Rule2024-18547
Special Conditions: Bell Textron Inc. (Bell) Model 525 Helicopter; Static Longitudinal Stability Compliance
Primary source
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Published
August 22, 2024
Effective
August 22, 2024
Issuing agencies
Transportation DepartmentFederal Aviation Administration
Abstract
These special conditions are issued for the Bell Model 525 helicopter. This helicopter will have a novel or unusual design feature when compared to the state of technology envisioned in the airworthiness standards for transport category helicopters. This design feature is a four-axis full authority digital fly-by-wire (FBW) flight control system (FCS) that provides for aircraft control through pilot input or coupled auto pilot modes. The applicable airworthiness regulations do not contain adequate or appropriate safety standards for this design feature. These 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 163 (Thursday, August 22, 2024)</title>
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[Federal Register Volume 89, Number 163 (Thursday, August 22, 2024)]
[Rules and Regulations]
[Pages 67851-67852]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2024-18547]
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DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 29
[Docket No. FAA-2024-0895; Special Conditions No. 29-057-SC]
Special Conditions: Bell Textron Inc. (Bell) Model 525
Helicopter; Static Longitudinal Stability Compliance
AGENCY: Federal Aviation Administration (FAA), DOT.
ACTION: Final special conditions.
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SUMMARY: These special conditions are issued for the Bell Model 525
helicopter. This helicopter will have a novel or unusual design feature
when compared to the state of technology envisioned in the
airworthiness standards for transport category helicopters. This design
feature is a four-axis full authority digital fly-by-wire (FBW) flight
control system (FCS) that provides for aircraft control through pilot
input or coupled auto pilot modes. The applicable airworthiness
regulations do not contain adequate or appropriate safety standards for
this design feature. These 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: Effective August 22, 2024.
FOR FURTHER INFORMATION CONTACT: Gregory Thumann, Performance and
Environment Unit, AIR-621A, Technical Policy Branch, Policy and
Standards Division, Aircraft Certification Service, Federal Aviation
Administration, 1801 S Airport Road, Wichita, KS 67209; telephone and
fax (405) 666-1052; email <a href="/cdn-cgi/l/email-protection#b2f5c0d7d5ddc0cb9cf59ce6dac7dfd3dcdcf2d4d3d39cd5ddc4"><span class="__cf_email__" data-cfemail="de99acbbb9b1aca7f099f08ab6abb3bfb0b09eb8bfbff0b9b1a8">[email protected]</span></a>.
SUPPLEMENTARY INFORMATION:
Background
On December 15, 2011, Bell applied for a type certificate for a new
14 CFR part 29 transport category helicopter designated as the Model
525. Bell applied for multiple extensions to its certification
application, with the most recent occurring on September 21, 2023. The
helicopter is a medium twin-engine rotorcraft. The maximum takeoff
weight is 20,500 pounds, with a maximum capacity of 16 passengers and a
crew of 2.
Type Certification Basis
Under the provisions of 14 CFR 21.17, Bell must show that the Model
525 meets the applicable provisions of part 29, as amended by
Amendments 29-1 through 29-55 thereto. The Bell Model 525 certification
basis date is December 31, 2019.
If the Administrator finds that the applicable airworthiness
regulations (i.e., 14 CFR part 29) do not contain adequate or
appropriate safety standards for the Bell Model 525 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, 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 Bell Model 525 helicopter must comply with the 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. 21.17(a)(2).
Novel or Unusual Design Feature
The Bell Model 525 helicopter will incorporate the following novel
or unusual design feature: a four-axis full authority digital FBW FCS
that provides aircraft control through pilot input or coupled auto
pilot modes in addition to degraded modes.
Discussion
For a conventional rotorcraft having mechanical linkages from the
primary cockpit flight controls to the rotor, static longitudinal
stability means that a pull force on the controller (i.e., cyclic) will
result in a reduction in speed relative to the trim speed, and a push
force will result in a higher speed relative to the trim speed.
Longitudinal stability is required by the regulations for the following
reasons:
<bullet> Airspeed change cues are provided to the pilot through
increased and decreased forces on the controller.
<bullet> Short periods of unattended control of the rotorcraft do
not result in significant changes in attitude, airspeed, or load
factor.
<bullet> A predictable pitch response is provided to the pilot.
<bullet> An acceptable level of pilot workload, to attain and
maintain trim speed and altitude, is provided to the pilot.
<bullet> Longitudinal stability provides gust stability.
