[Federal Register Volume 76, Number 102 (Thursday, May 26, 2011)]
[Rules and Regulations]
[Pages 30523-30529]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2011-13022]
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DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 25
[Docket No. NM400; Special Conditions No. 25-388A-SC]
Special Conditions: Boeing Model 747-8/-8F Airplanes, Interaction
of Systems and Structures
AGENCY: Federal Aviation Administration (FAA), DOT.
ACTION: Final special conditions.
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SUMMARY: These special conditions amend Special Conditions No. 25-388-
SC for the Boeing Model 747-8/-8F airplanes. These special conditions
were previously issued July 29, 2009, and became effective September
10, 2009. These special conditions are being amended to include
additional criteria addressing the Outboard Aileron Modal Suppression
System. The 747-8/-8F will have novel or unusual design features when
compared to the state of technology envisioned in the airworthiness
standards for transport category airplanes. These design features
include their effects on the structural performance. 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.
Additional special conditions will be issued for other novel or unusual
design features of the 747-8/-8F airplanes.
DATES: Effective Date: June 27, 2011.
FOR FURTHER INFORMATION CONTACT: Carl Niedermeyer, FAA, Airframe &
Cabin Safety Branch, ANM-115, Transport Airplane Directorate, Aircraft
Certification Service, 1601 Lind Avenue SW., Renton, Washington 98057-
3356; telephone (425) 227-2279; e-mail [email protected].
SUPPLEMENTARY INFORMATION:
[[Page 30524]]
Background
On November 4, 2005, The Boeing Company, P.O. Box 3707, Seattle, WA
98124, applied for an amendment to Type Certificate Number A20WE to
include the new Model 747-8 passenger airplane and the new Model 747-8F
freighter airplane. The Model 747-8 and the Model 747-8F are
derivatives of the 747-400 and the 747-400F, respectively. Both the
Model 747-8 and the Model 747-8F are four-engine jet transport
airplanes that will have a maximum takeoff weight of 970,000 pounds and
new General Electric GEnx -2B67 engines. The Model 747-8 will have two
flight crew and the capacity to carry 605 passengers. The Model 747-8F
will have two flight crew and a zero passenger capacity, although
Boeing has submitted a petition for exemption to allow the carriage of
supernumeraries.
These special conditions were originally issued July 29, 2009, and
published in the Federal Register on August 12, 2009 (74 FR 40479).
Type Certification Basis
Under the provisions of Title 14, Code of Federal Regulations (14
CFR) 21.101, Boeing must show that Model 747-8 and 747-8F airplanes
(hereafter referred as 747-8/-8F) meet the applicable provisions of
part 25, as amended by Amendments 25-1 through 25-117, except for
earlier amendments as agreed upon by the FAA. These regulations will be
incorporated into Type Certificate No. A20WE after type certification
approval of the 747-8/-8F.
In addition, the certification basis includes other regulations,
special conditions and exemptions that are not relevant to these
proposed special conditions. Type Certificate No. A20WE will be updated
to include a complete description of the certification basis for these
model airplanes.
If the Administrator finds that the applicable airworthiness
regulations (i.e., 14 CFR part 25) do not contain adequate or
appropriate safety standards for the 747-8/-8F 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, or should any other model already
included on the same type certificate be modified to incorporate 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 747-8/-8F must comply with the fuel vent and exhaust
emission requirements of 14 CFR part 34 and the noise certification
requirements of 14 CFR part 36.
Special conditions, as defined in Sec. 11.19, are issued under
Sec. 11.38, and become part of the type certification basis under
Sec. 21.101.
Novel or Unusual Design Features
The Boeing Model 747-8/8F is equipped with systems that affect the
airplane's structural performance, either directly or as a result of
failure or malfunction. That is, the airplane's systems affect how it
responds in maneuver and gust conditions, and thereby affect its
structural capability. These systems may also affect the aeroelastic
stability of the airplane. Such systems represent a novel and unusual
feature when compared to the technology envisioned in the current
airworthiness standards. A special condition is needed to require
consideration of the effects of systems on the structural capability
and aeroelastic stability of the airplane, both in the normal and in
the failed state.
