regarding hypoxia during flight is lack of oxygen to the brain
since it is particularly vulnerable to oxygen deprivation. Hypoxic
hypoxia, which is related to altitude, is the only type of hypoxia
discussed in this NPRM.
---------------------------------------------------------------------------

\40\ Federal Aviation Administration (2015), Aircraft operations
at altitudes above 25,000 feet mean sea level or Mach numbers
greater than .75 (Advisory Circular [AC] 61-107B CHG 1), available
at: <a href="https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_61-107B_CHG_1_FAA.pdf">https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_61-107B_CHG_1_FAA.pdf</a>.
---------------------------------------------------------------------------

Hypoxic hypoxia refers to an insufficient oxygen supply due to the
reduced partial pressure of oxygen (PO<INF>2</INF>) in the inhaled air
as barometric pressure decreases with increasing altitude. Though
PO<INF>2</INF> in the inhaled air decreases with increasing altitude
(i.e., the gas molecules spread further apart), the percentage of
oxygen remains unchanged at 21 percent. Hypoxia can result from the
loss of cabin pressure due to an aircraft decompression and can
significantly reduce mental and physical performance of aircraft
occupants at altitude due to decreased blood oxygen saturation levels
(S<INF>p</INF>O<INF>2</INF>). Hypoxia poses the greatest potential
physiological threat during high altitude flight.
The human body is well-adapted to the Physiological Efficient Zone
(i.e., sea level to 10,000 feet) where pressure altitudes are
sufficient to maintain S<INF>p</INF>O<INF>2</INF> levels for normal
cognitive and physical performance. Aircraft cabin pressurization
creates and maintains this protective zone for cabin occupants in
flight. Ten thousand feet is an important physiological threshold \41\
where most healthy individuals start to display mild hypoxic symptoms,
such as drowsiness/fatigue, dizziness, and mild headache. Between
10,000 and approximately 15,000 feet, the cardiopulmonary system
compensates for reduced oxygen availability by increasing both heart
rate and the rate and depth of breathing. Above 15,000 feet--the
maximum cabin pressure altitude at which passenger oxygen masks
automatically deploy--these reflexive cardiopulmonary responses are no
longer sufficient to compensate fully, leading to a steady decline in
brain oxygen saturation and more pronounced hypoxic symptoms, including
cognitive and physical impairment.42 43 At 25,000 feet most
people become profoundly hypoxic and lose consciousness within three to
five minutes unless given highly concentrated
oxygen.44 45 46 Brain cells cannot store oxygen and rapidly
use the oxygen delivered to them.
---------------------------------------------------------------------------

\41\ This corresponds to the inflection point and sharp decrease
in the sigmoid-shaped oxygen-hemoglobin dissociation curve at
approximately 60 mmHg PO<INF>2</INF> and 87%-88%
S<INF>p</INF>O<INF>2</INF>.
\42\ Guyton, A.C., & Hall, J.E., Aviation, high-altitude, and
space physiology. In A.C. Guyton & J.E. Hall (Eds.), Textbook of
medical physiology, 11th ed., pp. 537-541, Elsevier (2006).
\43\ McArdle, W.D., Katch, F.I., & Katch, V.L., Exercise
physiology, nutrition, energy, and human performance (8th ed.),
Wolters Kluwer (2015).
\44\ Green, N., Gaydos, S., Hutchison, E., & Nicol, E., Acute
hypoxia and hyperventilation, In N. Green, S. Gaydos, E. Hutchison,
& E. Nicol (Eds.), Handbook of aviation and space medicine, pp. 51-
60, CRC Press (2019).
\45\ Pickard, J.S., & Gradwell, D.P., Respiratory physiology and
protection against hypoxia, In J.R. Davis, R. Johnson, J. Stepanek,
& J.A. Fogarty (Eds.), Fundamentals of aerospace medicine, 4th ed.,
pp. 20-45, Lippincott Williams & Wilkins (2008).
\46\ Federal Aviation Administration, Aircraft operations at
altitudes above 25,000 feet mean sea level or Mach numbers greater
than .75, Advisory Circular 61-107B with Change 1 (2015), available
at: <a href="https://www.faa.gov/regulations_policies/advisory_circulars/index.cfm/go/document.list/?q=AC+61-107B&statusID=2">https://www.faa.gov/regulations_policies/advisory_circulars/index.cfm/go/document.list/?q=AC+61-107B&statusID=2</a>.
---------------------------------------------------------------------------

If hypoxia is allowed to progress at 25,000 feet, objective signs
of impairment begin to appear such as mental confusion, poor judgment,
muscle incoordination, and, ultimately, unconsciousness. Subjectively,
the hypoxic symptoms most frequently felt are fatigue, headache,
dizziness, poor visual acuity, hot and cold flashes, numbness, and
tingling. In most people, hypoxia induces a sense of euphoria, which
makes it difficult to convince them to don the oxygen mask and treat
themselves by breathing 100 percent supplemental oxygen.\47\ If hypoxia
occurs at 40,000 feet due to an explosive (one to three seconds) or
rapid decompression (three to 60 seconds), unconsciousness is highly
likely due to the sudden increase in cabin pressure altitude and
administration of 100 percent oxygen under positive pressure is needed
within seconds. Moreover, administration of 100 percent oxygen is
needed within three to four minutes to prevent irreversible brain cell
damage or death.48 49
---------------------------------------------------------------------------

\47\ Dart, T., Pilmanis, A., & Wurmstein, A., High altitude
physiology [Symposium session], High Altitude Symposium, Medical
University of South Carolina, Charleston, SC (Aug 12-14, 2024).
\48\ Guyton, A.C., & Hall, J.E., Aviation, high-altitude, and
space physiology, In A.C. Guyton & J.E. Hall (Eds.), Textbook of
medical physiology, 11th ed., pp. 537-541, Elsevier (2006).
\49\ Pickard, J.S., & Gradwell, D.P., Respiratory physiology and
protection against hypoxia, In J.R. Davis, R. Johnson, J. Stepanek,
& J.A. Fogarty (Eds.), Fundamentals of aerospace medicine, 4th ed.,
pp. 20-45, Lippincott Williams & Wilkins (2008).
---------------------------------------------------------------------------

ii. Time of Useful Consciousness
FAA considered the safety risk and data around the TUC,\50\ which
is the amount of time from the moment a person is deprived of adequate
oxygen to when the person is no longer able to think clearly or take
effective action to correct the situation prior to losing consciousness
(i.e., quickly don an oxygen mask in the event of a cabin
depressurization). FAA analyzed results from altitude chamber studies
conducted from the early 1940s \51\ into the early 1970s.\52\ These
studies

[[Page 60538]]

provided experimental evidence for the TUC values listed in Table 2 and
form the basis of the current pilot oxygen use rules.
---------------------------------------------------------------------------

\50\ Time of Useful Consciousness (TUC) or Effective Performance
Time (EPT). This is the period of time from interruption of the
oxygen supply, or exposure to an oxygen-poor environment, to the
time when an individual is no longer capable of taking proper
corrective and protective action (e.g., donning an oxygen mask) (AC
61-107B CHG 1). It is not the time to total unconsciousness.
The definition of ``Time of Useful Consciousness (TUC)'' and
``Effective Performance Time (EPT)'' is the same and the two terms
are used interchangeably; however, EPT is used for pilots and flight
attendants with assigned operational flight duties and TUC is used
for passengers/individuals with no assigned operational flight
duties. TUC is the more common term and will be used throughout the
remainder of this document.
\51\ Comfort, E., & Wilson, J.W., Some factors affecting time
consciousness at high altitudes (U.S. Air Force Technical Report No.
5970), U.S. Air Force Air Materiel Command (1950).
\52\ Ernsting, J., Denison, D.M., Byford, G.H., & Fryer, D.I.,
Hypoxia induced by rapid decompression from 8,000 feet to 40,000
feet--the influence of rate of decompression (Flying Personnel
Research Committee Report No. AD-A009 006), Royal Air Force
Institute of Aviation Medicine (1973).
---------------------------------------------------------------------------

Table 2 compares current rules, TUC, and the proposed rule changes
for part 91 and part 135. As stated previously, TUC is the amount of
time from the moment a person is deprived of adequate oxygen to when
they are no longer able to think clearly or take effective corrective
action (e.g., don an oxygen mask). TUC does not mean the onset of
unconsciousness. Impaired performance may be immediate, and the prompt
use of 100 percent oxygen is critical.

