Proposed Rule2024-09660
Special Conditions: Airbus Model A321neo Extra-Long Range (XLR) Airplane; Cabin Evacuation-Protection From Fuel Tank Explosion Due to External Fuel-Fed Ground Fire
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Published
May 7, 2024
Issuing agencies
Transportation DepartmentFederal Aviation Administration
Abstract
This action proposes special conditions for the Airbus Model A321neoXLR airplane. This airplane will have a novel or unusual design feature when compared to the technology envisaged by the airworthiness standards for transport category airplanes. This design feature is an integral rear center tank (RCT). The applicable airworthiness regulations do not contain adequate or appropriate safety standards for fire-safety performance of fuel-tank skin or structure in a post-crash external fuel-fed ground fire. These proposed special conditions contain the additional safety standards that the Administrator considers necessary to establish a level of safety equivalent to that established by the existing airworthiness standards.
Full Text
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<title>Federal Register, Volume 89 Issue 89 (Tuesday, May 7, 2024)</title>
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[Federal Register Volume 89, Number 89 (Tuesday, May 7, 2024)]
[Proposed Rules]
[Pages 38004-38007]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2024-09660]
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DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 25
[Docket No. FAA-2023-2412; Notice No. 25-23-06-SC]
Special Conditions: Airbus Model A321neo Extra-Long Range (XLR)
Airplane; Cabin Evacuation--Protection From Fuel Tank Explosion Due to
External Fuel-Fed Ground Fire
AGENCY: Federal Aviation Administration (FAA), DOT
ACTION: Notice of proposed special conditions.
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SUMMARY: This action proposes special conditions for the Airbus Model
A321neoXLR airplane. This airplane will have a novel or unusual design
feature when compared to the technology envisaged by the airworthiness
standards for transport category airplanes. This design feature is an
integral rear center tank (RCT). The applicable airworthiness
regulations do not contain adequate or appropriate safety standards for
fire-safety performance of fuel-tank skin or structure in a post-crash
external fuel-fed ground fire. These proposed special conditions
contain the additional safety standards that the Administrator
considers necessary to establish a level of safety equivalent to that
established by the existing airworthiness standards.
DATES: Send comments on or before June 21, 2024.
ADDRESSES: Send comments identified by Docket No. FAA-2023-2412 using
any of the following methods:
Federal eRegulations Portal: Go to <a href="http://www.regulations.gov">www.regulations.gov</a> and follow
the online instructions for sending your comments electronically.
Mail: Send comments to Docket Operations, M-30, U.S. Department of
Transportation (DOT), 1200 New Jersey Avenue SE, Room W12-140, West
Building Ground Floor, Washington, DC, 20590-0001.
Hand Delivery or Courier: Take comments to Docket Operations in
Room W12-140 of the West Building Ground Floor at 1200 New Jersey
Avenue SE, Washington, DC, between 9 a.m. and 5 p.m., Monday through
Friday, except Federal holidays.
Fax: Fax comments to Docket Operations at 202-493-2251.
Docket: Background documents or comments received may be read at
<a href="http://www.regulations.gov">www.regulations.gov</a> at any time. Follow the online instructions for
accessing the docket or go to Docket Operations in Room W12-140 of the
West Building Ground Floor at 1200 New Jersey Avenue SE, Washington,
DC, between 9 a.m. and 5 p.m., Monday through Friday, except Federal
holidays.
FOR FURTHER INFORMATION CONTACT: Douglas Bryant, Engine and Propulsion
Section, AIR-625, Technical Policy Branch, Policy and Standards
Division, Aircraft Certification Service, Federal Aviation
Administration, 2200 South 216th Street, Des Moines, Washington 98198;
telephone and fax 206-231-3166; email <a href="/cdn-cgi/l/email-protection#13777c66747f72603d7d3d71616a727d67537572723d747c65"><span class="__cf_email__" data-cfemail="a4c0cbd1c3c8c5d78aca8ac6d6ddc5cad0e4c2c5c58ac3cbd2">[email protected]</span></a>.
SUPPLEMENTARY INFORMATION:
Comments Invited
The FAA invites interested people to take part in this rulemaking
by sending written comments, data, or views. The most helpful comments
reference a specific portion of the proposed special conditions,
explain the reason for any recommended change, and include supporting
data.
