Special Conditions: Boeing Model 787-8 Airplane; Lightning Protection of Fuel Tank Structure To Prevent Fuel Tank Vapor Ignition, 71346-71351 [2010-29409]

Agencies

• Federal Aviation Administration
• DEPARTMENT OF TRANSPORTATION

[Federal Register Volume 75, Number 225 (Tuesday, November 23, 2010)]
[Rules and Regulations]
[Pages 71346-71351]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2010-29409]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. NM415; Special Conditions No. 25-414-SC]


Special Conditions: Boeing Model 787-8 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 the Boeing Model 787-8 
airplane. This airplane will have novel or unusual design features when 
compared to the state of technology envisioned in the airworthiness 
standards for transport category airplanes. The Boeing Model 787-8 
airplane will incorporate a fuel tank nitrogen generation system (NGS) 
that actively reduces flammability exposure within the main fuel tanks 
significantly below that required by the fuel tank flammability 
regulations. Among other benefits, this 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: December 23, 2010.

FOR FURTHER INFORMATION CONTACT: Mike Dostert, FAA, ANM-112, Transport 
Airplane Directorate, Aircraft Certification Service, 1601 Lind Avenue, 
SW., Renton, Washington 98057-3356; telephone (425) 227-2132; facsimile 
(425) 227-1149.

SUPPLEMENTARY INFORMATION:

Background

    On March 28, 2003, The Boeing Company applied for an FAA type 
certificate for its new Boeing Model 787-8 passenger airplane. The 
Boeing Model 787-8 airplane will be a new design, two-engine turbo-jet 
transport category airplane with a two-aisle cabin configuration. The 
maximum takeoff weight will be 484,000 pounds, and it will carry a 
maximum of 381 passengers.

Type Certification Basis

    Under provisions of Title 14, Code of Federal Regulations (14 CFR) 
21.17, Boeing must show that Boeing Model 787-8 airplanes (hereafter 
referred to as ``the 787'') meet the applicable provisions of 14 CFR 
part 25, as amended by Amendments 25-1 through 25-117, with three 
exceptions. Sections 25.809(a) and 25.812 will remain as amended by 
Amendment 25-115, and Sec.  25.981, which will be as amended by 
Amendment 25-125 in accordance with 14 CFR 26.37.
    If the Administrator finds that the applicable airworthiness 
regulations (i.e., part 25) do not contain adequate or appropriate 
safety standards for the 787 because of novel or unusual design 
features, special conditions are prescribed under provisions of 14 CFR 
21.16.
    In addition to the applicable airworthiness regulations and special 
conditions, the 787 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. Finally, the FAA must also issue a finding of 
regulatory adequacy under Sec.  611 of Public Law 92-574, the ``Noise 
Control Act of 1972.''
    The FAA issues special conditions, as defined in 14 CFR 11.19, in 
accordance with Sec.  11.38, and they become part of the type 
certification basis under Sec.  21.17(a)(2).
    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 features, the special conditions would also 
apply to the other model under Sec.  21.101.

Novel or Unusual Design Features

    The 787 will have a fuel tank NGS that is intended to control fuel 
tank flammability. This NGS is designed to provide a level of 
performance that will reduce the warm day fleet average wing fuel tank 
flammability significantly below the maximum wing fuel tank 
flammability limits set in Sec.  25.981(b), as amended by Amendment 25-
125. This high level of wing fuel tank NGS performance is an unusual 
design feature not envisioned at the time the regulations in the 787 
certification basis were promulgated.

Existing Regulations

    The certification basis of the 787 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, and it includes revised 
flammability limits for the wing fuel tanks and new specific 
limitations on flammability of normally emptied fuel tanks located 
within the fuselage contour as defined in Sec.  25.981(b), as amended 
by Amendment 25-125. (Section 25.981(c) contains an alternative to 
meeting paragraph (b)--vapor ignition mitigation--that is not 
applicable to the 787 design.)

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 the probability 
of an ignition source being present is extremely remote. (Amendment 25-
102 included Sec.  25.981(c) that 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

[[Page 71347]]

requirement defines three types of scenarios that must be addressed in 
order 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 construction conventional 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 787 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 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 can be physically 
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.
    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 787, Boeing 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. 
Boeing has concluded from this examination that providing multiple 
fault tolerance for some structural elements is not practical. Boeing 
has also identified some areas of the 787 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 Boeing in detail and has agreed that providing fault 
tolerance beyond that in the proposed 787 design for these areas would 
be impractical.
    As a result of the 787 and other certifications 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: https://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 wing main 
tanks on the 787 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 787 equivalent conventional unheated aluminum wing fuel 
tank would not exceed a fleet average flammability time of 3 percent, 
the actual composite 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 proposed 787 design includes a wing tank NGS 
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 proposed wing tank NGS on the 787 provides performance 
that meets part 25, Appendix M, the FAA has determined that the risk 
reduction

[[Page 71348]]

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 Boeing shows that providing fault tolerance is 
impractical, these special conditions would require Boeing to show that 
a fuel tank vapor ignition event due to the summed risk of all non-
fault-tolerant design features is extremely improbable. Boeing would be 
required to show that this safety objective is met by the proposed 
design using a structured system safety assessment similar to that 
currently used for demonstrating compliance with Sec. Sec.  25.901 and 
25.1309.