Table 2--Time of Useful Consciousness & Current and Proposed Rules Comparison
----------------------------------------------------------------------------------------------------------------
Time of useful
consciousness: Time of useful
Current rule Altitude following a consciousness: Proposed rule
(feet) gradual following a rapid
decompression decompression
----------------------------------------------------------------------------------------------------------------
18,000 20 to 30 minutes... 10 to 15 minutes...
22,000 10 minutes......... 5 to 6 minutes.....
(FL250).......................... 25,000 3 to 5 minutes..... 1.5 to 2.5 minutes.
Part 135: One Pilot..............
28,000 2.5 to 3 minutes... 1 to 1.5 minutes...
30,000 1 to 2 minutes..... 30 to 60 seconds...
(FL350).......................... 35,000 30 to 60 seconds... 15 to 30 seconds... (FL350)
Part 135: Two Pilots............. Part 135: One
Part 91: One pilot............... Pilot.
(FL410).......................... 40,000 15 to 20 seconds... Nominal/Likely (FL 410)
Part 91: Two Pilots.............. immediate loss of Part 135: Two
consciousness. Pilots.
43,000 9 to 12 seconds.... Nominal/Likely
immediate loss of
consciousness.
50,000 9 to 12 seconds.... Nominal/Likely (No Limit)
immediate loss of Part 91: w/Quick
consciousness. Donning Masks.
----------------------------------------------------------------------------------------------------------------
** ``nominal'' = in name only, very short amount of time.

Importantly, the TUC times in Table 2 are based on altitude chamber
research studies during which the subjects were at rest or performing
very simple tasks, not the cognitively or physically demanding tasks
associated with flying an aircraft at high altitude while managing an
emergency situation. In research studies during which subjects
performed more complex or physically demanding tasks, TUC was much
shorter.53 54 55 Therefore, it is strongly recommended that
pilots assume the lesser TUC time at each flight level is limiting.
---------------------------------------------------------------------------

\53\ Noble, J., Jones, J.G., & Davis, E.J., Cognitive function
during moderate hypoxaemia, Anesthesia and Intensive Care, 21(2),
pp. 180-184 (1993).
\54\ Kelman, G.R., & Crow, T.J., Impairment of mental
performance at a simulated altitude of 8,000 feet, Aerospace
Medicine, 40(9), pp. 981-982 (1969).
\55\ Harding, R.M. (revised by Gradwell, D.P.), Hypoxia and
hyperventilation, In D.P. Gradwell & D.J. Rainford (Eds.),
Ernsting's aviation and space medicine, 5th ed., pp. 59, CRC Press
(2016).
---------------------------------------------------------------------------

These studies were performed in anticipation that subsonic
transport aircraft would fly at higher altitudes (i.e., FL400-FL500) to
take advantage of less congested airspace and there were concerns about
the physiological protection and survival of pilots and passengers in
the event of a high-altitude depressurization.\56\ In the late 1960s to
early 1970s, the focus of this work shifted to physiological protection
in supersonic aircraft.
---------------------------------------------------------------------------

\56\ The Boeing 307 Stratoliner, the first commercial use
aircraft with a pressurized cabin, entered revenue service in July
1940 with a cruising altitude of 15,000 to 20,000 feet. Pressurized
jet aircraft in the 1960s could reach cruising altitudes of 30,000
to 40,000 feet. Grant, R.G., Flight: 100 years of aviation, DK
Publishing, Inc. (2002).
---------------------------------------------------------------------------

Important results from these studies include:
<bullet> TUC decreases with altitude, and a rapid decompression
dramatically reduces TUC. In general, TUC is reduced by 50 percent
following a rapid decompression to altitudes between 25,000 feet and
43,000 feet. Above 43,000 feet, TUC is reduced to the time it takes for
blood to circulate from the lungs to the brain; approximately nine
seconds from the start of a rapid decompression to the loss of
functional capability.57 58
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\57\ Guyton, A.C., & Hall, J.E., Aviation, high-altitude, and
space physiology, In A.C. Guyton & J.E. Hall (Eds.), Textbook of
medical physiology, 11th ed., pp. 537-541, Elsevier (2006).
\58\ Luft, U.C., Clamann, H.G., & Optiz, E., The latency of
hypoxia on exposure to altitude above 50,000 feet, The Journal of
Aviation Medicine, 22(2), 117-136 (1951).
---------------------------------------------------------------------------

<bullet> In chamber rapid decompressions to altitudes at and below
30,000 feet 59 60 and controlled in-flight aircraft
depressurizations where the cabin pressure altitude did not exceed
30,000 feet,\61\ most pilots were able to don their oxygen mask and
return their hands to the flight controls or initiate an emergency
descent within the TUC of 30-60 seconds. Although these recognition
reaction times and mask donning were fast enough to prevent severe
hypoxic symptoms at 30,000 feet, similar reaction times at higher
altitudes will lead to pilot incapacitation.62 63
---------------------------------------------------------------------------

\59\ O'Connor, W.F., & Pendergrass, G.E., Effects of
decompression on operator performance (Federal Aviation Agency
Report No. AM 66-10), Office of Aviation Medicine, Federal Aviation
Agency (1966).
\60\ Hoffler, G.W., Turner, H.S., Wick, R.L., & Billings, C.E.,
Behavior of naive subjects during rapid decompression from 8,000 to
30,000 feet, Aerospace Medicine, 45(2), pp. 117-122 (1974).
\61\ Bennett, G., Reactions and performance of pilots following
decompression, Aerospace Medicine, 32(2), pp. 134-136 (1961).
\62\ Ernsting, J., The ideal relationship between inspired
oxygen concentration and cabin altitude, Aerospace Medicine, 34(11),
pp. 991-997 (1963).
\63\ Donaldson, R.T., Carter, E.T., Billings, C.E., & Hitchcock,
F.A., Acute hypoxia during rapid decompression and emergency descent
in a commercial jet aircraft, Aerospace Medicine, 31(10), pp. 842-
851 (1960).
---------------------------------------------------------------------------

<bullet> In rapid decompressions (two to 12 seconds) from 8,000
feet to 40,000 feet, the oxygen mask must be donned, sealed, and
supplying 100 percent oxygen within eight seconds before the onset of
impairing hypoxia symptoms (e.g., dimmed/blurry vision, mental
confusion). If mask donning is delayed by 10 seconds or more, yet still
accomplished, the pilot will become severely hypoxic and lose
consciousness within 15 seconds and remain unconscious for an
additional 20-40 seconds despite having the mask in place and supplying
100 percent

[[Page 60539]]

oxygen.\64\ The severe hypoxia and loss of consciousness are due to gas
expansion of the 80 percent nitrogen in the lungs from breathing
ambient cabin air prior to depressurization. The removal (i.e.,
clearance) of this nitrogen takes time, during which the PO<INF>2</INF>
in the lungs is too low to oxygenate the blood to support brain
function. Even if the first breath post-depressurization is 100 percent
oxygen, it still takes approximately 20-40 seconds to clear the
nitrogen from the lungs to oxygenate the blood adequately.
---------------------------------------------------------------------------

