[Federal Register Volume 81, Number 236 (Thursday, December 8, 2016)]
[Rules and Regulations]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2016-29431]
DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 29
[Docket No. FAA-2016-6939; Notice No. 29-038-SC]
Special Conditions: Bell Helicopter Textron, Inc. (BHTI), Model
525 Helicopters; Interaction of Systems and Structures.
AGENCY: Federal Aviation Administration (FAA), DOT.
ACTION: Final special conditions.
SUMMARY: These special conditions are issued for the BHTI Model 525
helicopter. This helicopter will have a novel or unusual design feature
associated with fly-by-wire flight control system (FBW FCS) functions
that affect the structural integrity of the rotorcraft. 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: These special conditions are effective January 9, 2017.
FOR FURTHER INFORMATION CONTACT: Martin R. Crane, Aviation Safety
Engineer, Safety Management Group, Rotorcraft Directorate, FAA, 10101
Hillwood Pkwy, Fort Worth, TX 76177; telephone (817) 222-5110; email
On December 15, 2011, BHTI applied for a type certificate for a new
transport category helicopter designated as the Model 525. The aircraft
is a medium twin engine rotorcraft. The design maximum takeoff weight
is 20,000 pounds, with a maximum capacity of 16 passengers and a crew
The BHTI Model 525 helicopter will be equipped with a FBW FCS. The
control functions of the FBW FCS and its related systems affect the
structural integrity of the rotorcraft. Current regulations do not take
into account loads for the rotorcraft due to the effects of systems on
structural performance including normal operation and failure
conditions with strength levels related to probability of occurrence.
Special conditions are needed to account for these features.
Type Certification Basis
Under the provisions of 14 CFR 21.17, BHTI must show that the Model
525 helicopter meets the applicable provisions of part 29, as amended
by Amendment 29-1 through 29-55 thereto. The BHTI Model 525
certification basis date is December 15, 2011, the date of application
to the FAA.
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 BHTI 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 BHTI 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 BHTI Model 525 helicopter will incorporate the following novel
or unusual design features: FBW FCS, and its related systems (stability
augmentation system, load alleviation system, flutter control system,
and fuel management system), with control functions that affect the
structural integrity of the rotorcraft. Current regulations are
inadequate for considering the effects of these systems and their
failures on structural performance. The general approach of accounting
for the effect of system failures on structural performance would be
extended to include any system where partial or complete failure, alone
or in combination with any other system's partial or complete failure,
would affect structural performance.
Active flight control systems are capable of providing automatic
responses to inputs from sources other than the pilots. Active flight
control systems have been expanded in function, effectiveness, and
reliability to the point that FBW FCS systems are being installed on
new rotorcraft. As a result of these advancements in flight control
technology, 14 CFR part 29 does not provide a basis to achieve an
acceptable level of safety for rotorcraft so equipped. Certification of
these systems requires issuing special conditions under the provisions
of Sec. 21.16.
In the past, traditional rotorcraft flight control system designs
have incorporated power-operated systems, stability or control
augmentation with limited control authority, and autopilots that were
certificated partly under Sec. 29.672 with guidance from Advisory
Circular 29-2C, Section AC 29.672. These systems are integrated into
the primary flight controls and are given sufficient control authority
to maneuver the rotorcraft up to its structural design limits in 14 CFR
part 29 subparts C and D. The FBW FCS advanced technology with its full
authority necessitates additional requirements to account for the
interaction of control systems and structures.
The regulations defining the loads envelope in 14 CFR part 29 do
not fully account for the effects of systems on structural performance.
Automatic systems may be inoperative or they may operate in a degraded
mode with less than full system authority and associated built-in
protection features. Therefore, it is necessary to determine the
structural factors of safety and operating margins such that the
probability of structural failures due to application of loads during
FBW FCS malfunctions is not greater than that found in rotorcraft
equipped with traditional flight control systems. To achieve this
objective and to ensure an acceptable level of safety, it is necessary
to define the failure conditions and their associated frequency of
Traditional flight control systems provide two states, either fully
functioning or completely inoperative. These conditions are readily
apparent to the flight crew. Newer active flight control systems have
failure modes that allow the system to function in a degraded mode
without full authority and associated built-in protection features. As
these degraded modes are not readily apparent to the flight crew,
monitoring systems are required to provide an annunciation of degraded
A notice of proposed special conditions for the BHTI Model 525
helicopter FBW FCS and its related systems was published in the Federal
Register on May 27, 2016 (81 FR 33606). We did not receive any
As discussed above, these special conditions are applicable to the
BHTI Model 525 helicopter. Should BHTI apply at a later date for a
change to the type certificate to include another model incorporating
the same novel or unusual design feature, the special conditions would
apply to that model as well.
This action affects only certain novel or unusual design features
on one model of rotorcraft. It is not a rule of general applicability.
