[Federal Register Volume 79, Number 173 (Monday, September 8, 2014)]
[Rules and Regulations]
[Pages 53129-53133]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2014-21245]
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DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 25
[Docket No. FAA-2013-1002; Special Conditions No. 25-530-SC]
Special Conditions: Airbus Model A350-900 Airplane; Lightning
Protection of Fuel-Tank Structure To Prevent Fuel-Tank Vapor Ignition
AGENCY: Federal Aviation Administration (FAA), DOT.
ACTION: Final special conditions.
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SUMMARY: These special conditions are issued for Airbus Model A350-900
airplanes.
These airplanes will have a novel or unusual design feature that
will incorporate a nitrogen generation system (NGS) for all fuel tanks,
to actively reduce flammability exposure within the fuel tanks
significantly below that required by the fuel-tank flammability
regulations. Among other benefits, the NGS significantly reduces the
potential for fuel-vapor ignition caused by lightning strikes. The
applicable airworthiness regulations do not contain adequate or
appropriate safety standards for this design feature. These special
conditions contain the additional safety standards that the
Administrator considers necessary to establish a level of safety
equivalent to that established by the existing airworthiness standards.
DATES: Effective Date: October 8, 2014.
[[Page 53130]]
FOR FURTHER INFORMATION CONTACT: Doug Bryant, Propulsion/Mechanical
Systems, ANM-112, Transport Airplane Directorate, Aircraft
Certification Service, 1601 Lind Avenue SW., Renton, Washington 98057-
3356; telephone (425) 227-2384; facsimile (425) 227-1320.
SUPPLEMENTARY INFORMATION:
Background
On August 25, 2008, Airbus applied for a type certificate for their
new Model A350-900 airplane. Later, Airbus requested, and the FAA
approved, an extension to the application for FAA type certification to
November 15, 2009. The Model A350-900 airplane has a conventional
layout with twin wing-mounted Rolls-Royce Trent XWB engines. It
features a twin-aisle, 9-abreast, economy-class layout, and
accommodates side-by-side placement of LD-3 containers in the cargo
compartment. The basic Model A350-900 airplane configuration
accommodates 315 passengers in a standard two-class arrangement. The
design cruise speed is Mach 0.85 with a maximum take-off weight of
602,000 lbs. The Model A350-900 series airplane has a composite wing
and fuel-tank structure constructed of carbon-fiber-reinforced plastic
materials.
Type Certification Basis
Under Title 14, Code of Federal Regulations (14 CFR) 21.17, Airbus
must show that the Model A350-900 airplane meets the applicable
provisions of 14 CFR part 25, as amended by Amendments 25-1 through 25-
129.
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 Model A350-900 airplane because of
a novel or unusual design feature, special conditions are prescribed
under 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 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 Model A350-900 airplane 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. The FAA must issue a
finding of regulatory adequacy under Sec. 611 of Public Law 92-574,
the ``Noise Control Act of 1972.''
The FAA issues special conditions, as defined in 14 CFR 11.19,
under Sec. 11.38, and they become part of the type-certification basis
under Sec. 21.17(a)(2).
Novel or Unusual Design Features
The Airbus Model A350-900 airplane will incorporate the following
novel or unusual design features: Fuel-tank NGS that is intended to
control fuel-tank flammability for all fuel tanks. This NGS is designed
to provide a level of performance that applies the more stringent
standard for warm-day flammability performance applicable to normally
emptied tanks within the fuselage contour from Sec. 25.981(b), and 14
CFR part 25 appendix M, to all fuel tanks of the Model A350-900
airplane. This high level of NGS performance for all fuel tanks is a
novel or unusual design feature not envisioned at the time the
regulations applying to the Model A350-900 airplane certification basis
were issued.
Discussion
The certification basis of the Airbus Model A350-900 airplane
includes Sec. 25.981, as amended by Amendment 25-125, as required by
14 CFR 26.37. This amendment includes the ignition-prevention
requirements in Sec. 25.981(a), as amended by Amendment 25-102. It
includes revised flammability limits for all fuel tanks, and new
specific limitations on flammability for all fuel tanks as defined in
Sec. 25.981(b), as amended by Amendment 25-125.