\64\ Bryan, C.A., & Leach, W.G., Physiologic effects of cabin
pressure failure in high altitude passenger aircraft, Aerospace
Medicine, 31(4), pp. 267-275 (1960); Blockley, W.V., & Hanifan,
D.T., An analysis of the oxygen protection problem at flight
altitudes between 40,000 and 50,000 feet, Federal Aviation Agency
Final Report on Contract No. FA-955, Psychological Research
Associates (1961); Barron, C.I., & Cook, T.J., Effects of variable
decompressions to 45,000 feet, Aerospace Medicine, 36(5), pp. 425-
430 (1965); Ernsting, J., McHardy, G.J.R., & Roxburgh, H.L., The
choice of gas mixture for breathing in high performance aircraft
(Flying Personnel Research Committee Report No. FPRC/1142), Royal
Air Force Institute of Aviation Medicine (1960); Ernsting, J., The
ideal relationship between inspired oxygen and cabin altitude,
Aerospace Medicine, 34(11), pp. 991-997 (1963); Ernsting, J., The
physiological effects of failure of the pressure cabins of passenger
aircraft flying at altitudes between 35,000 feet and 65,000 feet
(Flying Personnel Research Committee Report No. FPRC/313), Royal Air
Force Institute of Aviation Medicine (1965); and Ernsting, J.,
Denison, D.M., Byford, G.H., & Fryer, D.I., Hypoxia Induced by rapid
decompression from 8,000 to 40,000 feet--the influence of rate of
decompression (Flying Personnel Research Committee Report No. AD-
A009 006), Royal Air Force Institute of Aviation Medicine (1973).
---------------------------------------------------------------------------

<bullet> In rapid decompressions to 40,000 feet and above,
breathing oxygen continuously for several minutes prior to
decompression (to saturate the lungs and upper respiratory tract) is
needed to prevent severe hypoxia and loss of consciousness. An air-
oxygen mixture of at least 40 percent oxygen must be breathed at 8,000
feet cabin pressure altitude before decompression, with 100 percent
oxygen supplied within two seconds (essentially the first breath
immediately after depressurization) to prevent mental and physical
performance decrements. This equates to breathing an air-oxygen mixture
of at least 40 percent whenever the aircraft is above FL350.\65\
---------------------------------------------------------------------------

\65\ Ernsting, J., The physiological effects of failure of the
pressure cabins of passenger aircraft flying at altitudes between
35,000 feet and 65,000 feet (Flying Personnel Research Committee
Report No. FPRC/313), Royal Air Force Institute of Aviation Medicine
(1965).
---------------------------------------------------------------------------

Although the majority of these studies were conducted in the 1960s,
altitude chamber research continued in support of rulemaking activities
similar to this NPRM. A 1990 study by Marotte et al.\66\ evaluated the
effect of decompression in transport aircraft from cabin pressure
altitudes of 8,000 feet to final cabin pressure altitudes ranging from
16,000 feet to 45,000 feet. Their findings indicated that delays in
mask donning times correlated with significant decreases in
S<INF>p</INF>O<INF>2</INF> and the onset of hypoxia symptoms. They
concluded oxygen mask wear and use should be mandated below FL410 and
recommended they be mandated at FL380 or above.
---------------------------------------------------------------------------

\66\ Marotte, H., Toure, C., Clere, J.M., & Vieillefond, H.,
Rapid decompression of a transport aircraft cabin: protection
against hypoxia, Aviation, Space, and Environmental Medicine, 61(1),
pp. 21-27 (1990).
---------------------------------------------------------------------------

iii. Time of Useful Consciousness and the Proposed Oxygen Use Rule
Changes
Mental and physical impairment and eventual incapacitation from
hypoxia are the greatest physiological threats at high altitude in the
event of an aircraft decompression, be it gradual, explosive, or rapid.
Explosive and rapid decompressions are extremely rare events, but still
possible.\67\ Gradual decompressions are more frequent operationally
but potentially more dangerous--a gradual decompression may go
unnoticed and the resultant hypoxia unrecognized.
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\67\ See Section III.C.8. See also Safety Risk Management
report.
---------------------------------------------------------------------------

In the worst-case explosive decompression scenario at high
altitude, for which FAA has found no recorded instances during the 10-
year review period, if the cabin pressure altitude nearly instantly
equalizes to the aircraft's current flight level, human physiology is
the limiting factor. A significant advantage of wearing an oxygen mask
at high altitude is to prevent severe hypoxia and loss of consciousness
by facilitating nitrogen gas clearance from the lungs before exposure
to explosive or rapid decompression. The proposed rules relax the
requirement to wear an oxygen mask, thus negating this beneficial
effect; however, explosive and rapid decompressions of an aircraft are
extremely rare events. Commenters should be fully aware of the
potential severe physiological consequences of depressurization events
at high altitudes, as well as the unlikelihood of an occurrence when
responding to this NPRM.

D. Other factors

i. United States Military Oxygen Mask Requirements and Pressurization
Events
In addition to the safety risk assessment, FAA requested
information from each branch of the U.S. military regarding their
operating requirements and any data they could provide on
pressurization events in aircraft used in both military and civilian
operations (e.g., Gulfstream V, known as the C-37), as the U.S.
military uses several aircraft based on civilian designs.
The U.S. Army publishes Army Regulation 95-1, Aviation Flight
Regulations, which specifies regulations for flightcrews in pressurized
aircraft that mirror current FAA part 91 oxygen requirements.
The U.S. Navy (USN) NATOPS General Flight Operation Instructions
Manual, CNAF M-3710.7, only requires one pilot to wear an oxygen mask
above FL400 if quick-donning oxygen masks are installed.
Air Force Manual 11-202 Volume 3, Flight Operations, permits
operations up to and including FL500 in pressurized airplanes without
requiring at least one pilot wearing an oxygen mask, provided the
airplane is equipped with an automatic emergency descent mode.
Otherwise, one pilot is required to always wear an oxygen mask when
operating above FL410. This marks a change from the Air Force's
response to the 1982 proposal, where the Air Force opposed raising the
altitudes at which oxygen masks would be required.
USN responded to FAA's request for data on pressurization events in
aircraft used by both USN and civilian operators. USN provided data for
29 events in total over a 28-year period. Of those 29 events, seven
were in aircraft that are like those used by civilian business aircraft
operators, the Gulfstream IV designated as the C-20D and C-20G. In all
events, there were no reported injuries or fatalities, and the aircraft
landed safely.
ii. Aircraft Design Modernization for Safety Mitigations
In the 1986 withdrawal of the 1982 NPRM, FAA noted that advances in
aircraft engines and airframe designs are only one component of high-
altitude flight, citing the physiological limitations of the human body
and the effects of low atmospheric pressure. At the time, FAA
determined it was in the interest of public safety not to raise the
minimum altitude at which at least one pilot at the controls was
required to wear and use an oxygen mask in parts 91, 121, and 135. As
part of that determination, FAA recognized the available data on
decompression events was inadequate to justify a change to the minimum
altitude requirements for supplemental oxygen masks. However, there
have been advancements in design features since 1986 that help reduce
the risk of high-altitude decompressions.

[[Page 60540]]

