Special Conditions: Bell Textron Inc. (Bell) Model 525 Helicopter; Static Longitudinal Stability Compliance, 67851-67852 [2024-18547]

Agencies

• Federal Aviation Administration
• DEPARTMENT OF TRANSPORTATION

[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 [www.gpo.gov]
[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 [email protected].

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:
     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 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

[[Page 67852]]

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