Ignition Source Prevention
Section 25.981(a)(3) requires applicants to show that an ignition
source in the fuel-tank system could not result from any single
failure, from any single failure in combination with any latent failure
condition not shown to be extremely remote, or from any combination of
failures not shown to be extremely improbable. This requirement was
originally adopted in Amendment 25-102, and it requires the assumption
that the fuel tanks are always flammable when showing that the
probability of an ignition source being present is extremely remote.
(Amendment 25-102 included Sec. 25.981(c), which required minimizing
fuel-tank flammability, and this was defined in the preamble as being
equivalent to unheated aluminum fuel tanks located in the wing.) This
requirement defines three types of scenarios that must be addressed to
show compliance with Sec. 25.981(a)(3). The first scenario is that any
single failure, regardless of the probability of occurrence of the
failure, must not cause an ignition source. The second scenario is that
any single failure, regardless of the probability of occurrence, in
combination with any latent failure condition not shown to be at least
extremely remote, must not cause an ignition source. The third scenario
is that any combination of failures not shown to be extremely
improbable must not cause an ignition source. Demonstration of
compliance with this requirement would typically require a structured,
quantitative safety analysis. Design areas that have latent failure
conditions typically would be driven by these requirements to have
multiple fault tolerance, or ``triple redundancy.'' This means that
ignition sources are still prevented even after two independent
failures.
Flammability Limits
Section 25.981(b) states that no fuel-tank fleet-average
flammability exposure may exceed 3 percent of the flammability-exposure
evaluation time calculated using the method in part 25, Appendix N, or
the fleet-average flammability of a fuel tank within the wing of the
airplane being evaluated, whichever is greater. If the wing is not a
conventional, unheated aluminum wing, the analysis must be based on an
assumed equivalent, conventional construction, unheated, aluminum wing.
In addition, for fuel tanks that are normally emptied during operation
and that have any part of the tank located within the fuselage contour,
the fleet-average flammability for warm days (above 80 [deg]F) must be
limited to 3 percent, as calculated using the method in part 25,
Appendix M.
Application of Existing Regulations Inappropriate Due to Impracticality
Since the issuance of Sec. 25.981(a)(3), as amended by Amendment
25-102, the FAA has conducted certification projects in which
applicants found it impractical to meet the requirements of that
regulation for some areas of lightning protection for fuel tank
structure. Partial exemptions were issued for these projects. These
same difficulties exist for the Airbus Model A350-900 airplane project.
The difficulty of designing multiple-fault-tolerant structure, and
the difficulty of detecting failures of hidden structural-design
features in general, makes compliance with Sec. 25.981(a)(3) uniquely
challenging and impractical for certain aspects of the electrical
bonding of structural elements. Such bonding is needed to prevent
occurrence of fuel-tank ignition sources from lightning strikes. The
effectiveness and fault tolerance of electrical-bonding
[[Page 53131]]
features for structural joints and fasteners is partially dependent on
design features that cannot be effectively inspected or tested after
assembly without damaging the structure, joint, or fastener. Examples
of such features include a required interference fit between the shank
of a fastener and the hole in which the fastener is installed; metal
foil or mesh imbedded in composite material; a required clamping force
provided by a fastener to pull two structural parts together; and a
required faying surface bond between the flush surfaces of adjacent
pieces of structural material, such as in a wing-skin joint, or a
mounting-bracket installation. In addition, other features that
physically can be inspected or tested may be located within the fuel
tanks. Therefore, it is not practical to inspect for failures of those
features at short intervals. Examples of such failures include
separation or loosening of cap seals over fastener ends, and actual
structural failures of internal fasteners. This inability to
practically detect manufacturing errors and failures of structural-
design features critical to lightning protection results in degraded
conditions that occur and remain in place for a very long time,
possibly for the remaining life of the airplane. The complex
construction techniques associated with composite structure can make
these aspects particularly challenging.
