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**Date:** Proposed

**To:** See Distribution

**From:** Manager, Transport Airplane Directorate, Aircraft
Certification Service

**Prepared by:** Victor Wicklund, ANM-112

**Subject:** Policy on Issuance of Special Conditions and Exemptions
Related to Lightning Protection of Fuel Tank Structure

**Memo No.:** ANM-112-08-002

**Regulatory Reference:** 14 CFR 25.981, 14 CFR 26.33, 26.37, Special
Conditions Issued for Boeing Model 737 and 747 Fuel Tank Flammability
Reduction Means (FRM)

**Policy Reference:** AC 25.981-1C, AC 25.981-2A, AC 20-53, and AC
20-155

**Summary**

This memorandum provides additional Federal Aviation Administration
(FAA) policy regarding the fuel tank safety provisions of Title 14 Code
of Federal Regulations (14 CFR) 25.981(a)(3), as amended by Amendment
25-102, for lightning protection of fuel tank structure. This policy was
developed because we determined that compliance with current regulatory
standards applicable to lightning protection of fuel tank structure may
be shown to be impractical in some cases for some areas of structural
design. In addition, the FAA has now issued standards for fuel tank
flammability reduction that may offset some reduction in the ignition
prevention standards of § 25.981(a)(3). The FAA therefore plans further
rulemaking to address the practicality issues with § 25.981(a)(3). In
the interim, the FAA will consider the use of the changed product rule
(14 CFR 21.101), special conditions, or exemptions to set an appropriate
certification basis for new and changed type designs. This policy memo
provides the conditions under which special conditions or exemptions
should be applied, and the alternative requirements that will be applied
in lieu of § 25.981(a)(3) when special conditions or exemptions are
used. These alternative requirements consider the fact that, based on
technical advances that were not envisioned at the time of Amendment
25-102, the FAA has issued new regulations setting specific performance
standards for fuel tank flammability, including a revised § 25.981(b) at
Amendment 25-125. This policy memo also discusses the conditions under
which the exception provision of § 21.101 will be used for type design
changes to allow application of § 25.981(a) at an amendment level
earlier than Amendment 25-102.

# Definition of Key Terms

In the policy statement below, the formatting (*italics*, plain text, or
\[square brackets\]) and terms used ("must," "should," or "recommend")
have a specific meaning that is explained in Attachment 1.

For the purpose of this policy, "fuel tank structure" is considered to
include structural members, such as airplane skins, joints, ribs, spars,
stringers, and associated fasteners, brackets, coatings and sealant.

**Current Regulations and Advisory Material**

Prior to Amendment 25-102 to part 25, § 25.954 was the regulation
applied to lightning protection of fuel tanks. As it was normally
applied, that regulation only required prevention of ignition of vapors
in the tank with no consideration of anticipated design failures, aging,
wear, or maintenance errors for airplane structure. After a catastrophic
fuel tank explosion accident on a large transport airplane in 1996, the
FAA pursued a dual approach to precluding fuel tank explosions by (1)
reducing the probability of ignition sources occurring in the fuel
tanks, and (2) reducing the flammability of the fuel tanks.

At the time Amendment 25‑102 was being prepared (i.e., 1998-2001), the
FAA and industry were still investigating the dynamics of tank
flammability and the fleet average flammability exposure for transport
airplane fuel tanks. Evaluation of the technical and economic viability
of fuel tank inerting systems for commercial transport airplanes was
also in its early stages at that time. As a result, the FAA adopted
changes to § 25.981(a) that applied the existing safety analysis
principles of §§ 25.901(c) and 25.1309(b) to fuel tank system ignition
sources.

Amendment 25-102 also added § 25.981(c), which required that fuel tank
flammability be minimized, or that effects of an explosion be mitigated
such that any damage from a fire or explosion would not prevent
continued safe flight and landing. This requirement was intended to be
separate from, and in addition to, the requirement to prevent ignition
sources.