In 1996, FAA amended Sec. 25.841(a), Pressurized Cabins, to
require that cabin pressure altitude not exceed 40,000 feet \68\ for
any duration, or 25,000 feet for more than two minutes, after any
failure that has not been shown to be extremely improbable.\69\ This
two-minute threshold at 25,000 feet is within the three to five minutes
of TUC following a gradual decompression and within the median of TUC
following a rapid decompression (see Table 2, Time of Useful
Consciousness & Current and Proposed Rules Comparison). For transport
category airplanes certified under Sec. 25.841(a), it is very unlikely
that the cabin pressure will ever go above 40,000 feet during the
plane's lifetime.
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\68\ At 40,000 feet, TUC is 15 to 20 seconds following a gradual
decompression and is nominal following a rapid decompression. See
Table 2, Time of Useful Consciousness & Current and Proposed Rules
Comparison and the related discussion in section III.C.3.ii of this
rulemaking.
\69\ As described in 14 CFR 25.4(c)(4) and FAA AC 25.1309-1B,
the term ``extremely improbable'' means a failure condition that is
not anticipated to occur during the total operational life of all
aircraft of a given type.
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For general aviation airplanes certified under part 23 prior to
2012, FAA established special conditions for those intended to operate
above 40,000 feet. Although the special conditions were issued
individually for each aircraft model, many of the requirements were
common across multiple models and were documented in the Type
Certification Data Sheet. In general, to gain FAA approval under these
special conditions, design approval holders must have demonstrated that
following any probable malfunction or failure of the pressurization
system--combined with any undetected latent failures--the cabin
pressure altitude would not exceed 25,000 feet. For other additional
potential failures, design approval holders must have demonstrated that
the cabin pressure altitude would not exceed 40,000 feet.
In 2012, FAA revised Sec. 23.841, Pressurized cabins, to
incorporate the minimum design standards that had been previously
established via special conditions. Amendment 23-62 mandated that
airplanes certificated to operate above 45,000 feet must ensure that
the cabin pressure altitude would not exceed 40,000 feet after
decompression from any failure condition not shown to be extremely
improbable. In 2017, FAA replaced Sec. 23.841 with Sec. 23.2320,
Occupant physical environment, and the means of compliance was detailed
in American Society for Testing and Materials F3227, Standard
Specification for Environmental Systems in Aircraft. FAA defines an
``extremely improbable'' failure condition for commuter airplanes as
those failure conditions so unlikely that they are not anticipated to
occur during the entire operational life of all airplanes of one
type.\70\ In commuter aircraft certified to Sec. 23.841 at amendment
23-62, or Sec. 23.2320 at amendment 23-64 or equivalent, cabin
pressure altitudes exceeding 40,000 feet are unlikely to occur during
the service life of the aircraft fleet.
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\70\ FAA Advisory Circular (AC) 23.1309-1E, Safety Analysis and
Assessment for Part 23 Airplanes (Nov. 17, 2011), available at:
<a href="https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_23_1309-1E.pdf">https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_23_1309-1E.pdf</a>.
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Updated part 23 and part 25 design requirements serve as safety
mitigations by maintaining a controlled cabin environment that protects
passengers and crew from the risks associated with high-altitude
flight, even if the minimum flight level for pilot supplemental oxygen
is raised. By ensuring that cabin pressure altitude does not exceed
critical thresholds, the regulations help prevent hypoxia and other
altitude-related health issues, providing a vital safety net in
scenarios where pilots may operate at higher altitudes without
supplemental oxygen. This ensures a safe and breathable environment,
reducing the risk of incapacitation due to insufficient oxygen levels.
In addition to the aircraft design itself, the standards governing
pilot oxygen masks and regulators have become more robust since the
1982 rulemaking efforts and warrant consideration in this discussion.
Aircraft certified to operate at high altitudes usually have quick-
donning oxygen masks installed for the flightcrew on the flightdeck.
The oxygen masks and regulators typically meet the minimum performance
standards defined in Technical Standard Order TSOC78a and Technical
Standard Order C89a. During normal operations, oxygen is delivered to
the user at varying concentrations based on cabin pressure altitude. An
oxygen mask regulator setting is also available to supply 100 percent
oxygen regardless of altitude. Furthermore, aircraft that fly at
altitudes where it is not extremely improbable for a decompression
event to expose the flightcrew to cabin pressure altitudes exceeding
34,000 feet are typically equipped with pressure-demand oxygen systems
on the flightdeck. Pressure-demand systems provide oxygen under
pressure to force oxygen into the user's lungs and bloodstream,
improving oxygen absorption and reducing the risk of hypoxia, even at
high altitudes.
As discussed in section III.C. of this rulemaking, a thorough
review of pressurization events in the last 10 years was conducted.
During that review, the positive effects of these advancements in
aircraft design were apparent. Most events occurred during climb-out,
and the flightcrew identified the problem before reaching an altitude
at which hypoxia would have become a significant concern. Although the
exact cause of the pressurization problem was not always reported,
often it was a door seal or other minor failure which resulted in a
gradual decompression of the cabin. Even in events that occurred at
high altitude, many of the reports reviewed noted that both flightcrew
members donned their masks after the first indication of a problem,
which was often after reaching altitudes at which oxygen mask use is
required by current regulations. FAA attributes the rarity of
catastrophic decompression events, in part, to the advances in aircraft
design discussed previously.

E. Related Regulatory Actions

The FAA Reauthorization Act of 2018 \71\ directed FAA to issue a
final rule revising Sec. 121.333(c)(3) to apply only to flight
altitudes above FL410. At the time of the enactment of this Act, Sec.
121.333(c)(3) required if for any reason it is necessary for one pilot
to leave their station at the controls of an airplane when operating at
flight altitudes above FL250, the remaining pilot at the controls must
put on and use their oxygen mask until the other pilot has returned to
their duty station. FAA published a final rule in response to the
Congressional mandate amending the part 121 flight level threshold at
which one pilot at the controls must put on and use an oxygen mask
while the other pilot leaves their control station from FL250 to
FL410.\72\ The 2020 revision to Sec. 121.333 rendered part 121 less
restrictive than parts 91 and 135 pilot supplemental oxygen rules.
Although the changes proposed in this NPRM to parts 91 and 135 would
alleviate that incongruity, part 135 would still be more restrictive
than part 121. This discrepancy is acceptable to FAA because the cabin
volume of most airplanes operated under part 121 is significantly
greater than that of those operated under part 135, and airplanes
operated under part 121 are therefore less likely to experience a rapid
decompression.
---------------------------------------------------------------------------

\71\ Section 579 of Public Law 115-254.
\72\ Oxygen Mask Requirement: Supplemental Oxygen for Emergency
Descent and for First Aid Turbine Engine Powered Airplanes with
Pressurized Cabins, 85 FR 16897 (Mar. 25, 2020).

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[[Page 60541]]

IV. Proposal

As discussed in the Executive Summary, section 834(a) of the Act
directs FAA to issue an NPRM on whether to revise the requirements for
paragraphs (3) and (4) of Sec. 135.89(b) to raise the required flight
level at which pilots must don an oxygen mask to FL410. Section 834(b)
of the Act further states that FAA shall consider applicable safety
data and risks, including data from incidents and accidents and NTSB
investigations and recommendations, as discussed in section III.C. of
this preamble.
FAA has weighed the considerations for oxygen mask requirements in
paragraphs (3) and (4) of Sec. 135.89(b) based on the available data
and the safety risk assessment. FAA has determined there is sufficient
reason to raise the altitude requirement for donning a supplemental
oxygen mask from FL350 to FL410 in Sec. 135.89(b)(3), as directed by
the Act. However, FAA has determined that the data and safety
considerations when applied to Sec. 135.89(b)(4) do not justify
raising the minimum altitude at which a lone pilot must don a
supplemental oxygen mask from FL250 to FL410.
Instead, FAA has determined that raising the minimum altitude in
Sec. 135.89(b)(4) to FL350 is acceptable based on safety data. FAA
requests comments and pertinent information on whether the appropriate
flight level for this requirement should be FL350 or FL410. In
addition, in considering proposed changes to Sec. 135.89(b), FAA also
decided that Sec. 91.211(b)(1)(ii) should be amended to align the
supplemental oxygen mask requirements in part 91 with safety
expectations, considering the proposed changes to part 135. These
decisions are based on the limited TUC available above FL350, the
available data on pressurization events, and the Safety Continuum's
mandate to ensure part 135 certificate holders are held to a higher
level of safety than general aviation flights conducted under part 91.