Accounting for such long failure-latency periods in the system
safety analysis, required by Sec. 25.981(a)(3), would require multiple
fault tolerance in the structural lightning-protection design. As part
of the design-development activity for the Model A350-900 airplane,
Airbus has examined possible design provisions to provide multiple
fault tolerance in the structural design to prevent ignition sources
from occurring in the event of lightning attachment to the airplane in
critical locations. Airbus has concluded from this examination that
providing multiple fault tolerance for some structural elements is not
practical. Airbus has also identified some areas of the Model A350-900
airplane design where it is impractical to provide even single-fault
tolerance in the structural design to prevent ignition sources from
occurring in the event of lightning attachment after a single failure.
The FAA has reviewed this examination with Airbus in detail and has
agreed that providing fault tolerance beyond that in the Model A350-900
airplane design for these areas would be impractical.
As a result of the Airbus Model A350-900 airplane and other
certification projects, the FAA has now determined that compliance with
Sec. 25.981(a)(3) is impractical for some areas of lightning
protection for fuel-tank structure, and that application of Sec.
25.981(a)(3) to those design areas is therefore inappropriate. The FAA
plans further rulemaking to revise Sec. 25.981(a)(3). As appropriate,
the FAA plans to issue special conditions or exemptions for
certification projects progressing before the revision is complete.
This is discussed in FAA Memorandum ANM-112-08-002, Policy on Issuance
of Special Conditions and Exemptions Related to Lightning Protection of
Fuel Tank Structure, dated May 26, 2009.\1\
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\1\ The memorandum may be viewed at: http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgPolicy.nsf/0/12350AE62D393B7A862575C300709CA3?OpenDocument&Highlight=anm-112-08-002
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Application of Existing Regulations Inappropriate Due to Compensating
Feature That Provides Equivalent Level of Safety
Section 25.981(b) sets specific standards for fuel-tank
flammability as discussed above under ``Flammability Limits.'' Under
that regulation, the fleet-average flammability exposure of all fuel
tanks on the Model A350-900 airplane may not exceed 3 percent of the
flammability-exposure evaluation time calculated using the method in
part 25, Appendix N, or the fleet-average flammability of a wing main
tank within an equivalent construction, conventional, unheated,
aluminum wing fuel tank, whichever is greater. The typical fleet-
average fuel-tank flammability of fuel tanks located in the wing ranges
between 1 and 5 percent. If it is assumed that a Model A350-900
airplane equivalent, conventional, unheated, aluminum wing fuel tank
would not exceed a fleet-average flammability time of 3 percent, the
actual composite Model A350-900 airplane wing-fuel-tank design would be
required to comply with the 3 percent fleet average flammability
standard, and therefore a means to reduce the flammability to 3 percent
would be required. However, the Model A350-900 airplane design includes
NGS for all fuel tanks that will also be shown to meet the additional,
more-stringent warm-day average flammability standard in part 25,
Appendix M, which is only required for normally emptied fuel tanks with
some part of the tank within the fuselage contour. Fuel tanks that meet
this requirement typically have average fuel-tank flammability levels
well below the required 3 percent.
Since the NGS for all fuel tanks on the Model A350-900 airplane
provides performance that meets part 25, Appendix M, the FAA has
determined that the risk reduction provided by this additional
performance will provide compensation for some relief from the
ignition-prevention requirements of Sec. 25.981(a)(3) while still
establishing a level of safety equivalent to that established in the
regulations.
In determining the appropriate amount of relief from the ignition-
prevention requirements of Sec. 25.981(a), the FAA considered the
original overall intent of Amendment 25-102, which was to ensure the
prevention of catastrophic events due to fuel-tank vapor explosion.
These special conditions are intended to achieve that objective through
a prescriptive requirement that fault tolerance (with respect to the
creation of an ignition source) be provided for all structural
lightning protection design features where providing such fault
tolerance is practical, and through a performance-based standard for
the risk due to any single-failure vulnerability that exists in the
design. In addition, for any structural lightning-protection design
features for which Airbus shows that providing fault tolerance is
impractical, these special conditions require Airbus to show that a
fuel-tank vapor-ignition event, due to the summed risk of all non-
fault-tolerant design features, is extremely improbable. Airbus would
be required to show that the design meets this safety objective using a
structured system-safety assessment similar to that currently used for
demonstrating compliance with Sec. Sec. 25.901 and 25.1309.