While Amendment 25-102 added § 25.981(c) to require minimization of
flammability or mitigation of damage from vapor ignition events, the
primary focus of Amendment 25-102 was the prevention of ignition sources
since significantly reducing the level of fuel tank flammability was
believed to be impractical at that time. The amended ignition prevention
requirements in § 25.981(a)(3) require consideration of factors such as
aging, wear, and maintenance errors as well as the existence of single
failures, combinations of failures not shown to be extremely improbable,
and single failures in combination with latent failures to account for
the cause of many ignition sources in fuel tanks and deficiencies in the
existing regulations. This amendment to § 25.981, which applies to the
fuel tank system, requires the designs be protected from lightning with
failure tolerant features.

The policy for showing compliance with the ignition prevention
requirements of § 25.981 was based on the long-standing practice that
fuel tank vapor should be assumed to be flammable at any time
(probability of flammability = 1). However, in the preamble to Amendment
25-102, the FAA also suggested that fuel tank flammability exposure
might be considered in the future in meeting the requirements of §
25.981(a)(3) for ignition prevention. The FAA stated --

"However, if technological changes are developed, such as full-time fuel
tank inerting, and prove to be a superior method of eliminating the risk
of fuel tank ignition, the FAA could consider a change in this
philosophy in future rulemaking."

Amendment 25-102 introduced § 25.981(a)(3) --

> **25.981(a):** *" No ignition source may be present at each point in
> the fuel tank or fuel tank system where catastrophic failure could
> occur due to ignition of fuel or vapors. This must be shown by:*
>
> *"(3) Demonstrating that an ignition source could not result from each
> single failure, from each single failure in combination with each
> latent failure condition not shown to be extremely remote, and from
> all combinations of failures not shown to be extremely improbable. The
> effects of manufacturing variability, aging, wear, corrosion, and
> likely damage must be considered."*

This specific requirement affecting fuel systems is based on the
combined requirements of §§ 25.901(c) and 25.1309(b). As stated in AC
25.981-1C, "in order to eliminate any ambiguity as to the restrictions
on latent failures, § 25.981(a)(3) explicitly requires that any
anticipated latent failure condition not leave the airplane one failure
away from a catastrophic fuel tank ignition."

The policy for showing compliance with § 25.981(a)(3) described in AC
25.981-1C also stated that applicants should assume that a lightning
attachment could occur at any time (probability of lightning = 1). In
addition, industry and FAA practice has been to assume that a defined
worst-case lightning waveform would be associated with every lightning
attachment.

After promulgation of Amendment 25-102, the FAA and industry continued
research and discussion of the measurement and modeling of fuel tank
flammability, and development of practical means to reduce or eliminate
flammability in transport airplane fuel tanks, which eventually led to
the certification of practical retrofit designs for center wing fuel
tank nitrogen generation systems on two existing transport airplane
models. Those systems use nitrogen enriched air that is generated
onboard the airplane to displace oxygen in the fuel tank. This results
in inerting the fuel tank throughout most of the flight and ground
operations. An applicant for a new type certificate involving composite
wing structure included a nitrogen generation system for all fuel tanks
in the proposed design, including the main fuel tanks.

Amendment 25-125 (part of the fuel tank flammability reduction (FTFR)
rule issued in 2008) revised § 25.981(b) and (c) to introduce specific
performance based standards for the maximum flammability allowed in
various fuel tanks. Amendment 25-125 maintained the alternative adopted
by Amendment 25-102 allowing ignition mitigation means. Amendment 25-125
established a new fleet average flammability exposure limit of 3 percent
for all fuel tanks, or that of an equivalent conventional unheated
aluminum fuel tank. Fuel tanks that are not main fuel tanks and that
have any portion located within the fuselage contour must be limited to
3 percent fleet average exposure and 3 percent warm day exposure. As an
alternative, an applicant may install a means to mitigate the effects of
a fuel tank vapor ignition event in fuel tanks such that no damage
caused will prevent continued safe flight and landing. Amendment 25-125
did not change the ignition prevention standards of § 25.981(a).

