Document ID: FAA-2021-0065-0004
Agency: faa
Document Type: Rule
Title: Special Conditions: Bell Textron Inc. Model 525 Helicopter; Fly-By-Wire Flight Control System
Posted Date: 2022-01-19T05:00Z

[Federal Register Volume 87, Number 12 (Wednesday, January 19, 2022)]
[Rules and Regulations]
[Pages 2689-2692]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2022-00862]

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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 29

[Docket No. FAA-2021-0065; Special Conditions No. 29-054-SC]

Special Conditions: Bell Textron Inc. Model 525 Helicopter; Fly-
By-Wire Flight Control System

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final special conditions.

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SUMMARY: These special conditions are issued for the Bell Textron Inc. 
(Bell) Model 525 helicopter. This helicopter will have a novel or 
unusual design feature associated with a fly-by-wire (FBW) flight 
control system (FCS). 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 February 18, 2022.

FOR FURTHER INFORMATION CONTACT: John VanHoudt, FAA, Dynamic Systems 
Section, AIR-627, Technical Innovation Policy Branch, Policy and 
Innovation Division, Aircraft Certification Service, 10101 Hillwood 
Parkway, Fort Worth, TX 76177-1524; telephone and fax 817-222-5193; 
email [email protected].

SUPPLEMENTARY INFORMATION:

Background

    On December 15, 2011, Bell applied for a type certificate for a new 
transport category helicopter, designated as the Model 525, under Title 
14, Code of Federal Regulations (CFR) part 29. Bell applied for 
multiple extensions, with the most recent occurring on November 12, 
2020. The date of the updated type certification basis is December 31, 
2016, based upon the applicant's proposed type certificate issuance 
date of December 31, 2021. The Model 525 is a

[[Page 2690]]

medium twin-engine rotorcraft. The design maximum takeoff weight is 
20,500 pounds, with a maximum capacity of 19 passengers and a crew of 
two.
    The Bell Model 525 helicopter will be equipped with a four axis 
full authority digital FBW FCS that provides for aircraft control 
through pilot input and coupled flight director modes. The design of 
the Bell Model 525 FBW controls, which provides no direct hydro-
mechanical linkage between the primary cockpit flight controls or 
inceptors and the main and tail rotor actuators, is a first for 
commercial rotorcraft use. Therefore, the regulations do not contain 
adequate or appropriate safety standards for this new design feature.
    The rotorcraft industry is producing new generations of 
helicopters, and gradually increasing size, speed, load capacity, and 
technical sophistication. In recent years, an accelerated trend has 
occurred using rotorcraft for a wide range of commercial and industrial 
applications. This has resulted in increased complexity of modern 
control systems and increased use of automation in flight control 
systems, including the implementation of advanced flight control 
systems such as FBW FCS.
    Section 29.671(c), which provides requirements for transport 
category rotorcraft control systems, does not contain adequate or 
appropriate safety standards for this new design feature. Section 
29.671(c) requires, in part, a means to allow the pilot to determine 
that full control authority is available prior to flight. This command 
control authority is typically achieved by verifying movement of the 
control quadrant through an unassisted mechanical pilot-initiated 
manipulation of the primary flight controls prior to flight. Although 
this approach does not guarantee that 100% maximum control movement of 
the flight controls has been achieved prior to flight, it has been 
deemed appropriate for mechanical flight control systems.
    Unlike traditional mechanical flight control systems, the FBW FCS 
reduces the opportunity for jamming of the flight controls due to 
mechanical bind, improper servo adjustment resulting from faulty 
maintenance, or presence of a foreign object in the control mechanism 
that will impair safety. This reduced exposure for jams is due to the 
replacement of the mechanical linkages between the primary cockpit 
flight controls or inceptors and the main and tail rotor actuators with 
digital signal processing wiring. However, the FBW FCS does increase 
the potential for latent failures or faults that could impair full 
control authority, unless a means exists to ensure the FBW FCS is fully 
functional and free of control authority impairment prior to flight. A 
FBW system may have the ability to verify full control authority 
without having to move the primary flight controls.
    Although part 29 does not contain adequate or appropriate safety 
standards for this novel or unusual design feature, 14 CFR 25.671, 
amendment 25-23, provides these requirements for transport category 
airplanes. Accordingly, these special conditions are based on Sec.  
25.671 to provide requirements for a FBW FCS on the Bell Model 525 
helicopter. Section 25.671(c) provides the same level of safety as 
intended by Sec.  29.671(c) when employing a FBW FCS by including 
requirements for jamming and failure analysis. These special conditions 
require a comprehensive safety analysis of the aircraft's FBW FCS to 
include failures due to command logic (software), mechanical and 
electronic interfaces to other systems, jamming, and maintenance. 
Therefore, in conjunction with Sec.  29.671(a) and (b), these special 
conditions incorporate provisions from Sec.  25.671(c) to establish a 
level of safety equivalent to that established in the regulations.