Discussion
The Boeing 747-8F airplane exhibits an aeroelastic mode of
oscillation that is self-excited and does not completely damp out after
an external disturbance. The sustained oscillation (also known as a
limit cycle oscillation or limit cycle flutter) is caused by an
unstable aeroelastic mode that is prevented from becoming a divergent
oscillation due to one or more nonlinearities that exist in the
airplane.
While the sustained oscillation is not divergent, the FAA considers
it to be an aeroelastic instability. Boeing has proposed the addition
of an Outboard Aileron Modal Suppression (OAMS) system to the fly-by-
wire (FBW) flight control system to reduce, but not eliminate, the
amplitude of the sustained oscillation and control the aeroelastic
instability.
Section 25.629 requires the airplane to be free of any aeroelastic
instability, including flutter. It also requires the airplane to remain
flutter free after certain failures. The regulations do not anticipate
the use of systems that control flutter modes but do not completely
suppress them. The use of the OAMS system is a novel and unusual design
feature that the airworthiness standards do not adequately address. The
FAA believes such systems can be used to ensure that limit cycle (non-
divergent) flutter is kept to safe levels. Therefore, the FAA proposes
a special condition that addresses this particular sustained
oscillation characteristic and provides the necessary standards that
permit the use of such active flutter control systems.
Discussion of Comments
Notice of proposed special conditions No. 25-11-09-SC for Boeing
747-8/-8F airplanes was published in the Federal Register on March 16,
2011 (76 FR 14341). The standards in Section A were modified to
incorporate the reference to Section C and remove ``flutter control
systems'' from the applicability. Section B was already adopted in
Special Conditions 25-388-SC and was included for reference. Comments
were invited on the amended Section A and the proposed text of Section
C, Outboard Aileron Modal Suppression System. Several comments were
received from one commenter.
Concerns With the Philosophy of Controlling Aeroelastic Instability
(Flutter) With an Active Control System
The commenter, Leth and Associates, LLC, expressed reservations
with the philosophy of controlling an aeroelastic instability (flutter)
with an active control system under the current rules and regulations,
specifically Sec. 25.629. The commenter's position is that a safety
issue is being solved by introducing more risk with the addition of an
active suppression system. The commenter also expressed concern that
the acceptance of this remedy to a design problem will encourage
manufacturers to use a similar approach in solving potentially more
onerous design flaws in the future. The commenter acknowledged that
active flutter suppression systems may be introduced in advanced
designs of the future, and stated that it is incumbent on the
regulatory authorities to introduce regulations that clearly address
the safety requirement of such systems. The commenter recommended that
until these amended rules are in place to address aeroservoelastic
systems, it is ill advised to accept ad hoc solutions to safety issues
by adding more risk. The commenter further recommended that until such
time as the rules have been changed, flutter prevention should rely on
true and tested methods, using passive means of stabilization. The
commenter did not offer any specific changes to the special conditions.
However, the commenter suggested issues that should be addressed during
future rulemaking.
Although the FAA agrees with many of the statements and sentiments
[[Page 30525]]
expressed by the commenter, we believe that the special conditions and
the agreed upon means of compliance between the FAA and the applicant,
adequately address the commenter's concerns. The FAA does not agree
that the acceptance of the use of the OAMS system and the type of
sustained oscillation exhibited by the Boeing Model 747-8F need wait
until new general rulemaking is completed. Special conditions are a
form of rulemaking and are issued when the existing applicable
airworthiness standards do not contain adequate or appropriate safety
standards because of novel or unusual design features of the product to
be type certificated. The phrase ``novel or unusual'' applies to design
features of the product to be certificated when compared to the
applicable airworthiness standards. This allows the FAA to make
adjustments for individual type certificate projects by developing
special conditions where novel or unusual design features are present.
The special conditions addressing the OAMS system and the existence
of the limit cycle flutter mode were formulated based on the
characteristics observed during flight testing of the 747-8F and
Boeing's proposed solution to the problem. The FAA is requiring that
the type of sustained oscillation covered by the special conditions
must not be a hazard to the airplane nor its occupants with the active
system inoperative or failed. This is assured by compliance with the
requirements in the special condition.
The FAA is taking a conservative approach to the introduction of
active flutter suppression systems on transport category airplanes. At
this point in time, the FAA is not prepared to certify active flutter
suppression systems that control divergent flutter modes, or limit
cycle flutter modes that do not meet the requirements of Section C,
paragraphs 2.(a) and 2.(c), of these special conditions with the active
system inoperative or failed.