Amendment 26-2 (also part of the FTFR rule issued in 2008) added
regulations requiring compliance with the flammability standards in the
new version of § 25.981(b) for certain existing type designs, for
certain type design change programs, for pending new type certificate
programs, and for future new production of existing type design
airplanes after September 20, 2010. At the same time, Amendments
121‑340, 125-55 and 129-46 added operational requirements for
installation and operation of flammability reduction means on certain
airplanes manufactured after 1991. As stated in the FTFR rule preamble,
these changes were made because the FAA recognized that measures in
Amendment 25-102 aimed at ignition source prevention would not alone be
sufficient to prevent future fuel tank explosions on transport
airplanes. That preamble stated,

> Predicting the effectiveness of ignition prevention actions is
> challenging, since many ignition sources are the result of human
> error, which cannot be precisely predicted or quantitatively
> evaluated. Despite extensive efforts by the FAA and industry to
> prevent ignition sources, we continue to learn of new ignition
> sources. Some of these ignition sources are attributable to failures
> on the part of engineering organizations to identify potential
> ignition sources and provide design changes to prevent them. Others
> are attributable to actions by production, maintenance, and other
> operational personnel, who inadvertently compromise wiring and
> equipment producing ignition sources. Regardless of the causes, we
> believe that ignition prevention actions, while necessary, are
> insufficient to eliminate ignition sources.

# Relevant Past Practice

Existing composite and aluminum structural technology can generally
provide the ability to withstand single faults and still prevent
ignition sources in the event of lightning attachment for most areas of
the design. However, systems with potentially catastrophic failure modes
typically meet requirements such as §§ 25.901 and 25.1309 through a
design architecture that can withstand multiple independent failures
without a catastrophic effect. As it applies to fuel tank lightning
protection for basic airplane structure, compliance with § 25.981(a)(3)
would typically necessitate a design with three highly reliable,
independent, and redundant protective features to prevent ignition
sources. However, for certain certification programs conducted since
Amendment 25‑102 became effective, applicants have shown that it was
impractical to provide multiple redundant protective features for some
aspects of aircraft structural lightning protection using
state-of-the-art technology for both composite fuel tank structure and
traditional aluminum fuel tank structure.

A design architecture that can withstand single failures, but not
multiple failures, that could result in an ignition source could
potentially comply with § 25.981(a)(3) when combined with either regular
inspections at sufficiently short intervals, or when combined with a
monitoring device to verify the functionality of the protective
features. However, inspections and monitoring features for the various
structural design features may not be feasible. Confirming the continued
functionality of structural lightning protection features can be
difficult because of the significant challenges of providing continual
health monitoring of the features. It can also be difficult because of
the limited capability to inspect the features without significantly
impacting aircraft performance, efficiency, or other features critical
to safety. While some concepts involving monitoring aids attached
external to the wings have been considered, the protective features are
frequently integral to the fuel tank structure or internal to the fuel
tanks. Inspection of features inside fuel tanks requires access to the
fuel tanks, which is usually only scheduled a few times during the life
of the airplane. Increasing the frequency of internal fuel tank
inspections could also have the undesirable effect of increasing the
possibility of damaging the lightning protection features or other
design features during the inspection process.

Finally, applicants have also argued that it is not practical to provide
effective fault tolerance for certain design areas of fuel tank
structure. Certain known single failure conditions can create an
ignition source in the event of a critical lightning attachment. These
areas include cracking of structural elements, complete fracture of a
bolt with high tension loading that causes gapping or departure of one
or both ends of the bolt from the bolt hole and simultaneously
compromises sealant over the bolt, and failures of sealant where the
sealant is the primary feature needed to prevent an ignition source.
Design changes to provide fault tolerance for these design areas to
prevent ignition sources have been found to be impractical.

After examining some of these problematic design areas with applicants,
the FAA has agreed that it can be impractical to meet the specific
requirements of § 25.981(a)(3) for certain areas of structural design,
and that exemptions, (or exceptions under § 21.101, when applicable)
should be considered for design features where the applicant shows
compliance to be impractical. To date, two applicants with airplanes
using conventional aluminum structure have requested exemptions from
§ 25.981(a)(3) for structural lightning protection and have received
partial exemptions. For another applicant for winglet installation
design changes, the FAA found under § 21.101 that an exception applied,
allowing application of an earlier amendment of § 25.981(a) for fuel
tank structure.