Discussion of the Final Special Conditions

    Given these novel 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 787. However, without the Sec.  25.981(a)(3) 
provisions, the remaining applicable regulations in the 787 
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 787. 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 Boeing can show 
that providing fault tolerance is impractical, and associated 
requirements, is discussed below.) The other core requirement is that 
Boeing 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 for only a relatively small portion of the fleet operational 
life.
    For any non-fault-tolerant features proposed in the design, Boeing 
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 Boeing shows that providing fault 
tolerant structural lighting protection features is impractical, non-
fault-tolerant features will be allowed provided Boeing 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. Boeing 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 proposed 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

[[Page 71349]]

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 an equivalent level of safety to direct compliance with 
Sec.  25.981(a)(3) will be achieved for the 787 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 tank. 
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 787. Paragraph 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 the wing tanks of the 787.
    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 exceeds 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.
    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).

Discussion of Comments

    Notice of proposed special conditions No. 25-09-11-SC for the 
Boeing Model 787-8 airplanes was published in the Federal Register on 
October 14, 2009 (74 FR 52698). Several comments were received from two 
commenters (Cessna and NATCA).

Cessna 1

    Cessna requested additional wording be added to the discussion of 
the proposed special conditions to clarify the fuel tank flammability 
requirements proposed in the special conditions would only be applied 
specifically to special conditions. Cessna referred to FAA Policy Memo 
ANM-112-08-002 and noted the flammability levels of Appendix M are not 
defined as a precondition for petitions for exemptions. Cessna proposed 
the following text:

    ``Since the proposed wing tank NGS on the 787 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.''

    The additional wording proposed by the commenter clarifies that the 
safety level provided by the special conditions is equivalent to that 
established in the regulation. Part 21 only allows the FAA to propose 
special conditions when equivalent safety to the applicable 
airworthiness standards has been demonstrated. We agree with the 
accuracy of the commenters proposed text and modified the wording of 
the discussion in the special conditions as suggested by the commenter.
    As we have already stated in FAA Policy Memo ANM-112-08-002 (Policy 
on Issuance of Special Conditions and Exemptions Related to Lightning 
Protection of Fuel Tank Structure), for traditional airplanes that do 
not have active flammability reduction systems, where the applicant 
shows that full compliance with Sec.  25.981 is impractical, we intend 
to allow a similar reduction in the number of ignition-prevention 
features using the exemption process. Exemptions are needed because 
reducing the number of ignition-prevention features without reducing 
the fuel-tank flammability does not provide equivalent safety to Sec.  
25.981.
    No change to the proposed special conditions was made as a result 
of this comment.

Cessna 2

    Cessna recommended that the alternative requirements for special 
conditions and exemptions to Sec.  25.981(a)(3) include considerations 
for both structure and systems, with regards to both lightning and 
electrostatics protection. They supported their comment with the 
rationale that electrostatic protection methods rely upon bonding 
techniques similar to those employed for lightning protection, and pose 
similar practicality issues. Each additional redundant bonding 
provision is itself another potential failure mode, and the over-
complication of increased redundancy presents maintenance and 
operational issues.
    Cessna requested that the proposed Special Condition No. 1, 
Definitions, be changed to broaden the applicability of the special 
conditions to include ``systems internal to the fuel tank.'' We have 
already addressed this comment in developing FAA Policy Memo ANM-112-
08-002. The public comments to FAA Policy Memo ANM-112-08-002 and our 
disposition of those comments are available at https://rgl.faa.gov. 
Click on ``Policy,'' then search (By Policy Number) for ANM-112-08-002. 
The commenter has provided no new information, and no change was made 
to the proposed special conditions as a result of this comment.

Cessna 3

    Cessna recommended the FAA include reference to guidance material 
developed by the Society of Automotive Engineers (SAE) AE-2 Lightning 
Committee directly in exemptions and special conditions. The FAA 
participated on the SAE committee that prepared the guidance material. 
However, at this time the FAA has not completed its review of the AE-2 
guidance. We will review the proposed guidance material and publish it 
for comment if we determine it to be a viable means of showing 
compliance to special conditions or exemptions. In the mean time, this 
guidance is not necessary for the adoption of, or compliance with, 
these special conditions.

NATCA 1

    The National Air Traffic Controller Association (NATCA) requested 
the proposed special conditions be withdrawn since they believe the 
information provided in the special condition's Background section does 
not support the FAA finding that the

[[Page 71350]]

proposed special conditions provide equivalent safety to the existing 
part 25 safety standards for transport airplanes.
    We have already addressed this request to not publish the proposed 
special conditions in developing FAA Policy Memo ANM-112-08-002. For 
the reasons stated in that policy memo and the associated disposition 
of comments, we believe these special conditions do establish an 
equivalent level of safety.