Given these novel or unusual design features, and the compliance
challenges noted earlier in this document, the FAA has determined that
application of Sec. 25.981(a)(3) is inappropriate in that it is
neither practical nor necessary to apply the ignition-source-prevention
provisions of Sec. 25.981(a)(3) to the specific fuel-tank structural
lightning-protection features of the Airbus Model A350-900 airplane.
However, without the Sec. 25.981(a)(3) provisions, the remaining
applicable regulations in the Model A350-900 airplane certification
basis would be inadequate to set an appropriate standard for fuel-tank
ignition prevention. Therefore, in accordance with provisions of Sec.
21.16, the FAA has determined that, instead of Sec. 25.981(a)(3),
alternative fuel-tank structural lighting-protection requirements be
applied to fuel-tank lightning-protection features that are integral to
the airframe structure of the
[[Page 53132]]
Model A350-900 airplane. These alternative requirements are intended to
provide the level of safety intended by Sec. 25.981(a)(3), based on
our recognition, as discussed above, that a highly effective NGS for
the fuel tanks makes it unnecessary to assume that the fuel tank is
always flammable. As discussed previously, the assumption that the fuel
tanks are always flammable was required when demonstrating compliance
to the ignition-prevention requirements of Sec. 25.981(a)(3).
One resulting difference between these special conditions and the
Sec. 25.981(a)(3) provisions they are meant to replace is the outcome
being prevented--fuel-vapor ignition versus an ignition source. These
special conditions acknowledge that the application of fuel-tank-
flammability performance standards will reduce fuel-tank flammability
to an extent that it is appropriate to consider the beneficial effects
of flammability reduction when considering design areas where it is
impractical to comply with Sec. 25.981(a)(3).
One of the core requirements of these special conditions is a
prescriptive requirement that structural lightning-protection design
features must be fault tolerant. (An exception, wherein Airbus can show
that providing fault tolerance is impractical, and associated
requirements, is discussed below.) The other core requirement is that
Airbus must show that the design, manufacturing processes, and
Airworthiness Limitations section of the Instructions for Continued
Airworthiness include all practical measures to prevent, and detect and
correct, failures of structural-lightning protection features due to
manufacturing variability, aging, wear, corrosion, and likely damage.
The FAA has determined that, if these core requirements are met, a
fuel-tank vapor-ignition event, due to lightning, is not anticipated to
occur in the life of the airplane fleet. This conclusion is based on
the fact that a critical lightning strike to any given airplane is
itself a remote event, and on the fact that fuel tanks must be shown to
be flammable only for a relatively small portion of the fleet
operational life.
For any non-fault-tolerant features in the design, Airbus must show
that eliminating these features or making them fault tolerant is
impractical. The requirements and considerations for showing it is
impractical to provide fault tolerance are described in FAA Memorandum
ANM-112-08-002. This requirement is intended to minimize the number of
non-fault-tolerant features in the design.
For areas of the design where Airbus shows that providing fault-
tolerant structural lighting-protection features is impractical, non-
fault-tolerant features will be allowed, provided Airbus can show that
a fuel-tank vapor-ignition event, due to the non-fault-tolerant
features, is extremely improbable when the sum of probabilities of
those events, due to all non-fault-tolerant features, is considered.
Airbus will be required to submit a structured, quantitative assessment
of fleet-average risk for a fuel-tank vapor-ignition event due to all
non-fault-tolerant design features included in the design. This will
require determination of the number of non-fault-tolerant design
features, estimates of the probability of the failure of each non-
fault-tolerant design feature, and estimates of the exposure time for
those failures. This analysis must include failures due to
manufacturing variability, aging, wear, corrosion, and likely damage.