In addition, special conditions to be used in lieu of § 25.981(a)(3) may
be developed for proposed designs that include features to reduce
flammability significantly below the level required by § 25.981(b) at
Amendment 25-125. Special conditions are currently under development for
a proposed design using composite wing structure with a flammability
reduction system for all fuel tanks.

The following policy was developed to promote a standardized approach to
these special conditions and exemptions.

# Policy 

The FAA now has a better understanding of the flammability exposure
level of unheated aluminum wing tanks and the performance and
availability of systems providing flammability reduction means. As
evidenced by the issuance of the FTFR rule and as discussed in the
preamble for that rule, the FAA has now determined that there is at
least one practical method available to significantly reduce risk due to
fuel tank flammability. In addition, the FAA has now determined that
application of § 25.981(a)(3) to fuel tank structural lightning
protection can be impractical for certain areas of structural design,
and is therefore inappropriate for design features where the applicant
shows compliance is impractical. Further rulemaking to revise § 25.981
to address this issue is under consideration. The FAA will seek public
comment on any rulemaking through the normal rulemaking process.

In the interim, until any such rulemaking is effective, the FAA will
propose special conditions or grant exemptions with conditions to define
appropriate airworthiness requirements for fuel tank systems and changes
to fuel tank systems. For design changes where it is determined to be
appropriate, the FAA will allow an exception to apply § 25.981(a) at an
earlier amendment level under the provisions of § 21.101. Specific
policy regarding the application of § 21.101 for type design changes
involving fuel tank structure is discussed below.

Because of the impracticality issues discussed above, the FAA is also
considering proposals to depart from certain aspects of the policy
currently contained in AC 25.981-1C for structural lightning protection.
That AC calls for applicants to assume the probability of lightning and
the probability of flammability are equal to 1. However, unlike what was
intended under that AC, the safety standards imposed via these special
conditions and exemptions are stated in terms of likelihood of a fuel
tank vapor ignition event, and not the likelihood of an ignition source
alone. In order for a vapor ignition event to occur due to a structural
failure (or combination of structural failures), that failure must be
combined with a critical lightning attachment and with flammable
conditions in the fuel tank.

Therefore, for practicality reasons, the FAA has determined that it is
acceptable in this case to treat the required analysis as similar to a
multiple failure probabilistic analysis performed under § 25.1309(b).
Specifically, when complying with the proposed special conditions and
exemptions described below, the FAA considers it acceptable to consider
the probability of lightning attachment, the distribution of lightning
strike energy, and the probability of fuel tank flammability when
performing the required safety analysis. The FAA has not yet approved
specific methods to quantify the probability of lightning attachment or
the distribution of lightning strike energy. Therefore, specific
agreements on the quantification of these factors should be coordinated
with the Transport Airplane Directorate (TAD) through the issue paper
process. The FAA has adopted part 25 Appendix N as the required method
to quantify fuel tank flammability when showing compliance to the
flammability exposure requirements of Amendment 25-125. Alternative
methods to quantify fuel tank flammability may also be acceptable for
the purpose of performing the safety analyses required by special
conditions or exemptions issued under this policy. Any such alternative
methods should be coordinated with the Transport Airplane Directorate
(TAD) through the issue paper process. This specific policy should not
be interpreted as changing other current FAA policy regarding use of the
probability of environmental or operational conditions in performing
safety analyses.

[Eligibility for Consideration Under This Policy]{.underline}

The relief from § 25.981(a)(3) provided by this policy is intended to be
limited to areas of fuel tank structure for which compliance with
§ 25.981(a)(3) is shown by the applicant and determined by the FAA to be
impractical. General design areas for which the TAD has determined
compliance with § 25.981(a)(3) can be impractical include structural
members and joints, fasteners, coatings, and sealants. Determinations of
practicality are somewhat subjective and can be dependent on the
proposed design. Practicality determinations that are outside of the
examples provided below should be submitted to the TAD for review to
ensure standardization.

