Special Conditions: Boeing Model 747-8/-8F Airplanes, Interaction of Systems and Structures, 30523-30529 [2011-13022]
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Federal Register / Vol. 76, No. 102 / Thursday, May 26, 2011 / Rules and Regulations
that this rule will not have a significant
economic impact on a substantial
number of small credit unions within
the meaning of the Regulatory
Flexibility Act, 5 U.S.C. 601–612.
Small Business Regulatory Enforcement
Fairness Act
The Small Business Regulatory
Enforcement Fairness Act (SBREFA) of
1996, Public Law 104–121, provides
generally for congressional review of
agency rules. A reporting requirement is
triggered in instances where NCUA
issues a final rule as defined by Section
551 of the Administrative Procedures
Act. 5 U.S.C. 551. The Office of
Information and Regulatory Affairs, an
office within the Office of Management
and Budget, has reviewed this rule and
determined that, for purposes of
SBREFA, this is not a major rule.
Paperwork Reduction Act
This rule does not contain a
‘‘collection of information’’ within the
meaning of section 3502(3) of the
Paperwork Reduction Act of 1995 (44
U.S.C. 3502(3)) and would not increase
paperwork requirements under the
Paperwork Reduction Act of 1995 or
regulations of the Office of Management
and Budget.
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Executive Order 13132
Executive Order 13132 encourages
independent regulatory agencies to
consider the impact of their actions on
state and local interests. In adherence to
fundamental federalism principles,
NCUA, an independent regulatory
agency as defined in 44 U.S.C. 3502(5),
voluntarily complies with the executive
order. This rule would not have
substantial direct effect on the states, on
the connection between the national
government and the states, or on the
distribution of power and
responsibilities among the various
levels of government. NCUA has
determined that this rule does not
constitute a policy that has federalism
implications for purposes of the
executive order.
The Treasury and General Government
Appropriations Act, 1999—Assessment
of Federal Regulations and Policies on
Families
NCUA has determined that this rule
would not affect family well-being
within the meaning of section 654 of the
Treasury and General Government
Appropriations Act, 1999, Public Law
105–277, 112 Stat. 2681 (1998).
List of Subjects in 12 CFR Part 740
Advertisements, Credit unions, Signs
and symbols.
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By the National Credit Union
Administration Board on May 19, 2011.
Mary F. Rupp,
Secretary of the Board.
For the reasons discussed above, the
NCUA Board amends 12 CFR part 740
as follows:
PART 740—ACCURACY OF
ADVERTISING AND NOTICE OF
INSURED STATUS
1. The authority citation for part 740
continues to read as follows:
■
30523
intended to convey information to the
consumer. * * *
*
*
*
*
*
(c) * * *
(7) Advertisements by radio that are
less than fifteen (15) seconds in time;
(8) Advertisements by television,
other than display advertisements, that
are less than fifteen (15) seconds in
time;
*
*
*
*
*
[FR Doc. 2011–12825 Filed 5–25–11; 8:45 am]
BILLING CODE 7535–01–P
Authority: 12 U.S.C. 1766, 1781, 1785, and
1789.
2. Amend § 740.1 by redesignating
current paragraphs (b) and (c) as
paragraphs (c) and (d), respectively, and
by adding a new paragraph (b) to read
as follows:
DEPARTMENT OF TRANSPORTATION
§ 740.1
[Docket No. NM400; Special Conditions No.
25–388A–SC]
■
Definitions.
*
*
*
*
*
(b) Advertisement as used in this part
means a commercial message, in any
medium, that is designed to attract
public attention or patronage to a
product or business.
*
*
*
*
*
■ 3. Amend § 740.5(a) as follows:
■ a. Revise paragraph (a).
■ b. Revise the third sentence of
paragraph (b).
■ c. Remove paragraph (c)(1) and
redesignate paragraphs (c)(2) through
(c)(12) as paragraphs (c)(1) through
(c)(11) respectively.
■ d. Revise redesignated paragraphs
(c)(7) and (c)(8).
The revisions read as follows:
§ 740.5 Requirements for the official
advertising statement.
(a) Each insured credit union must
include the official advertising
statement, prescribed in paragraph (b) of
this section, in all of its advertisements
including, but not limited to, annual
reports and statements of condition
required to be published by law, and on
its main Internet page, except as
provided in paragraph (c) of this
section. For annual reports and
statements of condition required to be
published by law, an insured credit
union must place the official advertising
statement in a prominent position on
the cover page of such documents or on
the first page a reader sees if there is no
cover page.