It is acceptable to consider the probability of fuel-tank
flammability, the probability of a lightning strike to the airplane,
the probability of a lightning strike to specific zones of the airplane
(for example, Zone 2 behind the nacelle, but not a specific location or
feature), and a distribution of lightning-strike amplitude in
performing the assessment, provided the associated assumptions are
acceptable to the FAA. The analysis must account for any dependencies
among these factors, if they are used. The assessment must also account
for operation with inoperative features and systems, including any
proposed or anticipated dispatch relief. This risk-assessment
requirement is intended to ensure that an acceptable level of safety is
provided given the non-fault-tolerant features in the design.
Part 25, Appendix N, as adopted in Amendment 25-125, in conjunction
with these special conditions, constitutes the standard for how to
determine flammability probability. In performing the safety analysis
required by these special conditions, relevant Sec. 25.981(a)(3)
compliance guidance is still applicable. Appropriate credit for the
conditional probability of environmental or operational conditions
occurring is normally limited to those provisions involving multiple
failures, and this type of credit is not normally allowed in evaluation
of single failures. However, these special conditions would allow
consideration of the probability of occurrence of lightning attachment
and flammable conditions when assessing the probability of structural
failures resulting in a fuel-tank vapor-ignition event.
The FAA understands that lightning-protection safety for airplane
structure is inherently different from lightning protection for
systems. We intend to apply these special conditions only to structural
lightning-protection features of fuel systems. We do not intend to
apply the alternative standards used under these special conditions to
other areas of the airplane-design evaluation.
Requirements Provide Equivalent Level of Safety
In recognition of the unusual design feature discussed above, and
the impracticality of requiring multiple fault tolerance for lightning
protection of certain aspects of fuel-tank structure, the FAA has
determined that a level of safety equivalent to direct compliance with
Sec. 25.981(a)(3) will be achieved for the Model A350-900 airplane by
applying these requirements. The FAA considers that, instead of only
concentrating on fault tolerance for ignition-source prevention,
significantly reducing fuel-tank flammability exposure, in addition to
preventing ignition sources, is a better approach to lightning
protection for the fuel tanks. In addition, the level of average fuel-
tank flammability achieved by compliance with these special conditions
is low enough that it is not appropriate or accurate to assume, in a
safety analysis, that the fuel tanks may always be flammable.
Section 25.981(b), as amended by Amendment 25-125, sets limits on
the allowable fuel-tank flammability for the Model A350-900 airplane.
Condition 2(a) of these special conditions applies the more-stringent
standard, for warm-day flammability performance applicable to normally
emptied tanks within the fuselage contour, from Sec. 25.981(b) and
part 25, Appendix M, to all of the fuel tanks of the Model A350-900
airplane.
Because of the more-stringent fuel-tank flammability requirements
in these special conditions, and because the flammability state of a
fuel tank is independent of the various failures of structural elements
that could lead to an ignition source in the event of lightning
attachment, the FAA has agreed that it is appropriate in this case to
allow treatment of flammability as an independent factor in the safety
analysis. The positive control of flammability, and the lower
flammability that is required by these special conditions, exceed the
minimum requirements of Sec. 25.981(b). This offsets a reduction of
the stringent standard for ignition-source prevention in Sec.
25.981(a)(3), which assumes that the fuel tank is flammable at all
times.
[[Page 53133]]
Given the stringent requirements for fuel-tank flammability, the
fuel-vapor ignition prevention, and the ignition-source prevention
requirements in these special conditions will prevent ``. . .
catastrophic failure . . . due to ignition of fuel or vapors,'' as
stated in Sec. 25.981(a). Thus, the overall level of safety achieved
by these special conditions is considered equivalent to that which
would be required by compliance with Sec. 25.981(a)(3) and (b).
These special conditions contain the additional safety standards
that the Administrator considers necessary to establish a level of
safety equivalent to that established by the existing airworthiness
standards.
Discussion of Comments
Notice of proposed special conditions No. 25-13-36-SC for Airbus
Model A350-900 series airplanes was published in the Federal Register
on December 19, 2013 (78 FR 76775). No comments were received, and the
special conditions are adopted as proposed.
Applicability
As discussed above, these special conditions apply to Airbus Model
A350-900 series airplanes. Should Airbus apply later for a change to
the type certificate to include another model incorporating the same
novel or unusual design feature, the special conditions would apply to
that model as well.