Table 3--Comparison of Current and Proposed FAA Rules
------------------------------------------------------------------------
Current FAA rule Proposed FAA rule
------------------------------------------------------------------------
14 CFR 135.89(b)(3) Two pilots at the 14 CFR 135.89 Two pilots at the
controls: Whenever a pressurized controls: Whenever a
aircraft is operated at altitudes pressurized aircraft is
above 35,000 feet MSL, at least one operated at altitudes above
pilot at the controls shall wear, FL410, at least one pilot at
secured and sealed, an oxygen mask the controls shall wear,
required by paragraph (b)(2)(i). secured and sealed, an oxygen
mask required by paragraph
(b)(2)(i).
14 CFR 135.89(b)(4) One pilot at the 14 CFR 135.89 One pilot at the
controls: If one pilot leaves a pilot controls: Whenever only one
duty station of an aircraft when pilot is at the controls of an
operating at altitudes above 25,000 aircraft when operating above
feet MSL, the remaining pilot at the FL350, that pilot shall put on
controls shall put on and use an and use an approved oxygen
approved oxygen mask until the other mask.
pilot returns to the pilot duty
station of the aircraft.
14 CFR 91.211(b)(1)(ii) Two pilots at 14 CFR 91.211(b)(1)(ii) One or
the controls: At flight altitudes two pilots at the controls: At
above FL350 unless one pilot at the flight altitudes above FL350
controls of the airplane is wearing unless one pilot at the
and using an oxygen mask that is controls of the aircraft is
secured and sealed and that either wearing and using an oxygen
supplies oxygen at all times or mask that is secured and
automatically supplies oxygen whenever sealed and that either
the cabin pressure altitude of the supplies oxygen at all times
airplane exceeds 41,000 feet (MSL), or automatically supplies
except that the one pilot need not oxygen whenever the cabin
wear and use an oxygen mask while at pressure altitude of the
or below FL410 if there are two pilots aircraft exceeds 14,000 feet
at the controls and each pilot has a (MSL), except that the one
quick-donning type of oxygen mask that pilot need not wear and use an
can be placed on the face with one oxygen mask if each pilot has
hand from the ready position within a quick-donning type of oxygen
five seconds, supplying oxygen and mask that can be placed on the
properly secured and sealed. face with one hand from the
ready position within five
seconds, supplying oxygen and
properly secured and sealed.
14 CFR 91.211(b)(2) One pilot at the
controls: Notwithstanding paragraph
(b)(1)(ii), if for any reason at any
time it is necessary for one pilot to
leave the controls of the aircraft
when operating at flight altitudes
above FL350, the remaining pilot at
the controls shall put on and use an
oxygen mask until the other pilot has
returned to that crewmember's station.
------------------------------------------------------------------------

A. Revising Part 135 Oxygen Mask Requirements With More Than One Pilot
on the Flightdeck

FAA is proposing to amend Sec. 135.89(b)(3) to raise the altitude
above which at least one pilot must wear a secured and sealed approved
quick-donning type oxygen mask from FL350 to FL410. Part 135 prescribes
the requirements for commuter and on-demand operations of each person
who is required to hold an air carrier certificate or operating
certificate under part 119. Section 135.89 outlines pilot requirements
for use of oxygen when conducting commercial operations in pressurized
and unpressurized aircraft. Currently, when a pressurized aircraft is
operated at altitudes above FL350, Sec. 135.89(b)(3) requires at least
one pilot at the controls to wear a quick-donning oxygen mask. These
FAA rules on pilot oxygen mask use are stricter than ICAO standards, as
explained in section III.B. of this preamble. Safety data shows that
revising these requirements will not have a noticeable negative impact
on safety, as discussed in section III.C. of this preamble. FAA finds
that with two pilots at the controls on the flightdeck, in the event of
a depressurization at or below FL410, as quick-donning masks can be
donned within five seconds as per regulatory requirements,\73\ it is
very likely that at least one pilot will be able to don an oxygen mask
successfully before useful consciousness is lost.\74\
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\73\ See 14 CFR 25.1447(c)(2). For general aviation aircraft
certified in accordance with 14 CFR part 23, similar guidance is
provided in industry consensus standard American Society for Testing
and Materials F3227 for aircraft certified for operation above
FL410.
\74\ See TUC table in Table 1.
---------------------------------------------------------------------------

In addition, FAA recognizes that other CAAs and the U.S. Air Force
currently have less restrictive regulations regarding donning oxygen
masks. This, in conjunction with the expenses incurred through
unnecessary oxygen tank servicing and concerns regarding effects on
pilots and crew coordination including reduced oxygen availability in
emergencies, disease transmission risk, difficulty in pilot
communications, and increased fatigue have led FAA to consider raising
the minimum altitudes

[[Page 60542]]

for oxygen mask donning. Therefore, FAA is proposing to amend Sec.
135.89(b)(3) to raise the altitude above which at least one pilot must
wear a secured and sealed approved quick-donning type oxygen mask from
FL350 to FL410.

B. Revising Part 135 Mask Requirement to Flight Level 350 if One Pilot
Is on the Flightdeck

In addition to the proposed revision to Sec. 135.89(b)(3), FAA is
proposing to revise Sec. 135.89(b)(4) so that if one pilot leaves a
pilot duty station, the remaining pilot at the controls shall put on
and use an approved oxygen mask when operating at altitudes above FL350
until the other pilot returns to the pilot duty station of the
aircraft. Section Sec. 135.89 contains requirements for pilots to use
oxygen when there is only one crewmember at the controls of the
aircraft. Currently, when a pressurized aircraft is operating at
altitudes above FL250, Sec. 135.89(b)(4) requires that if one pilot
leaves a pilot duty station the remaining pilot at the controls shall
put on and use an oxygen mask until the other pilot returns to the
pilot duty station of the aircraft.
Section 834 of the Act directs FAA to issue an NPRM concerning
whether to change Sec. 135.89(b)(4) to require a pilot to wear an
oxygen mask when one pilot leaves the pilot duty station when flying
above FL410. The Act further directs FAA to consider applicable safety
data, risks, and investigations and recommendations of NTSB. FAA
identified risks associated with changing Sec. 135.89(b)(4) to FL410.
This is due to the very short TUC that a lone pilot at the controls
would have to recognize the depressurization and take action to don an
oxygen mask. AC 61-107B CHG1, Aircraft Operations at Altitudes Above
25,000 feet Mean Sea Level or Mach Numbers Greater Than .75, lists
average TUC values in Figure 2-3. At 35,000 feet, the TUC is
approximately 30 to 60 seconds. In the event of a rapid decompression,
that time is cut in half, yielding a TUC of 15 to 30 seconds. Figure 2-
3 does not list a value for 41,000 feet, but at 40,000 feet, the TUC is
15-20 seconds. A rapid decompression has a nominal TUC.
Due to the very brief time period available for the pilot to detect
a decompression and don an oxygen mask, 15 to 20 seconds following
gradual decompression and nominal or likely immediate loss of
consciousness following rapid decompression (see Table 2), FAA
considers an amendment to Sec. 135.89(b)(4) allowing a single pilot on
the flightdeck to not wear an oxygen mask until above FL410 would not
provide an acceptable level of safety in part 135 operations. As such,
FAA is proposing that if one pilot leaves a pilot duty station, the
remaining pilot at the controls shall put on and use an approved oxygen
mask when operating at altitudes above FL350 until the other pilot
returns to the pilot duty station of the aircraft.
FAA further notes that operations in pressurized aircraft may occur
when only one pilot is assigned to the flight. This is permissible in
aircraft that are type certificated for single pilot operations, when
the pilot-in-command is qualified for single pilot operations and the
certificate holder has been issued the appropriate operations
specifications. The current regulatory text only refers to situations
when two flightcrew members are assigned to the flight. FAA is
proposing to clarify that Sec. 135.89(b)(4) is applicable when only
one pilot is assigned to the flight and operating above FL350 in
addition to when one pilot leaves the controls. Due to the risk of
depressurization, if it is necessary for one pilot to use an oxygen
mask when a second pilot leaves the controls, it stands to reason that
in situations where only one pilot is assigned to a flight, that pilot
must wear an oxygen mask whenever the aircraft is above FL350.