No changes were made as a result of these comments and the special
conditions are adopted as proposed.
Applicability
As discussed above, these special conditions are applicable to
Boeing Model 747-8/-8F airplanes. Should Boeing apply at a later date
for a change to the type certificate to include another model
incorporating the same novel or unusual design features, these special
conditions would apply to that model as well under the provisions of
Sec. 21.101.
Conclusion
This action affects only certain novel or unusual design features
of the Boeing Model 747-8/-8F airplanes. It is not a rule of general
applicability.
List of Subjects in 14 CFR Part 25
Aircraft, Aviation safety, Reporting and recordkeeping
requirements.
The authority citation for this Special Condition 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 amendment to Special Conditions 25-388-SC
is issued as part of the type certification basis for the 747-8/-8F
airplanes.
A. General
The Boeing Model 747-8/8F airplanes are equipped with automatic
control systems that affect the airplane's 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 Subparts
C and D of part 25. Except as provided in Section C of these special
conditions, the following criteria must be used for showing compliance
with these special conditions for airplanes equipped with flight
control systems, autopilots, stability augmentation systems, load
alleviation 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 specific
system.
1. The criteria defined here only address the direct structural
consequences of the system responses and performances and cannot be
considered in isolation; however, they should be included in the
overall safety evaluation of the airplane. These criteria may in some
instances duplicate standards already established for this evaluation.
These criteria are only applicable to structural elements 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.
2. Depending on the specific characteristics of the airplane,
additional studies may be required that go beyond the criteria provided
in these special conditions in order to demonstrate the capability of
the airplane to meet other realistic conditions such as alternative
gust or maneuver descriptions for an airplane equipped with a load
alleviation system.
3. The following definitions are applicable to these special
conditions.
(a) Structural performance: Capability of the airplane to meet the
structural requirements of part 25.
(b) Flight limitations: Limitations that can be applied to the
airplane flight conditions following an in-flight occurrence and that
are included in the airplane flight manual (AFM) (e.g., speed
limitations, avoidance of severe weather conditions).
(c) Operational limitations: Limitations, including flight
limitations that can be applied to the airplane operating conditions
before dispatch (e.g., fuel, payload and Master Minimum Equipment List
(MMEL) limitations).
(d) Probabilistic terms: The probabilistic terms (probable,
improbable, extremely improbable) used in these special conditions are
the same as those used in Sec. 25.1309.
(e) Failure condition: The term failure condition is the same as
that used in Sec. 25.1309, however these special conditions apply only
to system failure conditions that affect the structural performance of
the airplane (e.g., system failure conditions that induce loads, change
the response of the airplane to inputs such as gusts or pilot actions,
or lower flutter margins). The system failure condition includes
consequential or cascading effects resulting from the first failure.
B. Effects of Systems on Structures
1. General. The following criteria will be used in determining the
influence of a system and its failure conditions on the airplane
structural elements.
2. System fully operative. With the system fully operative, the
following apply:
(a) Limit loads must be derived in all normal operating
configurations of the system from all the limit conditions specified in
subpart C (or used 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 airplane
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
[[Page 30526]]
conservative way when deriving limit loads from limit conditions.
(b) The airplane must meet the strength requirements of part 25
(i.e., 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
airplane has design features that will not allow it to exceed those
limit conditions.
(c) The airplane must meet the aeroelastic stability requirements
of Sec. 25.629.
3. System in the failure condition. For any system failure
condition not shown to be extremely improbable, the following apply:
(a) 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 failure.
(1) 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 ultimate loads to be considered for
design. The factor of safety (F.S.) is defined in Figure 1.
[GRAPHIC] [TIFF OMITTED] TR26MY11.004
(2) For residual strength substantiation, the airplane must be able
to withstand two thirds of the ultimate loads defined in subparagraph
3(a)(1). For pressurized cabins, these loads must be combined with the
normal operating differential pressure.
(3) Freedom from aeroelastic instability must be shown up to the
speeds defined in Sec. 25.629(b)(2). For failure conditions that
result in speeds beyond VC/MC, freedom from
aeroelastic instability must be shown to increased speeds, so that the
margins intended by Sec. 25.629(b)(2) are maintained.
(4) Failures of the system that result in forced structural
vibrations (oscillatory failures) must not produce loads that could
result in detrimental deformation of the affected structural elements.