Conclusion
This action affects only certain novel or unusual design features
on the Airbus Model A350-900 series 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 these special conditions is as follows:
Authority: 49 U.S.C. 106(g), 40113, 44701, 44702, 44704.
The Special Conditions
0
Accordingly, pursuant to the authority delegated to me by the
Administrator, the following special conditions are issued as part of
the type-certification basis for Airbus Model A350-900 series
airplanes.
1. Definitions
Most of the terms used in the special conditions described in
Alternative Fuel Tank Structural Lightning Protection Requirements
either have the common dictionary meaning or are defined in Advisory
Circular 25.1309-1A, System Design and Analysis, dated June 21, 1988.
The following definitions are the only terms intended to have a
specialized meaning when used in these special conditions:
(a) Basic Airframe Structure. Includes design elements such as
structural members, structural joint features, and fastener systems
including airplane skins, ribs, spars, stringers, etc., and associated
fasteners, joints, coatings, and sealant. Basic airframe structure may
also include those structural elements that are expected to be removed
for maintenance, such as exterior fuel-tank access panels and fairing-
attachment features, provided maintenance errors that could compromise
associated lightning-protection features would be evident upon an
exterior, preflight inspection of the airplane and would be corrected
prior to flight.
(b) Permanent System-Supporting Structure. Includes static,
permanently attached structural parts (such as brackets) that are used
to support system elements. It does not include any part intended to be
removed, or any joint intended to be separated, to maintain or replace
system elements or other parts, unless that part removal or joint
separation is accepted by the FAA as being extremely remote.
(c) Manufacturing Variability. Includes tolerances and variability
that the design and production specifications allow, as well as
anticipated errors or escapes from the manufacturing and inspection
processes.
(d) Extremely Remote. Conditions that are not anticipated to occur
to each airplane during its total life, but which may occur a few times
when considering the total operational life of all airplanes of one
type. Extremely remote conditions are those having an average
probability per flight hour on the order of 1 x 10-\7\ or
less, but greater than on the order of 1 x 10-\9\.
(e) Extremely Improbable. Conditions that are so unlikely that they
are not anticipated to occur during the entire operational life of all
airplanes of one type. Extremely improbable conditions are those having
an average probability per flight hour of the order of 1 x
10-\9\ or less.
2. Alternative Fuel-Tank Structural Lightning-Protection Requirements
For lightning-protection features that are integral to fuel-tank
basic airframe structure or permanent system-supporting structure, as
defined in this these special conditions Definitions, for which Airbus
shows and the FAA finds compliance with Sec. 25.981(a)(3) to be
impractical, the following requirements may be applied in lieu of the
requirements of Sec. 25.981(a)(3):
(a) Airbus must show that the airplane design meets the
requirements of part 25, Appendix M, as amended by Amendment 25-125,
for all fuel tanks installed on the airplane.
(b) Airbus must show that the design includes at least two
independent, effective, and reliable lightning-protection features (or
sets of features) such that fault tolerance to prevent lightning-
related ignition sources is provided for each area of the structural
design to be shown compliant with these special conditions in lieu of
compliance with the requirements of Sec. 25.981(a)(3). Fault tolerance
is not required for any specific design feature if:
(1) For that feature, providing fault tolerance is shown to be
impractical, and
(2) Fuel-tank vapor ignition due to that feature and all other non-
fault-tolerant features, when their fuel-tank vapor-ignition event
probabilities are summed, is shown to be extremely improbable.
(c) Airbus must perform an analysis to show that the design,
manufacturing processes, and airworthiness limitations section of the
instructions for continued airworthiness include all practical measures
to prevent, and detect and correct, failures of structural lightning-
protection features due to manufacturing variability, aging, wear,
corrosion, and likely damage.
Issued in Renton, Washington, on August 15, 2014.
Jeffrey E. Duven,
Transport Airplane Directorate, Aircraft Certification Service.
[FR Doc. 2014-21245 Filed 9-5-14; 8:45 am]
BILLING CODE 4910-13-P