C. Revising Part 91 Oxygen Mask Requirements

Part 91 is the default set of flight rules for non-commercial and
general aviation aircraft in the United States. FAA is proposing to
revise Sec. 91.211(b) in two substantive ways. First, it would amend
Sec. 91.211(b)(ii) to expand the current exception from the
requirement that one pilot wear and use an oxygen mask. FAA proposes to
eliminate the current FL410 altitude limitation for the exception,
allowing pilots to fly above that previous altitude limit without
donning an oxygen mask as long as each pilot has a quick-donning type
of oxygen mask that can be placed on the face with one hand within five
seconds. FAA would also delete paragraph (b)(2) from Sec. 91.211
altogether, thereby removing the requirement that if one pilot leaves
the controls while the aircraft is flying above FL350, the other pilot
put on and use an oxygen mask. In other words, if the proposal is
finalized, no pilot would be required to wear and use an oxygen mask
above FL350 if quick-donning masks are available on the flight deck. If
the aircraft does not have available quick-donning oxygen masks, then
at least one pilot would need to wear and use an oxygen mask.
Although the proposed changes would not make Sec. 91.211 identical
to the corresponding ICAO standards, it would bring the regulation and
the standards into closer alignment than they are at present. The
proposed changes to part 91 would also uphold the Safety Continuum \75\
by tolerating more risk in general aviation operations than in part 135
operations. Therefore, FAA has chosen to use this NPRM as an
opportunity to propose changes to Sec. 91.211(b)(1)(ii). Please see
section III.A.4 for more details.
---------------------------------------------------------------------------

\75\ See The Safety Continuum--A Doctrine for Application (Sept.
2014), <a href="https://downloads.regulations.gov/FAA-2015-1621-0018/attachment_1.pdf">https://downloads.regulations.gov/FAA-2015-1621-0018/attachment_1.pdf</a>, and Section III.4. of this preamble for additional
discussion about safety continuum application to parts 91 and 135
pilot oxygen mask requirements.
---------------------------------------------------------------------------

D. Terminology Changes

In addition to the changes described previously, FAA also proposes
a few terminology changes. First, FAA proposes to change the term
``airplane'' to ``aircraft'' to remove ambiguity on whether the
requirements of Sec. 91.211 apply to all aircraft rather than only
applying to airplanes. Second, FAA proposes to replace references to
feet MSL when discussing altitude with the term ``flight level'' to
align altitude terminology in parts 91 and 135 with other parts of
title 14 where appropriate.

V. Regulatory Notices and Analyses

Executive Order (E.O.) 12866 (``Regulatory Planning and Review'')
and E.O. 13563 (``Improving Regulation and Regulatory Review'') require
agencies to regulate in the ``most cost-effective manner,'' to make a
``reasoned determination that the benefits of the intended regulation
justify its costs,'' and to develop regulations that ``impose the least
burden on society.'' The Office of Management and Budget has determined
this proposed rule is not a significant regulatory action as defined in
section 3(f) of E.O. 12866.

A. Regulatory Impact Analysis (RIA)

1. Introduction
FAA is proposing to update oxygen mask usage requirements for part
135 operations in compliance with the Act. Following a comprehensive
safety risk analysis, FAA has determined it is feasible to raise the
flight level for mandatory oxygen mask usage from FL250 to FL350 when a
single pilot is at the controls, and to FL410 when two pilots are on
the flightdeck at the controls with quick-donning masks available. In
addition, FAA proposes to eliminate the requirement for oxygen mask
usage above FL350 in part 91 operations, provided that quick-donning
masks are readily accessible. These

[[Page 60543]]

amendments aim to enhance operational flexibility while maintaining
safety standards.
2. Need for Regulation
The need for this proposed regulation stems from a congressional
mandate included in the Act. Specifically, section 834 of the Act
requires that FAA issue an NPRM on whether to revise the requirements
of 14 CFR 135.89(b)(3) and (4) to apply only above FL410, while
considering applicable safety data and risks. Although not required by
statute, FAA is also proposing additional deregulatory actions for part
91 oxygen mask requirements, since the same aircraft flown under part
135 are frequently operated under part 91. This keeps part 91
operations and part 135 operations in the appropriate frame of
reference with respect to the Safety Continuum and reduces the
regulatory burden on operations that are conducted under part 91 by
relaxing when oxygen masks need to be worn. Further information is
discussed in Section III of this preamble.
3. Baseline for the Analysis
Currently, when only one pilot is at the controls of a part 135
flight, that pilot must wear and use an oxygen mask above FL250. Part
91 requires one pilot to wear and use an oxygen mask if the pilot is
the only pilot at the controls above FL350. With two pilots at the
controls, one of those pilots must wear and use an oxygen mask above
FL350 if operating under part 135, and above FL410 in part 91
operations. Operators may choose to fly below these thresholds to avoid
using pilot oxygen, but would miss out on competing benefits to flying
higher such as decreased fuel burn and avoiding weather or
turbulence.\76\ FAA assumes for this analysis the benefits of flying
above the thresholds outweigh the costs of oxygen usage to operators,
and so the regulatory analysis baseline is that applicable aircraft are
currently operating above the flight levels at which using an oxygen
mask is required. FAA currently lacks data to assess the number of
applicable operations above the flight level thresholds, or if aircraft
are operating below the thresholds solely to conserve pilot oxygen, and
requests comment with supporting data.
---------------------------------------------------------------------------

\76\ An Aircraft Owners and Pilots Association (AOPA) article on
some of the benefits of flying at higher altitudes for part 91
aircraft can be found at <a href="https://www.aopa.org/news-and-media/all-news/2025/june/pilot/flying-smart-the-benefits-of-getting-high">https://www.aopa.org/news-and-media/all-news/2025/june/pilot/flying-smart-the-benefits-of-getting-high</a>.
---------------------------------------------------------------------------

4. Cost Savings
Under the regulatory baseline that aircraft are currently operating
above oxygen mask use altitude thresholds, the primary effect of the
proposed rule would be reduced utilization of pilot oxygen during
normal flights, having the resulting benefit of requiring less frequent
refill servicing. Oxygen tank size and capacity vary by model, and are
filled according to their pressure specification. By virtue of using
pressure to measure the refill of oxygen, it is difficult to measure
the exact amount of oxygen consumed, and so refill services are usually
charged as a flat fee. Cost can vary significantly by location, FAA
market research indicates it can range between $100 to $400 per
recharge, regardless of how much oxygen is refilled.\77\
---------------------------------------------------------------------------

\77\ For example, the Cessna Owner Organization notes that
oxygen refill pricing varies by location and often costs $150 or
more, found at <a href="https://cessnaowner.org/how-to-fill-your-own-oxygen-cylinder/">https://cessnaowner.org/how-to-fill-your-own-oxygen-cylinder/</a>.
---------------------------------------------------------------------------

Based on these estimates, FAA calculates the annual cost savings
range for operators by multiplying the servicing cost with the amount
of reduced oxygen recharges due to no longer needing to consume as much
oxygen during normal flight. Without data on the current total number
of oxygen refills or roughly how many flights typically occur between
servicing, FAA is unable to estimate the exact reduction in refills and
resulting cost savings this rule could generate. Instead, FAA estimates
what a percentage point reduction in affected operations no longer
requiring oxygen servicing saves applicable aircraft operators.
As a proxy for operations, FAA uses the total landings data for
turbojets and multiengine turboprops from the 2023 General Aviation and
Part 135 Activity Surveys.\78\ Although the 2023 landing counts are the
most current survey data, FAA acknowledges that the number of
operations, and resulting cost savings, varies from year-to-year and
may change in the future. FAA also notes this landing data is
potentially an overcount, as it may include two-engine turboprops that
are not capable of operating above the flight level thresholds
requiring oxygen use. Based on the landing data, each percentage point
reduction of the roughly 4.7 million general aviation and part 135
turbojet and multiengine turboprop operations per year no longer
requiring oxygen servicing would save operators $4.7 million to $18.6
million in oxygen servicing fees annually. FAA calculates this based on
one percent of the total operations, or 46,589 operations, multiplied
by the $100 to $400 servicing cost. FAA requests comment, with
supporting documentation, on the estimated price of oxygen servicing,
potential reduction in the frequency of oxygen servicing operators
would realize due to the proposed rule, and any other assumption or
estimate.
---------------------------------------------------------------------------

\78\ The 2023 total landings data for parts 91 and 135 turbojets
and multiengine turboprops is found in table 2.3 of the 2023 General
Aviation and Part 135 Activity Surveys, available at: <a href="https://www.faa.gov/data_research/aviation_data_statistics/general_aviation/cy2023">https://www.faa.gov/data_research/aviation_data_statistics/general_aviation/cy2023</a>.
---------------------------------------------------------------------------

5. Costs
The proposed rule would enable operation of aircraft under parts 91
and 135 to higher altitudes before requiring use of an oxygen mask,
provided there are quick-donning masks available to the pilots. As
discussed in section III.C. of the preamble, the likelihood of
depressurization is considered ``extremely improbable'' and therefore
would have no expected impacts to safety from the changes proposed in
this rule and no resulting costs. The only resulting estimated effect
of the proposed rule would be cost savings from reduced oxygen use and
frequency of refill servicing, as discussed in the benefits section.
With no other major changes from the baseline, FAA anticipates no
effects that would have notable costs and welcomes comment on this
assessment, along with any other assumption or estimate presented in
the RIA.