Type Certification Basis

    Under the provisions of 14 CFR 21.17, Bell must show that the Model 
525 helicopter meets the applicable provisions of part 29, as amended 
by Amendments 29 through 55 thereto. The Bell Model 525 certification 
basis date is December 31, 2016.
    If the Administrator finds that the applicable airworthiness 
regulations (i.e., 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 or similar 
novel or unusual design feature, the 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 noise 
certification requirements of 14 CFR part 36, and the FAA must issue a 
finding of regulatory adequacy under Sec.  611 of Public Law 92-574, 
the ``Noise Control Act of 1972.''
    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 Features

    The Bell Model 525 helicopter will incorporate the following novel 
or unusual design features: A FBW FCS.
    This new design feature has no direct hydro-mechanical linkage 
between the primary cockpit flight controls or inceptors and the main 
and tail rotor actuators, thereby eliminating the more complex elements 
of either a manual movement of the controls by the pilot, or another 
manual means.

Discussion

    These special conditions require that a means be available to show 
full control authority for all powered control systems.
    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.

Discussion of Comments

    The FAA issued Notice of Proposed Special Conditions No. 29-054-SC 
for the Bell Model 525 helicopter, which published in the Federal 
Register on January 29, 2021 (86 FR 7516). The FAA received one 
response, from the European Union Aviation Safety Agency (EASA).
    The FAA proposed the special conditions, which are based on current 
Sec.  25.671(c), in lieu of Sec.  29.671(c). EASA requested the FAA 
explain its rationale for replacing Sec.  29.671(c), which requires a 
means to allow either full movement of all primary flight controls or a 
determination by the pilot that full control authority is available 
prior to flight. EASA stated that although FBW reduces the risk of 
jamming, it does not alleviate the need to allow checking the full 
control movement prior to flight and thus a pre-flight check is still 
necessary.
    The FAA is not replacing the requirement for a pre-flight check. 
Instead, these special conditions include a requirement for a 
comprehensive safety analysis to ensure the FBW FCS is fully functional 
and free of control authority impairment prior to flight. The 
comprehensive safety analysis should address failures due to command 
logic (software), mechanical and electronic interfaces to other 
systems, jamming, and maintenance. The safety analysis should also 
identify the existence of any latent faults.

[[Page 2691]]