The pitch control movement of the controller (i.e., cyclic) for the
FBW FCS is an attitude command, which results in a rotor movement to
attain the commanded pitch attitude. The flight path commanded by the
initial cyclic input will remain stick-free until the pilot gives
another command. This control function is applied during normal control
laws within the approved flight envelope. The relevant regulations in
part 29, which are Sec. Sec. 29.173(b), 29.175 for visual flight rules
(VFR) operations, and Appendix B to part 29 sections IV and VII--
Airworthiness Criteria for Helicopter Instrument Flight, are inadequate
for the Bell 525 because the longitudinal flight control laws for the
Bell 525 provide neutral and negative static stability, rather than
positive static stability, within the normal operational envelope. As
detailed in Sec. 29.173(b) and considered in Advisory Circular (AC)
29.173A, ``Static Longitudinal Stability'' (AC 29.173A), which is
contained in AC 29-2C, ``Certification of Transport Category
Rotorcraft'' (AC 29-2C), and the positive control force stability
requirements in Appendix B to part 29, sections IV and VII, the slope
of the control position (i.e., cyclic) versus airspeed curve must be
positive (i.e., provide positive static stability) throughout the full
range of altitude for which certification is requested and with the
throttle and collective pitch held constant.
The 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.
In lieu of meeting the requirements of Sec. Sec. 29.173(b), 29.175
for VFR operations and the airworthiness criteria for helicopter
instrument flight requirements of Appendix B to part 29, sections IV
and VII, the special conditions require the rotorcraft to be shown to
have suitable longitudinal stability and acceptable rotorcraft handling
qualities. The suitable static longitudinal stability must be primarily
based on a positive control movement, which is described as ``control
sense of motion'' in AC 29.173A contained in AC 29-2C. Additionally,
the static
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longitudinal stability and rotorcraft handling qualities are determined
through an assessment of pilot workload, cues, and pilot compensation
for specific test procedures performed during the flight test
evaluation.
The language ``must be primarily based on a positive control
movement'' reflects a pilot's perception of aircraft control where the
first concern is that the control movements are primarily positive in
control movement. Once that is established, the pilot must observe that
the second concern of ``rotorcraft handling qualities'' is not degraded
or mis-aligned where the anticipated flight behavior is not what the
pilot is witnessing. The special conditions address the concern that
these highly computer-controlled control systems can cause the pilot to
become disconnected or out-of-sync with the aircraft's control. Such a
situation can lead to control input errors and undesirable feedback
that can in turn result in loss of control.
Discussion of Comments
The FAA issued Notice of Proposed Special Conditions No. 29-24-01-
SC for the Bell Model 525 helicopter, which was published in the
Federal Register on May 22, 2024 (89 FR 44928). The FAA received one
response from the applicant, Bell.
Bell observed that the language in proposed special conditions No.
29-24-01-SC could be interpreted as negating the entirety of section
VII of appendix B to part 29, which is not the FAA's intent. Bell
recommended clarifying the reference to specify section VII(a)(2)(iv)
of appendix B to part 29 and adding an additional descriptive phrase in
order to avoid an implied exclusion of the remaining applicable parts
of section VII. The FAA concurs with Bell's request and accepts the
suggested language as proposed by Bell with minor edits for appropriate
presentation.
Except as discussed above, the special conditions are adopted as
proposed.
Applicability
As discussed above, these special conditions are applicable to the
Bell Model Bell 525 helicopter. Should Bell apply at a later date for a
change to the type certificate to include another model incorporating
the same novel or unusual design feature, these special conditions
would apply to that model as well.
Under standard practice, the effective date of final special
conditions would be 30 days after the date of publication in the
Federal Register. However, as the certification date for the Bell Model
525 is imminent, the FAA finds that good cause exists to make these
special conditions effective upon publication.
Conclusion
This action affects only a certain novel or unusual design feature
on one model of helicopter. 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, 44704.
The Special Conditions
Accordingly, pursuant to the authority delegated to me by the
Administrator, the following special conditions are issued as part of
the type certification basis for the Bell Textron Inc. (Bell) Model 525
helicopter.
In lieu of meeting the requirements of Sec. Sec. 29.173(b), 29.175
for VFR operations and the airworthiness criteria for helicopter
instrument flight requirements of Appendix B to part 29, sections IV
and VII(a)(2)(iv), as relating to the aircraft's static longitudinal
stability requirements, the Federal Aviation Administration (FAA)
establishes the following special conditions as part of the type
certification basis for Bell Model 525 helicopters.
The rotorcraft must be shown to have suitable longitudinal
stability in any condition normally encountered in service, including
the effects of atmospheric disturbance. The showing of suitable static
longitudinal stability must be primarily based on a positive control
movement in addition to acceptable rotorcraft handling qualities, both
of which are determined by assessing pilot workload, cues, and pilot
compensation for specific test procedures during the flight test
evaluation.
Issued in Kansas City, Missouri, on August 14, 2024.
Patrick R. Mullen,
Manager, Technical Policy Branch, Policy and Standards Division,
Aircraft Certification Service.
[FR Doc. 2024-18547 Filed 8-21-24; 8:45 am]
BILLING CODE 4910-13-P
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</html>Indexed from Federal Register on August 22, 2024.
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