B. Regulatory Flexibility Act

The Regulatory Flexibility Act (RFA) of 1980, Public Law 96-354 (5
U.S.C. 601-612), as amended by the Small Business Regulatory
Enforcement Fairness Act of 1996 (Pub. L. 104-121) and the Small
Business Jobs Act of 2010 (Pub. L. 111-240), requires Federal agencies
to consider the effects of the regulatory action on small business and
other small entities and to minimize any significant economic impact.
The term ``small entities'' comprises small businesses and not-for-
profit organizations that are independently owned and operated and are
not dominant in their fields, and governmental jurisdictions with
populations of less than 50,000.
As described in the RIA above, FAA estimates the effect of this
proposed rule would be cost savings for operators requiring less
frequent oxygen refill servicing. If an agency determines that a
rulemaking will not result in a significant economic impact on a
substantial number of small entities, the head of the agency may so
certify under section 605(b) of the RFA. Therefore, as

[[Page 60544]]

provided in section 605(b) and based on the foregoing, the head of FAA
proposes certifying that this rulemaking will not result in a
significant economic impact for small entities. FAA welcomes comments
on the basis for this proposed certification.

C. International Trade Impact Assessment

The Trade Agreements Act of 1979 (Pub. L. 96-39), as amended by the
Uruguay Round Agreements Act (Pub. L. 103-465), prohibits Federal
agencies from establishing standards or engaging in related activities
that create unnecessary obstacles to the foreign commerce of the United
States. Pursuant to these Acts, the establishment of standards is not
considered an unnecessary obstacle to the foreign commerce of the
United States, so long as the standard has a legitimate domestic
objective, such as the protection of safety and does not operate in a
manner that excludes imports that meet this objective. The statute also
requires consideration of international standards and, where
appropriate, that they be the basis for U.S. standards.
FAA has assessed the potential effect of this proposed rule and
determined that it brings its regulations in line with international
practices and does not exclude imports that meet this objective. As a
result, FAA does not consider this proposed rule as creating an
unnecessary obstacle to foreign commerce.

D. Unfunded Mandates Assessment

The Unfunded Mandates Reform Act of 1995 (2 U.S.C. 1531-1538)
governs the issuance of Federal regulations that require unfunded
mandates. An unfunded mandate is a regulation that requires a State,
local, or Tribal government or the private sector to incur direct costs
without the Federal Government having first provided the funds to pay
those costs. FAA determined that the proposed rule would not result in
the expenditure of $187 million or more by State, local, or Tribal
governments, in the aggregate, or the private sector, in any one year.

E. Paperwork Reduction Act

The Paperwork Reduction Act of 1995 (44 U.S.C. 3507(d)) requires
that FAA consider the impact of paperwork and other information
collection burdens imposed on the public. FAA has determined that there
would be no new requirement for information collection associated with
this proposed rule.

F. International Compatibility

In compliance with U.S. obligations under the Chicago Convention,
FAA policy is to conform to ICAO SARPs to the maximum extent
practicable. Therefore, FAA is revising its regulations under parts 91
and 135 to better align with ICAO Annex 6 SARPs for oxygen use. Annex 6
mandates that flight crewmembers in pressurized airplanes above 25,000
feet (376 hPa) have access to an oxygen mask that supplies oxygen on
demand, but it does not require the masks to be worn continuously at
any altitude. Currently, Sec. 135.89 is stricter than the SARP. The
proposed amendment to Sec. 135.89 would still be more restrictive but
would bring FAA regulations closer in alignment to the SARP. The
proposed changes to Sec. 91.211 would fully align FAA requirements
with the SARP regarding mask usage. However, similar to the existing
Sec. 91.211, the proposed rule would remain less restrictive than the
SARP by not requiring the installation of quick-donning masks; instead,
it specifies that if such masks are not installed, one pilot must wear
a mask above FL350.

G. Environmental Analysis

The Department has analyzed the environmental impacts of this
notice of proposed rulemaking pursuant to the National Environmental
Policy Act of 1969 (NEPA) (42 U.S.C. 4321, et seq.). FAA has determined
this rule is categorically excluded pursuant to FAA Order 1050.1G.
Categorical exclusions are categories of actions FAA has determined
normally do not significantly affect the quality of the human
environment and therefore do not require either an environmental
assessment (EA) or environmental impact statement (EIS).\79\ In
analyzing the applicability of a categorical exclusion, FAA must also
consider whether extraordinary circumstances are present that would
warrant the preparation of an EA or EIS.\80\ This rulemaking, which
proposes to raise the altitudes at which a pilot is required to don an
oxygen mask for commuter and on demand operations, is categorically
excluded pursuant to FAA Order 1050.1G, Appendix B, Paragraph B-2.6(f),
which categorically excludes issuance of regulatory documents. FAA does
not anticipate any environmental impacts, and there are no
extraordinary circumstances present in connection with this rulemaking.
---------------------------------------------------------------------------

\79\ See DOT Order 5610.1D Sec. 9.
\80\ Id. Sec. 9(b).
---------------------------------------------------------------------------

VI. Executive Order Determinations

A. Executive Order 13132, Federalism

FAA has analyzed this proposed rule under the principles and
criteria of E.O. 13132, Federalism. FAA has determined that this action
would not have a substantial direct effect on the States, or the
relationship between the Federal Government and the States, or on the
distribution of power and responsibilities among the various levels of
government, and, therefore, would not have federalism implications.

B. Executive Order 13175, Consultation and Coordination With Indian
Tribal Governments

Consistent with E.O. 13175, Consultation and Coordination with
Indian Tribal Governments,\81\ and FAA Order 1210.20, American Indian
and Alaska Native Tribal Consultation Policy and Procedures,\82\ FAA
ensures that Federally Recognized Tribes (Tribes) are given the
opportunity to provide meaningful and timely input regarding proposed
Federal actions that have the potential to affect uniquely or
significantly their respective Tribes. At this point, FAA has not
identified any unique or significant effects, environmental or
otherwise, on Tribes resulting from this proposed rule.
---------------------------------------------------------------------------

\81\ 65 FR 67249 (Nov. 6, 2000).
\82\ FAA Order No. 1210.20 (Jan.28, 2004), available at
<a href="http://www.faa.gov/documentLibrary/media/1210.pdf">www.faa.gov/documentLibrary/media/1210.pdf</a>.
---------------------------------------------------------------------------

C. Executive Order 13211, Regulations That Significantly Affect Energy
Supply, Distribution, or Use

FAA analyzed this proposed rule under E.O. 13211, Actions
Concerning Regulations that Significantly Affect Energy Supply,
Distribution, or Use. FAA has determined the proposed rule change would
not be a ``significant energy action'' under the Executive order and
would be unlikely to have a significant adverse effect on the supply,
distribution, or use of energy.

D. Executive Order 13609, Promoting International Regulatory
Cooperation

E.O. 13609, Promoting International Regulatory Cooperation,
promotes international regulatory cooperation to (1) meet shared
challenges involving health, safety, labor, security, environmental,
and other issues and reduce, eliminate, or (2) prevent unnecessary
differences in regulatory requirements. FAA has analyzed this action
under the policy and agency responsibilities of E.O. 13609. FAA has
determined this action would reduce differences between U.S. aviation
standards and those of other civil

[[Page 60545]]

aviation authorities by aligning with the ICAO Standards for Use of
Oxygen found in Annex 6, Part I, Section 4.4.5. Currently, Sec. 135.89
is more restrictive than the ICAO standard. Amending Sec. 135.89 as
proposed would still be more restrictive than ICAO Annex 6, Part I, but
would bring it closer to the ICAO standard. Similarly, Sec. 91.211 is
currently more restrictive than the ICAO Standards for general aviation
outlined in Annex 6, Part II, Attachment 2.A. However, the proposed
amendment would align it more closely with the ICAO Standard.

