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

DEPARTMENT OF TRANSPORTATION <SUBAGY>Federal Aviation Administration</SUBAGY> <CFR>14 CFR Part 29</CFR> <DEPDOC>[Docket No. FAA-2024-0895; Notice No. 29-24-01-SC]</DEPDOC> <SUBJECT>Special Conditions: Bell Textron Inc. (Bell) Model 525 Helicopter; Static Longitudinal Stability Compliance</SUBJECT> <HD SOURCE="HED">AGENCY:</HD> Federal Aviation Administration (FAA), DOT. <HD SOURCE="HED">ACTION:</HD> Notice of proposed special conditions. <SUM> <HD SOURCE="HED">SUMMARY:</HD> 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. </SUM> <EFFDATE> <HD SOURCE="HED">DATES:</HD> Send comments on or before July 8, 2024. </EFFDATE> <HD SOURCE="HED">ADDRESSES:</HD> Send comments identified by Docket No. FAA-2024-0895 using any of the following methods: <E T="03">Federal eRegulations Portal:</E> Go to <E T="03">www.regulations.gov</E> and follow the online instructions for sending your comments electronically. <E T="03">Mail:</E> 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. <E T="03">Hand Delivery or Courier:</E> 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. <E T="03">Fax:</E> Fax comments to Docket Operations at 202-493-2251. <E T="03">Docket:</E> Background documents or comments received may be read at <E T="03">https://www.regulations.gov/</E> 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. <FURINF> <HD SOURCE="HED">FOR FURTHER INFORMATION CONTACT:</HD> 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 <E T="03">Gregory.G.Thumann@faa.gov.</E> </FURINF> <SUPLINF> <HD SOURCE="HED">SUPPLEMENTARY INFORMATION:</HD> <HD SOURCE="HD1">Comments Invited</HD> 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. <HD SOURCE="HD1">Privacy</HD> 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 <E T="03">www.regulations.gov,</E> including any personal information you provide. The FAA will also post a report summarizing each substantive verbal contact received about these special conditions. <HD SOURCE="HD1">Confidential Business Information</HD> 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 <E T="02">FOR FURTHER INFORMATION CONTACT</E> 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. <HD SOURCE="HD1">Background</HD> 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. <HD SOURCE="HD1">Type Certification Basis</HD> 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 ( <E T="03">i.e.,</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 § 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 § 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 § 11.38, and they become part of the type-certification basis under § 21.17(a)(2). <HD SOURCE="HD1">Novel or Unusual Design Feature</HD> 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. <HD SOURCE="HD1">Discussion</HD> 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 ( <E T="03">i.e.,</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 ( <E T="03">i.e.,</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 §§ 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 § 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 ( <E T="03">i.e.,</E> cyclic) versus airspeed curve must be positive ( <E T="03">i.e.,</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 requirem ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Preview showing 10k of 14k characters. Full document text is stored and available for version comparison. ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━