Therefore, the means to ensure the FBW FCS is fully functional and free 
of control authority impairment prior to flight is based on the results 
of the comprehensive safety analysis. The means to ensure the safety 
objective of the special conditions is met may consist of design, 
analysis, test, built in test, and limited pre-flight checks.
    EASA noted the proposed special conditions, although derived from 
Sec.  25.671(c), are not aligned with EASA's latest Certification 
Specifications (CS) 25.671 (Amendment 24).
    Under Sec.  21.16, special conditions prescribed by the FAA must 
establish a level of safety equivalent to that established in the FAA's 
existing regulations. Accordingly, the FAA based these special 
conditions on 14 CFR 25.671(c) and not on EASA's certification 
specifications.
    EASA requested the FAA clarify its use of the term ``continued safe 
flight and landing'' used in the proposed special conditions. EASA 
stated the term has a specific definition for flight control failures 
on large airplanes and asked whether the FAA will use a consistent 
definition for failure conditions under Sec.  29.1309. EASA also asked 
whether the FAA will provide a definition of ``continued safe flight 
and landing'' in the context of flight control failures.
    Advisory Circular 29-2C, Certification of Transport Category 
Rotorcraft (AC 29-2C), contains a definition for ``continued safe 
flight and landing.'' The FAA plans to use this definition for the 
purposes of these special conditions.
    EASA stated the proposed special conditions introduce the term 
``normal flight envelope,'' which is not present in EASA's CS 29 
regulation. EASA questioned whether it is relevant only to the Bell 
Model 525 and whether it means the same as ``operating'' envelope.
    When Sec.  25.671 was incorporated, the ``normal flight envelope'' 
was the aircraft approved operating limitations contained in the 
aircraft flight manual. This proposed special condition has the same 
intent. In order to provide clarity and consistency in the language 
between this special condition and Sec.  29.672, the wording will be 
revised to approved operating limitations.
    EASA asked what the FAA means by the proposed requirement that 
``probable failures have only minor effects.'' Specifically, EASA asked 
whether a probable failure is greater than 1E-5 per flight 
hour and whether ``no safety effect'' would be a noncompliance.
    In AC 29-2C, the upper part of the range previously applied to the 
term ``probable'' has been redefined as ``reasonably probable.'' 
Accordingly, the FAA has revised these special conditions by replacing 
``probable'' with ``reasonably probable.'' As provided in AC 29-2C, 
reasonably probable events are based on a probability on the order of 
between 10-3 to 10-5. If a failure is classified 
as ``no safety effect,'' then no further showing of compliance would be 
required.
    EASA requested the FAA change the language in paragraphs (1) and 
(2) of the proposed special conditions to reference failures as defined 
in Sec.  29.671(c)(3). EASA states its suggested language will avoid a 
gap between EASA CS 29.671(c)(1) and 29.671(c)(3).
    The FAA agrees and made the suggested change in the special 
conditions.
    EASA stated that if the FAA's special conditions have a no single 
failure criterion under Sec.  29.1309, then jams under Sec.  
29.671(c)(3) may need to be excluded. EASA referenced CS 25.1309 
(Amendment 24) for no single failure.
    EASA is correct; there is no criteria for single failure in Sec.  
29.1309. As such, the FAA has removed the ``single'' descriptor from 
the special conditions language to be consistent with Sec.  29.1309 
safety objectives. The FAA does not agree that jams under Sec.  
29.671(c)(3) need to be excluded. Any failure condition that can be 
shown to be extremely improbable isn't limited by failures that occur 
from a single source.
    EASA stated that using language from Sec.  25.671(c), which is 
applicable to transport category airplanes, is overly ambitious for 
rotorcraft. EASA asked several hypothetical questions concerning how an 
applicant would show compliance and requested the FAA provide further 
guidance.
    Section 29.671(c), which these special conditions replace as a 
certification requirement for the Model 525, requires either a means to 
allow full control movement of the primary flight controls prior to 
flight or a means that will allow the pilot to determine that full 
control authority is available prior to flight. The language utilized 
from Sec.  25.671(c) for these special conditions ensures verification 
of the control authority prior to flight via a comprehensive safety 
analysis. This analysis is necessary to address failures that could not 
be detected by full control movement of the digital primary flight 
controls.
    EASA requested the FAA clarify whether Sec.  29.691 is sufficient 
for an FBW system or whether specific guidance is needed for FBW flight 
controls after a power failure at entry into and during autorotation.
    The requirements in Sec.  29.691, and the accompanying guidance in 
AC 29-2C, are sufficient for an FBW system. Section 29.691 requires 
that the flight control design allow rapid entry into autorotation 
after a power failure. AC 29-2C provides that applicants may comply 
with this rule through an evaluation as part of the Type Inspection 
Authorization test program.
    EASA requested the FAA clarify the meaning of ``normally 
encountered'' in paragraph (3) of the proposed special conditions. 
Specifically, EASA asked whether there are jams that are not considered 
normal and are therefore excluded from the assessment. EASA further 
noted that the flight conditions listed in paragraph (3) of the 
proposed special conditions are contrary to the maneuvers required by 
Sec. Sec.  29.141 and 29.143.
    The FAA intended these special conditions to address jams 
encountered during any flight condition including transitions between 
flight conditions. The FAA has revised paragraph (3) accordingly.
    EASA requested the FAA clarify the relationship between the 
proposed special conditions and Sec.  29.685(a), which addresses flight 
control jamming. EASA noted the approach in Sec.  29.685(a) is 
different from the one proposed in the special conditions, as Sec.  
29.685(a) requires the design of the control system to prevent jamming. 
EASA states the proposed special conditions would not provide credit 
for jamming that may result in a condition where continued safe flight 
is guaranteed.
    Section 29.685(a) contains a design requirement for mechanical 
controls and is limited in scope. These special conditions are broader 
and include FBW primary flight controls that did not exist when Sec.  
29.685 was promulgated in 1964. Regarding EASA's statement about 
credit, paragraph (3) of these special conditions require reducing 
jamming in any phase of flight to a level capable of continued safe 
flight and landing.

Applicability

    These special conditions are applicable to the Bell Model 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.

Conclusion

    This action affects only a certain novel or unusual design feature 
on the Bell Model 525 helicopter. It is not a rule of general 
applicability.

[[Page 2692]]

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, 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. Model 525 
helicopter. Unless otherwise stated, the following special conditions 
will be used in lieu of Sec.  29.671(c).
    The rotorcraft must be shown by analysis and tests, to be capable 
of continued safe flight and landing after any of the following 
failures or jamming in the flight control system for any speed or 
altitude within the approved operating limitations, without requiring 
exceptional piloting skill or strength. Reasonably probable failures 
must have only minor effects.
    (1) Any failure, excluding a jam as listed in paragraph (3).
    (2) Any combination of failures not shown to be extremely 
improbable, excluding a jam as listed in paragraph (3).
    (3) Any jam in a control position encountered during any flight 
condition, including transitions, within the approved operating 
limitations, unless the jam is shown to be extremely improbable, or can 
be alleviated.

    Issued in Kansas City, Missouri, on January 12, 2022.
Patrick Mullen,
Manager, Technical Innovation Policy Branch, Policy and Innovation 
Division, Aircraft Certification Service.
[FR Doc. 2022-00862 Filed 1-18-22; 8:45 am]
BILLING CODE 4910-13-P