The FAA will consider all comments received by the closing date for
comments, and will consider comments filed late if it is possible to do
so without incurring delay. The FAA may change these special conditions
based on the comments received.
Privacy
Except for Confidential Business Information (CBI) as described in
the following paragraph, and other information as described in title
14, Code of Federal Regulations (14 CFR) 11.35, the FAA will post all
comments received without change to <a href="http://www.regulations.gov">www.regulations.gov</a>, including any
personal information you provide. The FAA will also post a report
[[Page 38005]]
summarizing each substantive verbal contact received about these
special conditions.
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
these special conditions contain commercial or financial information
that is customarily treated as private, that you actually treat as
private, and that is relevant or responsive to these special
conditions, it is important that you clearly designate the submitted
comments as CBI. Please mark each page of your submission containing
CBI as ``PROPIN.'' The FAA will treat such marked submissions as
confidential under the FOIA, and the indicated comments will not be
placed in the public docket of these special conditions. Send
submissions containing CBI to the individual listed in the For Further
Information Contact section. Comments the FAA receives, which are not
specifically designated as CBI, will be placed in the public docket for
these special conditions.
Background
On September 16, 2019, Airbus applied for an amendment to Type
Certificate No. A28NM to include the new Model A321neo XLR series
airplane. The Airbus Model A321neo XLR series airplane, which is a
derivative of the Model A321neo Airbus Cabin Flex (ACF) currently
approved under Type Certificate No. A28NM, is a twin-engine transport
category aircraft that seats up to 244 passengers and has a maximum
takeoff weight of 222,667 lbs.
Type Certification Basis
Under the provisions of title 14, Code of Federal Regulations (14
CFR) 21.101, Airbus must show that the Model A321neo XLR series
airplane meets the applicable provisions of the regulations listed in
Type Certificate No. A28NM, or the applicable regulations in effect on
the date of application for the change, except for earlier amendments
as agreed upon by the FAA.
If the Administrator finds that the applicable airworthiness
regulations (e.g., 14 CFR part 25) do not contain adequate or
appropriate safety standards for the Airbus Model A321neo XLR series
airplane because of a novel or unusual design feature, special
conditions are prescribed under the provisions of Sec. 21.16.
Special conditions are initially applicable to the model for which
they are issued. Should the type certificate for that model be amended
later to include any other model that incorporates the same novel or
unusual design feature, or should any other model already included on
the same type certificate be modified to incorporate the same novel or
unusual design feature, these special conditions would also apply to
the other model under Sec. 21.101.
In addition to the applicable airworthiness regulations and special
conditions, the Airbus Model A321neo XLR series airplane must comply
with the fuel venting and exhaust emission requirements of 14 CFR part
34 and the noise certification requirements of 14 CFR part 36.
The FAA issues special conditions, as defined in 14 CFR 11.19, in
accordance with Sec. 11.38, and they become part of the type
certification basis under Sec. 21.101.
Novel or Unusual Design Features
The Airbus Model A321neo XLR series airplane will incorporate the
following novel or unusual design feature:
An integral RCT.
Discussion
The proposed Airbus Model A321neo XLR series airplane incorporates
an integral RCT. This tank is a ``center'' fuel tank, that would, if
approved, be located in the airplane fuselage rather than in its wings.
The tank is a ``rear'' tank, that would be located aft of the center
wing fuel tank and behind the wheel bay; it would be in an area of the
lower section of the fuselage, partially replacing the aft cargo
compartment of the airplane from which this proposed model is derived.
The top of the tank would be directly below the floor of the passenger
cabin. The fuel tank would be ``integral'' to the airplane, in that its
walls would be part of the airplane structure. The exterior skin of the
airplane fuselage would constitute part of the walls of the fuel tank,
and these areas are usually separate boundaries (not integral) on other
fuselage fuel tanks. An integral fuel tank may be referred to as a
conformal fuselage structural fuel tank since boundaries of the fuel
tank ``conform'' with the airplane exterior. The integral RCT is
installed in a location that may be exposed to the direct effects of
post-crash ground, or pool, fuel-fed fires. An external fuel-fed ground
fire or external fuel-fed pool fire is also referred to as `external
ground fire'.