[Examples of FAA Practicality Determinations]{.underline}

Examples of design changes or features that have been determined to be
practical include:

-   Installation of arc/spark-free rivets and bolts in aluminum
    structure through strict control of clearance fits and installation
    practices;

-   Installation of arc/spark-free bolts in composite structure through
    strict control of bolt/hole fit and quality, and with additional
    design features to distribute current such as foil or mesh at the
    material surface;

-   Installation of sealant or cap seals over fastener heads/ends
    located inside fuel tanks to provide fault tolerance.

Examples of design areas and/or failure modes for which it has been
determined that it is impractical to provide fault tolerance include:

-   Fatigue cracking within structural elements such as spars, skins,
    stringers, and ribs;

-   Failure of fasteners highly loaded in tension that lead to
    separation of the fastener or part of the fastener from the hole or
    gapping of the fastener head or nut, and consequent failure of a cap
    seal;

-   Failures of sealant where the sealant is the primary feature needed
    to prevent an ignition source in the event of lightning attachment.

An example of a design change or feature to provide fault tolerance that
has been determined to be impractical:

-   Installation of double cap seals or structurally reinforced cap
    seals to retain a bolt that fails under tension.

Examples of design, manufacturing, and maintenance program features that
have been determined to be practical include:

-   A structured design review process to ensure that all relevant
    design features are reviewed to identify critical design areas,
    critical processes, and associated testing and analysis
    requirements;

-   Engineering review of the proposed design to identify failure modes
    that may occur due to manufacturing variability (including errors or
    escapes), maintenance errors, repairs or alterations, aging, wear,
    corrosion, or likely damage;

-   Engineering review of manufacturing processes to identify failure
    modes that may occur due to manufacturing variability (including
    errors or escapes);

-   Engineering review of service history records to identify failure
    modes that may occur due to manufacturing variability (including
    errors or escapes), maintenance errors, repairs or alterations,
    aging, wear, corrosion, or likely damage;

-   Provisions in instructions for continued airworthiness to identify
    critical design configuration control limitations (CDCCL), to
    require life limits or repetitive inspections for specific features
    to detect anticipated failures, and to perform maintenance on
    critical areas.

[Special Conditions]{.underline}

The FAA will propose special conditions under the provisions of § 21.16
to address lightning protection of fuel tank structure for fuel tank
systems with flammability reduction means that meet or exceed the
performance standards set in Appendix M to part 25 as added at Amendment
25-125 for all fuel tanks on the airplane. Since compliance with §
25.981(b) at Amendment 25-125 only requires compliance with the
performance requirements of Appendix M for normally emptied fuel tanks
within the fuselage contour, a flammability reduction system that
complies with the Appendix M requirements for all fuel tanks is
considered to be an unusual feature.

In addition, the FAA considers the requirements in § 25.981(a)(3) to be
inappropriate for fuel tank structural lightning protection features
where application of those requirements is shown to be impractical. The
FAA also finds that an equivalent level of safety can be achieved
through less stringent ignition source prevention requirements.
Specifically, meeting the Appendix M standard for all fuel tanks would
reduce fuel tank flammability significantly below the maximum level that
would otherwise apply to the main tanks on airplane designs that are
required to meet Amendment 25-125. This additional risk reduction would
be considered a compensating feature that offsets some relaxation of the
requirements contained in § 25.981(a)(3).

When special conditions in lieu of § 25.981(a)(3) are determined to be
appropriate, they will typically include requirements addressing the
following areas. The FAA has determined that these alternative
requirements would be applicable and sufficient for most designs.

1.  The airplane design must meet or exceed the requirements of Appendix
    M of part 25 at Amendment 25-125 for all fuel tanks installed on the
    airplane.

2.  A safety analysis must show that

    a.  catastrophic fuel tank vapor ignition in the fuel tank system
        due to lightning is extremely improbable,

    b.  the risk of catastrophic fuel tank vapor ignition during
        operation with the flammability reduction means (FRM)
        inoperative, and with any other system whose failure can
        increase flammability inoperative, is on the order of 1x10^-8^
        events per flight hour or less, and

    c.  for each failure mode of each particular type of design feature
        (such as a particular fastener or joint type), the occurrence of
        an ignition source due to each failure mode of the feature is
        extremely remote, assuming all instances of that feature (such
        as the particular fastener type) have failed in that mode.