(b) For continuation of flight, for an airplane in the system
failed state and considering any appropriate reconfiguration and flight
limitations, the following apply:
(1) The loads derived from the following conditions (or used in
lieu of the following conditions) at speeds up to VC/
MC, or the speed limitation prescribed for the remainder of
the flight, must be determined:
(i) The limit symmetrical maneuvering conditions specified in Sec.
25.331 and in Sec. 25.345.
(ii) The limit gust and turbulence conditions specified in Sec.
25.341 and in Sec. 25.345.
(iii) The limit rolling conditions specified in Sec. 25.349 and
the limit unsymmetrical conditions specified in Sec. Sec. 25.367 and
25.427(b) and (c).
(iv) The limit yaw maneuvering conditions specified in Sec.
25.351.
(v) the limit ground loading conditions specified in Sec. Sec.
25.473, 25.491 and 25.493.
(2) For static strength substantiation, each part of the structure
must be able to withstand the loads in paragraph (3)(b)(1) of the
special condition 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.
[[Page 30527]]
[GRAPHIC] [TIFF OMITTED] TR26MY11.005
Qj = (Tj)(Pj)
Where:
Tj = Average time spent in failure condition j (in hours)
Pj = Probability of occurrence of failure mode j (per
hour)
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.
(3) For residual strength substantiation, the airplane must be able
to withstand two thirds of the ultimate loads defined in paragraph
(3)(b)(1) of the special condition. For pressurized cabins, these loads
must be combined with the normal operating differential pressure.
(4) 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.
(5) Freedom from aeroelastic instability must be shown up to a
speed determined from Figure 3. Flutter clearance speeds V' and V'' may
be based on the speed limitation specified for the remainder of the
flight using the margins defined by Sec. 25.629(b).
[[Page 30528]]
[GRAPHIC] [TIFF OMITTED] TR26MY11.006
V' = Clearance speed as defined by Sec. 25.629(b)(2).
V'' = Clearance speed as defined by Sec. 25.629(b)(1).
Qj = (Tj)(Pj)
Where:
Tj = Average time spent in failure condition j (in hours)
Pj = Probability of occurrence of failure mode j (per
hour)
Note: If Pj is greater than 10-3 per
flight hour, then the flutter clearance speed must not be less than
V''.
(6) Freedom from aeroelastic instability must also be shown up to
V' in Figure 3 above, for any probable system failure condition
combined with any damage required or selected for investigation by
Sec. 25.571(b).
(c) Consideration of certain failure conditions may be required by
other sections of part 25 regardless of calculated system reliability.
Where 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.
4. Failure indications. For system failure detection and
indication, the following apply:
(a) The system must be checked for failure conditions, not
extremely improbable, that degrade the structural capability below the
level required by part 25 or significantly reduce the reliability of
the remaining 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 Certification Maintenance
Requirements (CMRs) 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.
(b) The existence of any failure condition, not extremely
improbable, during flight that could significantly affect the
structural capability of the airplane 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 airplane
strength and the loads of subpart C below 1.25, or flutter margins
below V'', must be signaled to the crew during flight.
5. Dispatch with known failure conditions. If the airplane is to be
dispatched in a known system failure condition that affects structural
performance, or affects the reliability of the remaining system to
maintain structural performance, then the provisions of these special
conditions must be met, including the provisions of paragraph 2 for the
dispatched condition, and paragraph 3 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. 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 Figures 2 and 3.
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.
C. Outboard Aileron Modal Suppression System
1. In general, these special conditions apply to fly-by-wire active
flutter suppression systems that are intended to operate on a certain
type of aeroelastic instability. This type of instability is
characterized by a low frequency, self-excited, sustained oscillation
of an aeroelastic vibration
[[Page 30529]]
mode that is shown to be a stable limit cycle oscillation (LCO), with
the system operative and inoperative. (An LCO is considered ``stable''
if it maintains the same frequency and amplitude for a given excitation
input and flight condition.) In addition, the type of sustained
oscillation covered by these special conditions must not be a hazard to
the airplane nor its occupants with the active system failed. These
systems must be shown to reduce the amplitude of the sustained
oscillation to acceptable levels and effectively control the
aeroelastic instability. Specifically, the following criteria address
the existence of such a sustained oscillation on the Boeing Model 747-
8/-8F airplanes and the Outboard Aileron Modal Suppression (OAMS)
system that will be used to control it.