The airworthiness standards applicable to the Model A321neo XLR do
not contain specific standards for post-crash fire-safety performance
of fuel-tank skin or structure. In addition, the integral RCT on the
A321neo XLR was not envisaged by the FAA when promulgating requirements
related to occupant protection when fuel tanks are exposed to external
fuel-fed fires. The FAA considered fuel tank designs in widespread use
on transport airplanes, including main fuel tanks and auxiliary fuel
tanks when promulgating requirements related to occupant protection.
Auxiliary fuel tanks are normally located in the center wing and within
cargo holds, and in such cases are sometimes referred to as an
auxiliary center tank (ACT).
Airplane manufacturers commonly incorporate a center wing fuel tank
as an auxiliary fuel tank to make fuel available for increasing the
flight range of the airplane. Continued expansion of range performance
requirements has resulted in airplane designs using other areas of the
airplane to carry fuel, such as incorporating fuel tanks in the
empennage and fuselage. The Airbus model A321neo XLR airplane includes
a center wing fuel tank, an integral RCT and the option for additional
ACTs within the fuselage. Unlike an integral RCT, a center wing fuel
tank and optional ACTs are not expected by the FAA or manufacturers to
be exposed to the direct effects of post-crash ground fire because the
fuel tank walls are not exterior airplane skin on the center fuel tank
or ACT designs.
Due to its unusual configuration, the A321neo XLR's integral RCT
will also not incorporate the insulation that usually lines the
fuselage skin of a modern transport category airplane. Therefore the
FAA has issued, after notice and comment, a set of special conditions
that address that novel or unusual aspect of the A321neo XLR's integral
RCT with regard to certain of the FAA's regulatory requirements for
thermal/acoustic insulation installations, specifically 14 CFR
25.856(b). Those special conditions, No. 25-825-SC, require that the
lower half of the fuselage spanning the longitudinal location of the
RCT resist penetration from an external fuel-fed fire, in order to
ensure that the design provides the same level of passenger protection
from such fires as do the FAA's existing regulations for such
insulation.\1\ The special conditions
[[Page 38006]]
proposed herein address a different flammability aspect of the A321neo
XLR's integral RCT.
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\1\ Special Conditions: Airbus Model A321neoXLR Airplane;
Passenger Protection from External Fire. 87 FR 74503 (Dec. 6, 2022).
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Pertinent to the fuel tank structure, post-crash-fire occupant
survivability is dependent on the time available for occupant
evacuation prior to fuel-tank breach or structural failure. Structural
failure can be a result of degradation in load-carrying capability
caused by a fuel-fed ground fire. Structural failure can also be a
result of over-pressurization caused by ignition of fuel vapors inside
the fuel tank.
Past experience indicates that occupant survivability following a
post-crash fire is greatly influenced by the size and intensity of any
fire that occurs. The ability of main fuel tanks, when they have
aluminum wing surfaces wetted by fuel on their interior surface, to
withstand post-crash-fire conditions, has been demonstrated by tests
conducted at the FAA William J. Hughes Technical Center.\2\ Results of
these tests have verified adequate dissipation of heat across wetted
aluminum fuel-tank surfaces so that localized hot spots do not occur,
thus minimizing the threat of explosion. This inherent capability of
aluminum to dissipate heat also allows the aircraft's lower surface,
which is also the fuel tank boundary, to retain its load-carrying
characteristics during a fuel-fed ground fire, and significantly delays
structural collapse or burn-through for a time interval that usually
exceeds evacuation times. In addition, as an aluminum fuel tank with
significant quantities of fuel inside is heated, fuel vapor accumulates
in the ullage space, exceeding the upper flammability limit relatively
quickly and thus reducing the threat of a fuel-tank explosion prior to
fuel-tank burn-through.
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\2\ Hill, R., and Johnson, G.R., ``Investigation of Aircraft
Fuel Tank Explosions and Nitrogen Inerting Requirements During
Ground Fires,'' FAA Report DOT/FAA/RD-75-119, October 1975.
Available via the FAA Technical Center website for Fire Safety at
<a href="http://www.fire.tc.faa.gov/">http://www.fire.tc.faa.gov/</a>.