List of Subjects in 14 CFR Part 29
Aircraft, Aviation safety, Reporting and recordkeeping
The authority citation for these special conditions is as follows:
Authority: 49 U.S.C. 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 Bell Helicopter Textron, Inc., Model
525 helicopters when a fly-by-wire flight control system is installed:
Interaction of Systems and Structures
For rotorcraft equipped with systems that affect structural
performance, either directly or as a result of a failure or
malfunction, the influence of these systems and their failure
conditions must be taken into account when showing compliance with the
requirements of Title 14, Code of Federal Regulations (14 CFR) part 29
subparts C and D.
The following criteria must be used for showing compliance with
these special conditions for rotorcraft equipped with flight control
systems, autopilots, stability augmentation systems, load alleviation
systems, flutter control systems, fuel management systems, and other
systems that either directly or as a result of failure or malfunction
affect structural performance. If these special conditions are used for
other systems, it may be necessary to adapt the criteria to the
(a) The criteria defined herein only address the direct structural
consequences of the system responses and performance. They cannot be
considered in isolation but should be included in the overall safety
evaluation of the rotorcraft. These criteria may in some instances
duplicate standards already established for this evaluation. These
criteria are only applicable to structure whose failure could prevent
continued safe flight and landing. Specific criteria that define
acceptable limits on handling characteristics or stability requirements
when operating in the system degraded or inoperative mode are not
provided in these special conditions.
(b) Depending upon the specific characteristics of the rotorcraft,
additional studies may be required that go beyond the criteria provided
in this special condition in order to demonstrate the capability of the
rotorcraft to meet other realistic conditions such as alternative gust
or maneuver descriptions for a rotorcraft equipped with a load
(c) The following definitions are applicable to these special
(1) Structural performance: Capability of the rotorcraft to meet
the structural requirements of 14 CFR part 29.
(2) Flight limitations: Limitations that can be applied to the
rotorcraft flight conditions following an in-flight occurrence and that
are included in the flight manual (e.g., speed limitations and
avoidance of severe weather conditions).
(3) Operational limitations: Limitations, including flight
limitations, which can be applied to the rotorcraft operating
conditions before dispatch (e.g., fuel, payload, and Master Minimum
Equipment List limitations).
(4) Probabilistic terms: The terms ``improbable'' and ``extremely
improbable'' are the same as those used in Sec. 29.1309.
(5) Failure condition: The term ``failure condition'' is the same
as that used in Sec. 29.1309; however, these special conditions apply
only to system failure conditions that affect the structural
performance of the rotorcraft (e.g., system failure conditions that
induce loads, change the response of the rotorcraft to inputs such as
gusts or pilot actions, or lower flutter margins).
Effects of Systems on Structures
(a) General. The following criteria will be used in determining the
influence of a system and its failure conditions on the rotorcraft
(b) System fully operative. With the system fully operative, the
(1) Limit loads must be derived in all normal operating
configurations of the system from all the limit conditions specified in
subpart C (or defined by special condition or equivalent level of
safety in lieu of those specified in subpart C), taking into account
any special behavior of such a system or associated functions or any
effect on the structural performance of the rotorcraft that may occur
up to the limit loads. In particular, any significant nonlinearity
(rate of displacement of control surface, thresholds or any other
system nonlinearities) must be accounted for in a realistic or
conservative way when deriving limit loads from limit conditions.
(2) The rotorcraft must meet the strength requirements of part 29
(static strength, residual strength), using the specified factors to
derive ultimate loads from the limit loads defined above. The effect of
nonlinearities must be investigated beyond limit conditions to ensure
the behavior of the system presents no anomaly compared to the behavior
below limit conditions. However, conditions beyond limit conditions
need not be considered when it can be shown that the rotorcraft has
design features that will not allow it to exceed those limit
(3) The rotorcraft must meet the flutter and divergence
requirements of Sec. 29.629.
(c) System in the failure condition. For all system failure
conditions shown to be not extremely improbable, the following apply:
(1) At the time of occurrence. Starting from 1-g level flight
conditions, a realistic scenario, including pilot corrective actions,
must be established to determine the loads occurring at the time of
failure and immediately after the failure.
(i) For static strength substantiation, these loads multiplied by
an appropriate factor of safety that is related to the probability of
occurrence of the failure are the ultimate loads that must be
considered for design. The factor of safety is defined in Figure 1.
[GRAPHIC] [TIFF OMITTED] TR08DE16.007
(ii) For residual strength substantiation, the rotorcraft must be
able to withstand two-thirds of the ultimate loads defined in paragraph
(c)(1)(i) of these special conditions.
(iii) Freedom from flutter and divergence must be shown under all
conditions of operation including:
(A) Airspeeds up to 1.11 VNE (power on and power off).
(B) Main rotor speeds from 0.95 multiplied by the minimum permitted
speed up to 1.05 multiplied by the maximum permitted speed (power on
and power off).
(C) The critical combinations of weight, center of gravity
position, load factor, and altitude.