E. Executive Order 14192, Unleashing Prosperity Through Deregulation

This proposed rule, if finalized as proposed, is expected to be an
E.O. 14192 deregulatory action.

VII. Additional Information

A. Comments Invited

FAA invites interested persons to participate in this rulemaking by
submitting written comments, data, or views. FAA also invites comments
relating to the economic, environmental, energy, or federalism impacts
that might result from adopting the proposals in this document. The
most helpful comments reference a specific portion of the proposal,
explain the reason for any recommended change, and include supporting
data. To ensure the docket does not contain duplicate comments,
commenters should submit only one time if comments are filed
electronically, or commenters should send only one copy of written
comments if comments are filed in writing.
FAA will file in the docket all comments it receives, as well as a
report summarizing each substantive public contact with FAA personnel
concerning this proposed rulemaking. Before acting on this proposal,
FAA will consider all comments it receives on or before the closing
date for comments. FAA will consider comments filed after the comment
period has closed if it is possible to do so without incurring expense
or delay. FAA may change this proposal in light of the comments it
receives.
Privacy: In accordance with 5 U.S.C. 553(c), DOT solicits comments
from the public to better inform its rulemaking process. DOT posts
these comments, without edit, including any personal information the
commenter provides, to <a href="https://www.regulations.gov">https://www.regulations.gov</a>, as described in the
system of records notice (DOT/ALL-14 FDMS), which can be reviewed at
<a href="https://www.dot.gov/privacy">https://www.dot.gov/privacy</a>.

B. Confidential Business Information

Confidential Business Information (CBI) is commercial or financial
information that is both customarily and actually treated as private by
its owner. Under the Freedom of Information Act (FOIA) (5 U.S.C. 552),
CBI is exempt from public disclosure. If your comments responsive to
this NPRM contain commercial or financial information that is
customarily treated as private, that you actually treat as private, and
that is relevant or responsive to this NPRM, it is important that you
clearly designate the submitted comments as CBI. Please mark each page
of your submission containing CBI as ``PROPIN.'' FAA will treat such
marked submissions as confidential under the FOIA, and they will not be
placed in the public docket of this NPRM. Submissions containing CBI
should be sent to the person in the FOR FURTHER INFORMATION CONTACT
section of this document. Any commentary FAA receives which is not
specifically designated as CBI will be placed in the public docket for
this rulemaking.

C. Electronic Access and Filing

A copy of this NPRM, all comments received, any final rule, and all
background material may be viewed online at <a href="https://www.regulations.gov">https://www.regulations.gov</a>
using the docket number listed above. Electronic retrieval help and
guidelines are available on the website. It is available 24 hours each
day, 365 days each year. An electronic copy of this document may also
be downloaded from the Office of the Federal Register's website at
<a href="https://www.federalregister.gov">https://www.federalregister.gov</a> and the Government Publishing Office's
website at <a href="https://www.govinfo.gov">https://www.govinfo.gov</a>. A copy may also be found at FAA's
Regulations and Policies website at <a href="https://www.faa.gov/regulations_policies">https://www.faa.gov/regulations_policies</a>.
Copies may also be obtained by sending a request to the Federal
Aviation Administration, Office of Rulemaking, ARM-1, 800 Independence
Avenue SW, Washington, DC 20591, or by calling (202) 267-9677.
Commenters must identify the docket or notice number of this
rulemaking.
All documents FAA considered in developing this proposed rule,
including economic analyses and technical reports, may be accessed in
the electronic docket for this rulemaking.

D. Small Business Regulatory Enforcement Fairness Act

The Small Business Regulatory Enforcement Fairness Act (SBREFA) of
1996 requires FAA to comply with small entity requests for information
or advice about compliance with statutes and regulations within its
jurisdiction. A small entity with questions regarding this document may
contact its local FAA official or the person listed under the FOR
FURTHER INFORMATION CONTACT heading at the beginning of the preamble.
To find out more about SBREFA on the internet, visit <a href="https://www.faa.gov/regulations_policies/rulemaking/sbre_act/">https://www.faa.gov/regulations_policies/rulemaking/sbre_act/</a>.

List of Subjects

14 CFR Part 91

Air carriers, Aircraft, Air taxis, Airmen, Aviation safety, Charter
flights.

14 CFR Part 135

Air taxis, Aircraft, Airmen, Aviation safety.

The Proposed Amendment

For the reasons discussed in the preamble, the Federal Aviation
Administration proposes to amend 14 CFR chapter I as follows:

PART 91--GENERAL OPERATING AND FLIGHT RULES

0
1. The authority citation for part 91 continues to read as follows:

Authority: 49 U.S.C. 106(f), 40101, 40103, 40105, 40113, 40120,
44101, 44111, 44701, 44704, 44709, 44711, 44712, 44715, 44716,
44717, 44722, 46306, 46315, 46316, 46504, 46506-46507, 47122, 47508,
47528-47531, 47534; Pub. L. 114-190, 130 Stat. 615 (49 U.S.C. 44703
note); sec. 828 of Pub. L. 118-63, 138 Stat. 1330 (49 U.S.C. 44703
note); articles 12 and 29 of the Convention on International Civil
Aviation (61 Stat. 1180), (126 Stat. 11).

0
2. Amend Sec. 91.211 by revising paragraph (b) to read as follows:

Sec. 91.211 Supplemental oxygen.

* * * * *
(b) Pressurized cabin aircraft. (1) No person may operate a civil
aircraft of U.S. registry with a pressurized cabin at flight altitudes
above flight level 250 unless at least a 10-minute supply of
supplemental oxygen, in addition to any oxygen required to satisfy
paragraph (a) of this section, is available for each occupant of the
aircraft for use in the event that a descent is necessitated by loss of
cabin pressurization.
(2) No person may operate a civil aircraft of U.S. registry with a
pressurized cabin at flight altitudes above flight level 350 unless one
pilot at the controls of the aircraft is wearing and using an oxygen
mask that is secured and sealed and that either supplies oxygen at all
times or automatically supplies oxygen

[[Page 60546]]

whenever the cabin pressure altitude of the aircraft exceeds 14,000
feet (MSL), except that the one pilot need not wear and use an oxygen
mask if each pilot has a quick-donning type of oxygen mask that can be
placed on the face with one hand from the ready position within five
seconds, supplying oxygen and properly secured and sealed.

PART 135--OPERATING REQUIREMENTS: COMMUTER AND ON-DEMAND OPERATIONS
AND RULES GOVERNING PERSONS ON BOARD SUCH AIRCRAFT

0
3. The authority citation for part 135 continues to read as follows:

Authority: 49 U.S.C. 106(f), 40113, 41706, 44701-44702, 44705,
44709, 44711-44713, 44715-44717, 44722, 44730, 45101-45105; Pub. L.
112-95, 126 Stat. 58 (49 U.S.C. 44730).

0
4. Amend Sec. 135.89 by revising paragraphs (b)(2) introductory text,
(b)(3) and (b)(4) to read as follows:

Sec. 135.89 Pilot requirements: Use of oxygen.

* * * * *
(b) * * *
(2) Whenever a pressurized aircraft is operated at altitudes above
flight level 250 through flight level 350, unless each pilot has an
approved quick-donning type oxygen mask--
* * * * *
(3) Whenever a pressurized aircraft is operated at altitudes above
flight level 410, at least one pilot at the controls must wear, secured
and sealed, an oxygen mask required by paragraph (b)(2)(i) of this
section.
(4) Whenever only one pilot is at the controls of an aircraft when
operating above flight level 350, that pilot shall put on and use an
approved oxygen mask.

Issued under authority provided by 49 U.S.C. 106(f), 44701(a),
and section 834 of Public Law 118-63 in Washington, DC.
Hugh J. Thomas,
Executive Director, Flight Standards Service.
[FR Doc. 2026-19584 Filed 9-23-26; 8:45 am]
BILLING CODE 4910-13-P

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