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The center wing tank and optional ACTs are surrounded by fuselage
structure and would not be directly exposed to a post-crash ground
fire. This inherent separation is also expected to significantly delay
structural collapse or burn-through and reduce the threat of explosion
for a time interval that usually exceeds evacuation times. Service
history of conventional aluminum airplanes has shown that fuel-tank
explosions caused by ground fires have been rare on airplanes
configured with flame arrestors in the fuel-tank vent lines. The Model
A321neo XLR integral RCT may or may not have equivalent capability of
past designs approved with existing regulations, due to the RCT design
and location being integral with the fuselage.
There are several Part 25 requirements that address fire-safety
performance of the fuel tanks and fuselage in the Model A321neo XLR
certification basis. However, these requirements do not directly or
adequately address standards for post-crash fire-safety performance of
fuel-tank skin or structure. These standards address failure conditions
or minimize the hazard to the occupants in the event ignition of
flammable fluids or vapors occurs. For example, Sec. 25.863 requires
applicants to minimize the probability of ignition and resultant
hazards if ignition occurs for flammable fluid systems on the airplane.
Another example is Sec. 25.981(a) which requires applicants to
demonstrate no ignition source may be present at each point in the fuel
tank or fuel tank system where catastrophic failure could occur due to
ignition of fuel or vapors. Specifically, Sec. 25.981(a)(1) requires
``determining the highest temperature allowing a safe margin below the
lowest expected autoignition temperature of the fuel in the fuel
tanks.'' Then Sec. 25.981(a)(2) requires ``demonstrating that no
temperature at each place inside each fuel tank where fuel ignition is
possible will exceed the temperature determined under paragraph (a)(1)
of this section. This must be verified under all probable operating,
failure, and malfunction conditions of each component whose operation,
failure, or malfunction could increase the temperature inside the
tank.'' In addition, Sec. 25.981(a)(3) requires ``except for ignition
sources due to lightning addressed by Sec. 25.954, demonstrating that
an ignition source could not result from each single failure, from each
single failure in combination with each latent failure condition not
shown to be extremely remote, and from all combinations of failures not
shown to be extremely improbable, taking into account the effects of
manufacturing variability, aging, wear, corrosion, and likely damage.''
These airworthiness requirements address ignition sources and are part
of the FAA's regulatory framework for preventing fires and explosions;
however, taken together, they do not adequately address the potential
for a post-crash external ground fire to affect the safety of airplane
occupants.
The FAA therefore determined that the airworthiness standards
applicable to the Model A321neo XLR airplane do not contain adequate
standards for post-crash fire-safety performance of fuel-tank skin or
structure. The FAA therefore proposes that special conditions are
needed for the Model A321neo XLR airplane, because the integral RCT
design, including location in the lower fuselage, is considered an
unusual or novel design feature that could expose the RCT to an
external ground fire. Factors influencing occupant survival time when a
fuel tank is exposed to a ground-fed fire are the structural integrity
of the tank; burn-through resistance; flammability of the tank; and the
presence of auto-ignition threats during exposure to a fire. As
previously discussed, the FAA issued Special Conditions No. 25-825-SC
were issued to address the novel or unusual aspect of the A321neo XLR's
integral RCT with regard to requirements for thermal/acoustic
insulation installations. The FAA considers the occupant survival time
related to the burn-through resistance of the integral RCT to be
adequately accounted for in those special conditions.
These proposed special conditions address standards for post-crash
fire-safety performance of fuel-tank skin or structure by proposing a
requirement to prevent the ignition of fuel vapor during an external
fuel-fed ground fire. These proposed special conditions include
accounting for the potential for hot surface ignition created by the
external fuel-fed fire. As described in FAA Advisory Circular 25.981-
1D, ``Fuel Tank Ignition Source Prevention Guidelines,'' hot surfaces
that can exceed the autoignition temperature of the flammable vapor
under consideration are considered to be ignition sources. The FAA
intends this proposed requirement to adequately protect the airplane
occupants from the consequences of an integral RCT exposed to an
external fuel-fed ground, or pool fire.
The intention of the proposed requirement for the design to prevent
ignition is for the applicant to show that ignition sources do not
occur, such as from a hot surface, due to the external heat applied to
the integral RCT from an external fuel-fed ground fire. Where
previously discussed, Sec. 25.981(a) requires applicants to
demonstrate that no ignition source may be present but does not
specifically address ignition due to an external fuel-fed ground fire.