*
*
*
*
*
(b) * * * The official advertising
statement must be in a size and print
that is clearly legible and may be no
smaller than the smallest font size used
in other portions of the advertisement
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Federal Aviation Administration
14 CFR Part 25
Special Conditions: Boeing Model 747–
8/–8F Airplanes, Interaction of Systems
and Structures
Federal Aviation
Administration (FAA), DOT.
ACTION: Final special conditions.
AGENCY:
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
Carl.Niedermeyer@faa.gov.
SUMMARY:
SUPPLEMENTARY INFORMATION:
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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 § 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 § 21.101.
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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
§ 11.19, are issued under § 11.38, and
become part of the type certification
basis under § 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
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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 § 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
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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.
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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 § 21.101.
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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,
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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 § 25.1309.
(e) Failure condition: The term failure
condition is the same as that used in
§ 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
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Federal Register / Vol. 76, No. 102 / Thursday, May 26, 2011 / Rules and Regulations
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
§ 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.
(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 § 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 § 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
§ 25.331 and in § 25.345.
(ii) The limit gust and turbulence
conditions specified in § 25.341 and in
§ 25.345.
(iii) The limit rolling conditions
specified in § 25.349 and the limit
unsymmetrical conditions specified in
§§ 25.367 and 25.427(b) and (c).
(iv) The limit yaw maneuvering
conditions specified in § 25.351.
(v) the limit ground loading
conditions specified in §§ 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.
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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
Federal Register / Vol. 76, No. 102 / Thursday, May 26, 2011 / Rules and Regulations
Where:
Tj = Average time spent in failure condition
j (in hours)
Pj = Probability of occurrence of failure mode
j (per hour)
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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.
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(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
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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 § 25.629(b).
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Qj = (Tj)(Pj)
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V′ = Clearance speed as defined by
§ 25.629(b)(2).
V″ = Clearance speed as defined by
§ 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)
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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 § 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
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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
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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
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Federal Register / Vol. 76, No. 102 / Thursday, May 26, 2011 / Rules and Regulations
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 § 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 § 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
§ 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
§ 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
VerDate Mar<15>2010
16:14 May 25, 2011
Jkt 223001
wear, repairs, and modifications
throughout the service life of the
airplane by:
(i) Analysis to the nominal aeroelastic
stability envelope specified in
§ 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 § 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
§§ 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 § 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
§ 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.
PO 00000
Frm 00021
Fmt 4700
Sfmt 4700
30529
(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
DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 39
[Docket No. FAA–2010–0994; Directorate
Identifier 2009–NE–39–AD; Amendment 39–
16707; AD 2011–11–08]
RIN 2120–AA64
Airworthiness Directives; Rolls-Royce
plc (RR) RB211–535 Series Turbofan
Engines
Federal Aviation
Administration (FAA), DOT.
ACTION: Final rule.
AGENCY:
We are adopting a new
airworthiness directive (AD) for the
products listed above. This AD results
from mandatory continuing
airworthiness information (MCAI)
issued by an aviation authority of
another country to identify and correct
an unsafe condition on an aviation
product. The MCAI describes the unsafe
condition as:
SUMMARY:
There have been several findings of
cracking at the firtrees of LP Turbine discs.
Fatigue crack initiation and subsequent crack
propagation at the firtree may result in
multiple LP Turbine blade release. The latter
may potentially be beyond the containment
capabilities of the engine casings. Thus,
cracking at the firtrees of LP Turbine discs
constitutes a potentially unsafe condition.
We are issuing this AD to detect cracks
in the low-pressure (LP) turbine stage 1,
2, and 3 discs, which could result in an
uncontained release of LP turbine
blades and damage to the airplane.
DATES: This AD becomes effective June
30, 2011.
ADDRESSES: The Docket Operations
office is located at Docket Management
Facility, U.S. Department of
Transportation, 1200 New Jersey
Avenue, SE., West Building Ground
E:\FR\FM\26MYR1.SGM
26MYR1
]]>Agencies
[Federal Register Volume 76, Number 102 (Thursday, May 26, 2011)]
[Rules and Regulations]
[Pages 30523-30529]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2011-13022]
=======================================================================
-----------------------------------------------------------------------
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.
-----------------------------------------------------------------------
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 Carl.Niedermeyer@faa.gov.
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