NATCA 2 & 3

    NATCA provided an alternative to the proposed special conditions. 
They requested the proposed special conditions be withdrawn and revised 
and suggested the following requirements replace those proposed by the 
FAA:
    (1) Eliminate the allowance for single failures that can result in 
an ignition source, unless the fuel tank is shown to have a 
flammability reduction means that prevents the tanks from becoming 
flammable or,
    (2) Do not allow dispatch of any airplane with the inerting system 
that is not functioning if the design does not have two independent 
features that will prevent an ignition source.
    NATCA provided comments in support of its suggested change to the 
special conditions discussed above that would not ``allow dispatch of 
any airplane with the inerting system that is not functioning if the 
design does not have independent features that will prevent an ignition 
source.'' They suggested a means of meeting their proposed special 
conditions could be achieved by ``a combination of eliminating the 
single failures through design improvements and limiting airplane 
operation on warmer days with the NGS inoperative could essentially 
eliminate the chance of a fuel tank explosion due to a lightning 
strike.'' They supported their comment by stating design improvements 
implemented by Boeing have reduced the number of ignition sources and 
further design improvements implemented on later production airplanes 
could eliminate single failures. They proposed that once the single 
failures were eliminated, the restriction on dispatch of airplanes with 
the inerting system inoperative could be removed. They stated this 
would be a practical way to implement new technology because a small 
number of airplane flights could be impacted by flight delays caused by 
an inoperative fuel tank inerting system.
    We have already addressed the proposal to restrict dispatch with 
the inerting system inoperative in developing FAA Policy Memo ANM-112-
08-002. In short, determining appropriate dispatch relief, if any, is 
the function of the Flight Operations Evaluation Board and not the 
function of special conditions.

NATCA 4

    NATCA requested extension of the comment period because guidance 
material regarding means of compliance with the proposed special 
conditions was not available to the public prior the closing of the 
comment period. We do not agree with the request to extend the comment 
period but do agree that public comment on future policy should be 
sought. These special conditions are specific to the 787 and means of 
compliance are dependent upon specific proprietary design details of 
the airplane that cannot be released to the public.

NATCA 5

    NATCA provided comments that the number of single failures on the 
787 had been reduced through design changes and that earlier exemptions 
issued by the FAA did not allow single failures. They questioned the 
FAA's determination that it is impractical to eliminate single failures 
in the 787 design. They offered specific examples of possible methods 
of preventing certain single failures discussed in the preamble to the 
proposed special conditions, including use of monitoring aids 
consisting of overlays that are on the outside the fuel tank where 
failure could be easily detected and therefore failure of the features 
would not be latent.
    From this comment the FAA infers the commenter believes preventing 
all single failures is practical. While NATCA is correct that 
previously issued exemptions did not explicitly allow for single 
failures, at the time those exemptions were issued, we were not aware 
of the particular failure modes that could result in single failures 
that could create ignition sources. As stated in the proposed special 
conditions and in the discussion in FAA Policy Memo ANM-112-08-002, we 
now recognize that eliminating all single failures in airplane 
structure using current state-of-the-art design practices is not always 
practical.
    The FAA therefore does not agree that the proposed allowance for 
single failure conditions should be eliminated.

NATCA 7

    NATCA requested that ``the FAA make available to the public all 
documentation supporting the impracticality findings for each ignition 
prevention feature that will not be fail-safe, as well as why it is 
impractical (costs) to issue special conditions requiring the 787 
inerting system be operating on warmer days on any airplane that has 
been produced with known single failures.'' No change to the special 
conditions was requested in this comment. General information 
supporting the impracticality of eliminating single failures, as well 
as considerations for operating airplanes with the NGS inoperative, was 
previously discussed in FAA Policy Memo ANM-112-08-002. The specific 
design issues associated with the design of the 787 are likely to be 
proprietary, but that determination can only be made in the context of 
a Freedom of Information Act request. The special conditions, with 
clarifications discussed above, are adopted as proposed.

Applicability

    As discussed above, these special conditions are applicable to the 
Boeing Model 787-8 airplane. 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.

Conclusion

    This action affects only certain novel or unusual design features 
of the Boeing Model 787-8 airplane. It is not a rule of general 
applicability.

List of Subjects in 14 CFR Part 25

    Aircraft, Aviation safety, Reporting and recordkeeping 
requirements.

0
The authority citation for these special conditions is as follows:

    Authority: 49 U.S.C. 106(g), 40113, 44701, 44702, 44704.

The Special Conditions

    Accordingly, pursuant to the authority delegated to me by the 
Administrator, the following special conditions are issued as part of 
the type certification basis for the Boeing Model 787-8 airplane.

1. Definitions

    Most of the terms used in Special Condition No. 2, 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 Special Condition No. 2:

[[Page 71351]]

    (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 Systems 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 
allowed by the design and production specifications 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 systems supporting structure, as 
defined in Special Condition No. 1, Definitions, for which The Boeing 
Company 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) The Boeing Company 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) The Boeing Company 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 proposed 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) The applicant 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 November 
15, 2010.

Ali Bahrami,
Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 2010-29409 Filed 11-22-10; 8:45 am]
BILLING CODE 4910-13-P