(iv) For failure conditions that result in excursions beyond
operating limitations, freedom from flutter and divergence must be
shown to increased speeds, so that the margins intended by paragraph
(c)(1)(iii) of these special conditions are maintained.
(v) Failures of the system that result in forced structural
vibrations (oscillatory failures) must not produce loads that could
result in detrimental deformation of primary structure.
(2) For the continuation of the flight. For the rotorcraft in the
system failed state, and considering all appropriate reconfiguration
and flight limitations, the following apply:
(i) The loads derived from the following conditions (or defined by
special conditions or equivalent level of safety in lieu of the
following conditions) at speeds up to VNE (power on and
power off) (or the speed limitation prescribed for the remainder of the
flight) and at the minimum and maximum main rotor speeds, if
applicable, must be determined:
(A) The limit maneuvering conditions specified in Sec. Sec. 29.337
(B) The limit gust conditions specified in Sec. 29.341.
(C) The limit yaw maneuvering conditions specified in Sec. 29.351.
(D) The limit unsymmetrical conditions specified in Sec. 29.427.
(E) The limit ground loading conditions specified in Sec. 29.473.
(ii) For static strength substantiation, each part of the structure
must be able to withstand the loads in paragraph (c)(2)(i) of these
special conditions multiplied by a factor of safety depending on the
probability of being in this failure state. The factor of safety is
defined in Figure 2.
[GRAPHIC] [TIFF OMITTED] TR08DE16.008
Qj = (Tj)(Pj)
Tj = Average time spent in failure condition j (in hours)
Pj = Probability of occurrence of failure mode j (per
Note: If Pj is greater than 10-\3\ per
flight hour, then a 1.5 factor of safety must be applied to all
limit load conditions specified in Subpart C.
(iii) For residual strength substantiation, the rotorcraft must be
able to withstand two-thirds of the ultimate loads defined in paragraph
(c)(2)(ii) of these special conditions.
(iv) If the loads induced by the failure condition have a
significant effect on fatigue or damage tolerance, then their effects
must be taken into account.
(v) Freedom from flutter and divergence must be shown up to 1.11
VNE (power on and power off).
(vi) Freedom from flutter and divergence must also be shown up to
1.11 VNE (power on and power off) for all probable system
failure conditions combined with any damage required or considered
under Sec. 29.571(g) or Sec. 29.573(d)(3).
(3) Consideration of certain failure conditions may be required by
other sections of 14 CFR part 29 regardless of calculated system
reliability. Where the failure analysis shows the probability of
these failure conditions to be less than 10-\9\, criteria
other than those specified in this paragraph may be used for structural
substantiation to show continued safe flight and landing.
(d) Failure indications. For system failure detection and
indication, the following apply:
(1) The system must be checked for failure conditions, not
extremely improbable, that degrade the structural capability below the
level required by 14 CFR part 29 or that significantly reduce the
reliability of the remaining operational portion of the system. As far
as reasonably practicable, the flight crew must be made aware of these
failures before flight. Certain elements of the control system, such as
mechanical and hydraulic components, may use special periodic
inspections, and electronic components may use daily checks, in lieu of
detection and indication systems to achieve the objective of this
requirement. These other means of detecting failures before flight will
become part of the certification maintenance requirements (CMRs) and
must be limited to components that are not readily detectable by normal
detection and indication systems, and where service history shows that
inspections will provide an adequate level of safety.
(2) The existence of any failure condition, shown to be not
extremely improbable, during flight that could significantly affect the
structural capability of the rotorcraft and for which the associated
reduction in airworthiness can be minimized by suitable flight
limitations, must be signaled to the flight crew. For example, failure
conditions that result in a factor of safety between the rotorcraft
strength and the loads of Subpart C below 1.25, or flutter and
divergence margins below 1.11 VNE (power on and power off),
must be signaled to the crew during flight.
(e) Dispatch with known failure conditions. If the rotorcraft is to
be dispatched in a known system failure condition that affects
structural performance, or that affects the reliability of the
remaining operational portion of the system to maintain structural
performance, then the provisions of these special conditions must be
met, including the provisions of paragraph (b) for the dispatched
condition and paragraph (c) for subsequent failures. Expected
operational limitations may be taken into account in establishing
Pj as the probability of failure occurrence for determining
the safety margin in Figure 1 of these special conditions. Flight
limitations and expected operational limitations may be taken into
account in establishing Qj as the combined probability of
being in the dispatched failure condition and the subsequent failure
condition for the safety margins in Figure 2 of these special
conditions. These limitations must be such that the probability of
being in this combined failure state and then subsequently encountering
limit load conditions is extremely improbable. No reduction in these
safety margins is allowed if the subsequent system failure rate is
greater than 10-\3\ per hour.
Issued in Fort Worth, Texas, on November 30, 2012.
Manager, Rotorcraft Directorate, Aircraft Certification Service.
[FR Doc. 2016-29431 Filed 12-7-16; 8:45 am]
BILLING CODE 4910-13-P