Special Conditions: Bell Textron Inc. (Bell) Model 525 Helicopter; Static Longitudinal Stability Compliance, 44928-44930 [2024-11158]

Agencies

• Federal Aviation Administration
• DEPARTMENT OF TRANSPORTATION

[Federal Register Volume 89, Number 100 (Wednesday, May 22, 2024)]
[Proposed Rules]
[Pages 44928-44930]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2024-11158]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 29

[Docket No. FAA-2024-0895; Notice No. 29-24-01-SC]


Special Conditions: Bell Textron Inc. (Bell) Model 525 
Helicopter; Static Longitudinal Stability Compliance

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Notice of proposed special conditions.

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SUMMARY: This action proposes special conditions 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 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 July 8, 2024.

ADDRESSES: Send comments identified by Docket No. FAA-2024-0895 using 
any of the following methods:
    Federal eRegulations Portal: Go to www.regulations.gov 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.

[[Page 44929]]

    Docket: Background documents or comments received may be read at 
https://www.regulations.gov/ 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: 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 [email protected].

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 www.regulations.gov, including any 
personal information you provide. The FAA will also post a report 
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 proposed special conditions. Send 
submissions containing CBI to the individual listed in the FOR FURTHER 
INFORMATION CONTACT section above. Comments the FAA receives, which are 
not specifically designated as CBI, will be placed in the public docket 
for these proposed special conditions.

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:
     Airspeed change cues are provided to the pilot through 
increased and decreased forces on the controller.
     Short periods of unattended control of the rotorcraft do 
not result in significant changes in attitude, airspeed, or load 
factor.
     A predictable pitch response is provided to the pilot.
     An acceptable level of pilot workload, to attain and 
maintain trim speed and altitude, is provided to the pilot.
     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

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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 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.
    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 proposed special conditions would 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 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 proposed 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.

Applicability

    As discussed above, these proposed special conditions are 
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 apply to the other model as well.

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 29

    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

    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 Federal Aviation Administration (FAA) proposes 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 Des Moines, Washington, on May 16, 2024.
Caspar K. Wang,
Manager, Technical Policy Branch, Policy and Standards Division, 
Aircraft Certification Service.
[FR Doc. 2024-11158 Filed 5-21-24; 8:45 am]
BILLING CODE 4910-13-P