[Federal Register Volume 74, Number 197 (Wednesday, October 14, 2009)]
[Proposed Rules]
[Pages 52698-52702]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: E9-24652]


=======================================================================
-----------------------------------------------------------------------

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. NM415; Notice No. 25-09-11-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: Notice of proposed special conditions.

-----------------------------------------------------------------------

SUMMARY: This action proposes special conditions 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 
proposed 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: We must receive your comments by November 30, 2009.

ADDRESSES: You must mail two copies of your comments to: Federal 
Aviation Administration, Transport Airplane Directorate, Attention: 
Rules Docket (ANM-113), Docket No. NM415, 1601 Lind Avenue, SW., 
Renton, Washington 98057-3356. You may deliver two copies to the 
Transport Airplane Directorate at the above address. You must mark your 
comments: Docket No. NM415. You may inspect comments in the Rules 
Docket weekdays, except Federal holidays, between 7:30 a.m. and 4 p.m.

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: 

Comments Invited

    We invite interested persons to take part in this rulemaking by 
sending written comments, data, or views. The most helpful comments 
reference a specific portion of the special conditions, explain the 
reason for any recommended change, and include supporting data. We ask 
that you send us two copies of written comments.
    We will file in the docket all comments we receive as well as a 
report summarizing each substantive public contact with FAA personnel 
concerning these proposed special conditions. You may inspect the 
docket before and after the comment closing date. If you wish to review 
the docket in person, go to the address in the ADDRESSES section of 
this notice between 7:30 a.m. and 4 p.m., Monday through Friday, except 
Federal holidays.
    We will consider all comments we receive by the closing date for 
comments. We will consider comments filed late if it is possible to do 
so without incurring expense or delay. We may change the proposed 
special conditions based on comments we receive.
    If you want the FAA to acknowledge receipt of your comments on this 
proposal, include with your comments a pre-addressed, stamped postcard 
on which the docket number appears. We will stamp the date on the 
postcard and mail it back to you.

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 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.''
    Special conditions, as defined in 14 CFR 11.19, are issued in 
accordance with Sec.  11.38 and become part of the type certification 
basis in accordance with 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 proposed 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

[[Page 52699]]

time the regulations in the proposed 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 Sec.  26.37. This amendment 
includes the ignition prevention requirements in Sec.  25.981(a), as 
amended by Amendment 25-102, and it includes specific limitations on 
fuel tank flammability 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 proposed 
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 was based on the assumption 
that fuel tanks are always flammable. This 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 any 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 promulgation 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 failures of structural 
design features critical to lightning protection results in any such 
failures that occur remaining in place for a very long time, and 
possibly for the remaining life of the airplane, prior to detection.
    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 proposed 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). The FAA plans to issue special conditions or 
exemptions, when appropriate, for certification projects in the 
interim. 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\
---------------------------------------------------------------------------

    \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.
---------------------------------------------------------------------------

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

[[Page 52700]]

construction conventional unheated aluminum wing fuel tank, whichever 
is greater. 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. 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.
    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).
    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. The 
proposed 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, the proposed 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 Proposed Requirements

    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 is proposing 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 tank is always flammable was part of the 
basis for the ignition prevention requirements of Sec.  25.981(a)(3).
    One resulting difference between these proposed 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 proposed 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 the proposed 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 proposed special conditions, constitutes the standard for 
how to determine flammability probability. In performing the safety 
analysis required by these proposed special conditions,

[[Page 52701]]

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 
proposed 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.

Proposed 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 
proposed 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 proposed 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).

Applicability

    These proposed special conditions are applicable to the 787-8. 
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 proposed special conditions would apply 
to that model as well.

Conclusion

    This action affects only certain novel or unusual design features 
of the 787. 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 Proposed Special Conditions

    Accordingly, the Federal Aviation Administration (FAA) proposes the 
following special conditions 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 AC 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.:
    (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

[[Page 52702]]

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 September 24, 2009.
Ali Bahrami,
Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. E9-24652 Filed 10-13-09; 8:45 am]
BILLING CODE 4910-13-P