To provide the same level of safety as provided by the relevant
regulations in this model's certification basis, Airbus must
demonstrate that the Model A321neo XLR series airplane has sufficient
post-crash fire-safety performance of fuel-tank skin or structure to
enable occupants to safely evacuate in the event that the integral RCT
is exposed to an external fuel-fed ground fire.
[[Page 38007]]
The FAA assessed post-crash-survival time during the adoption of
Sec. 25.856 and revisions to appendix F to part 25 at Amendment 25-111
for fuselage burn-through protection. Studies conducted by and on
behalf of the FAA indicated that following a survivable accident,
prevention of fuselage burn-through for approximately 5 minutes can
significantly enhance survivability.\3\
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\3\ Cherry, R. and Warren, K. ``Fuselage Burnthrough Protection
for Increased Postcrash Occupant Survivability: Safety Benefit
Analysis Based on Past Accidents, ``FAA Report DOT/FAA/AR-99/57,
September 1999 and R G W Cherry & Associates Limited, ``A Benefit
Analysis for Cabin Water Spray Systems and Enhanced Fuselage
Burnthrough Protection,'' FAA Report DOT/FAA/AR-02/49, April 7,
2003.
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The FAA would consider Airbus showing the design prevents ignition
of fuel tank vapors in the integral RCT during at least 5 minutes of
exposure to an external fuel-fed ground fire as a sufficient time
duration for the purposes of these special conditions. The time
duration of 5 minutes is consistent with the aforementioned studies
showing prevention of fuselage burn-through for approximately 5 minutes
enhances occupant survivability. The requirements of the proposed
special conditions and the time duration are consistent with the
European Union Aviation Safety Agency Special Conditions No. SC-
D25.863-01, Cabin Evacuation--Protection from Fuel Tank Explosion due
to External Fuel Fed Ground Fire applicable to integral RCTs.\4\
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\4\ SC-D25.863-01, Issue 2, dated 24 October 2023 <a href="https://www.easa.europa.eu/en/document-library/product-certification-consultations/final-special-condition-ref-sc-d25863-01-cabin">https://www.easa.europa.eu/en/document-library/product-certification-consultations/final-special-condition-ref-sc-d25863-01-cabin</a>.
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Airbus may consider a flammability reduction system or ignition
mitigation means that complies with Sec. 25.981 when showing
compliance with the proposed special conditions, provided the system's
performance is demonstrated to meet the proposed special conditions. As
discussed previously, showing compliance with only Sec. 25.981(b) is
insufficient to show post-crash fire-safety performance of fuel-tank
skin or structure. Airbus must also meet the proposed special
conditions.
The proposed special conditions contain the additional safety
standards that the Administrator considers necessary to establish a
level of safety equivalent to that established by the existing
airworthiness standards.
Applicability
As discussed above, these proposed special conditions are
applicable to the Airbus Model A321neo XLR series airplane for which
they are issued. Should the type certificate for that model be amended
later to include any other model that incorporates the same novel or
unusual design feature, or should any other model already included on
the same type certificate be modified to incorporate the same novel or
unusual design feature, these special conditions would apply to the
other model as well.
Conclusion
This action affects only certain novel or unusual design feature on
A321neo XLR series airplanes. It is not a rule of general
applicability.
List of Subjects in 14 CFR Part 25
Aircraft, Aviation safety, Reporting and recordkeeping
requirements.
Authority Citation
The authority citation for these special conditions is as follows:
Authority: 49 U.S.C. 106(f), 106(g), 40113, 44701, 44702, and
44704.
The Proposed Special Conditions
[squf] Accordingly, the Federal Aviation Administration (FAA)
proposes the following special conditions as part of the type
certification basis for Airbus Model A321neo XLR series airplanes.
Cabin Evacuation--Protection from Fuel Tank Explosion Due to
External Fuel-Fed Ground Fire.
The applicant must show the design prevents ignition of fuel tank
vapors (due to hot surface) from occurring in the integral rear center
tank during the time required for evacuation. The applicant's showing
must also demonstrate that the design provides sufficient time for a
safe evacuation of all occupants after the initiation of an external
fuel-fed ground fire.
Issued in Kansas City, Missouri, on April 29, 2024.
Patrick R. Mullen,
Manager, Technical Policy Branch, Policy and Standards Division,
Aircraft Certification Service.
[FR Doc. 2024-09660 Filed 5-6-24; 8:45 am]
BILLING CODE 4910-13-P
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