2. In lieu of the requirements contained in Sec. 25.629, the
existence of a sustained, or limit cycle, oscillation that is
controlled by an active flight control system is acceptable, provided
that the following requirements are met:
(a) OAMS System Inoperative: The sustained, or limit cycle,
oscillation must be shown by test and analysis to be stable throughout
the nominal aeroelastic stability envelope specified in Sec.
25.629(b)(1) with the OAMS system inoperative. This should include the
consideration of disturbances above the sustained amplitude of
oscillation.
(b) Nominal Conditions:
(1) With the OAMS system operative it must be shown that the
airplane remains safe, stable, and controllable throughout the nominal
aeroelastic stability envelope specified in Sec. 25.629(b)(1) by
providing adequate suppression of the aeroelastic modes being
controlled. All applicable airworthiness and environmental requirements
should continue to be complied with. Additionally, loads imposed on the
airplane due to any amplitude of oscillation must be shown to have a
negligible impact on structure and systems, including wear, fatigue and
damage tolerance. The OAMS system must function properly in all
environments that may be encountered.
(2) The applicant must establish by test and analysis that the OAMS
system can be relied upon to control and limit the sustained amplitude
of the oscillation to acceptable levels (per Sec. 25.251) and control
the stability of the aeroelastic mode. This should include the
consideration of disturbances above the sustained amplitude of
oscillation; maneuvering flight, icing conditions; manufacturing
variations; Master Minimum Equipment List (MMEL) items; spare engine
carriage; engine removed or inoperative ferry flights; and wear,
repairs, and modifications throughout the service life of the airplane
by:
(i) Analysis to the nominal aeroelastic stability envelope
specified in Sec. 25.629(b)(1), and
(ii) Flight flutter test to the VDF/MDF
boundary. These tests must demonstrate that the airplane has a proper
margin of damping for disturbances above the sustained amplitude of
oscillation at all speeds up to VDF/MDF, and that
there is no large and rapid reduction in damping as VDF/
MDF is approached.
(iii) The structural modes must have adequate stability margins for
any OAMS flight control system feedback loop at speeds up to the fail-
safe aeroelastic stability envelope specified in Sec. 25.629(b)(2).
(c) Failures, Malfunctions, and Adverse Conditions:
(1) For the OAMS system operative and failed, for any failure, or
combination of failures not shown to be extremely improbable, and
addressed by Sec. Sec. 25.629(d), 25.571, 25.631, 25.671, 25.672,
25.901(c) or 25.1309 that results in LCO, it must be established by
test or analysis up to the aeroelastic stability envelope specified in
Sec. 25.629(b)(2) that the LCO:
(i) Is stable and decays to an acceptable limited amplitude once an
external perturbing force is removed;
(ii) Does not result in loads that would cause static, dynamic, or
fatigue failure of structure during the expected exposure period;
(iii) Does not result in repeated loads that would cause an
additional failure due to wear during the expected exposure period that
precludes safe flight and landing;
(iv) Has, if necessary, sufficient indication of OAMS failure(s)
and crew procedures to properly address the failure(s);
(v) Does not result in a vibration condition on the flight deck
that is severe enough to interfere with control of the airplane,
ability of the crew to read the flight instruments, perform vital
functions like reading and accomplishing checklist procedures, or to
cause excessive fatigue to the crew;
(vi) Does not result in adverse effects on the flight control
system or on airplane stability, controllability, or handling
characteristics (including airplane-pilot coupling (APC) per Sec.
25.143) that would prevent safe flight and landing; and
(vii) does not interfere with the flight crew's ability to
correctly distinguish vibration from buffeting associated with the
recognition of stalls or high speed buffet.
(2) The applicant must show that particular risks such as engine
failure, uncontained engine, or APU rotor burst, or other failures not
shown to be extremely improbable, will not adversely or significantly
change the aeroelastic stability characteristics of the airplane.
(3) No MMEL dispatch is allowed with the OAMS system inoperative.
Issued in Renton, Washington, on May 20, 2011.
Ali Bahrami,
Manager, Transport Airplane Directorate, Aircraft Certification
Service.
[FR Doc. 2011-13022 Filed 5-25-11; 8:45 am]
BILLING CODE 4910-13-P