3.  The applicant must show that the design, manufacturing, and
    maintenance programs include all practical measures to prevent
    failures of structural lightning protection features due to
    manufacturing variability, aging, wear, corrosion, and likely
    damage. Any residual risk from the effects of manufacturing
    variability, aging, wear, corrosion, and likely damage must be
    accounted for in the safety analysis described in paragraph 2 above.

Paragraph 2a above requires a structured, quantitative assessment of
fleet average risk for a fuel tank vapor ignition event. It is
acceptable to consider the probability of fuel tank flammability, the
probability of lightning, and a distribution of lightning waveforms in
performing the assessment provided the associated assumptions are
acceptable to the FAA. The assessment must account for operation with
inoperative features and systems, including any proposed dispatch
relief.

Paragraph 2b requires an analysis similar to paragraph 2a, except that
it is an assessment of fleet average risk during operation with any
flammability reduction system inoperative.

Paragraph 2c is intended to limit the risk that could arise from a
design error or manufacturing defect that leads to many similar failures
existing on an individual airplane due to one cause.

[Exemptions for New Type Certificate Programs]{.underline}

For new type certificate programs, the FAA will consider granting
exemptions from § 25.981(a)(3) for lightning protection of fuel tank
structure on airplanes with fuel tank systems that do not comply with
that regulation, but which do comply with the applicable fuel tank
flammability performance standards of § 25.981(b) at Amendment 25-125.
To grant such an exemption, the FAA must find that the approval of the
proposed design would provide an acceptable level of safety, and would
be in the public interest.

The FAA does not expect to be able to make the findings required for an
exemption from § 25.981(a)(3) for a new aircraft type that does not meet
the requirements of § 25.981(b) at Amendment 25-125. Therefore, it is
not envisioned that the FAA would grant exemptions to both §§
25.981(a)(3) and 25.981(b) at Amendment 25-125 for a new type
certificate program.

*Title 14 CFR 11.81 lists the information that must be included in a
petition for exemption. A petitioner seeking an exemption under this
policy must include in their petition all of the information required by
that regulation*. In providing that information, petitioners should
address the following:

1.  In responding to § 11.81(c), petitioners seeking an exemption under
    this policy should refer to this policy memo as acknowledging that
    compliance with § 25.981(a)(3) is impractical for some areas of
    structural lightning protection design. The petitioner should
    identify the specific design features for which an exemption is
    sought. The petitioner should show that all practical measures have
    been taken to meet the requirements of § 25.981(a)(3) for the fuel
    tank structure. For the design features for which an exemption is
    sought, the petitioner should show what potentially compliant design
    changes were examined, and what design changes were ruled out based
    on impracticality.

2.  In responding to § 11.81(e), a petitioner seeking an exemption under
    this policy should address each of the safety requirements listed
    below and reference this policy memo as acknowledging that an
    acceptable level of safety is expected to be provided if those
    safety requirements are met. In addition, the petitioner should
    include any other information they believe supports a finding that
    an acceptable level of safety will be provided.

If the FAA determines that it is in the public interest to grant an
exemption, the FAA would typically apply the following ignition source
prevention conditions to ensure that an acceptable level of safety is
provided. The FAA has determined that these alternative requirements
would be applicable and sufficient for most designs.

1.  In lieu of compliance with the requirements of § 25.981(a)(3), the
    applicant must show that the design includes at least two
    independent, effective, and reliable lightning protection features
    (or sets of features) such that fault tolerance is provided for each
    area of the structural design area proposed to be exempt from the
    requirements of that regulation. Fault tolerance is not required for
    any specific design feature if:

    a.  providing fault tolerance is shown to be impractical for that
        feature, and

    b.  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.

2.  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.

[Exemptions for Certain Type Design Change Programs on Amendment 25-102
Airplanes]{.underline}

For type design change programs on Amendment 25-102 airplanes, the FAA
will consider granting exemptions from § 25.981(a)(3) as described above
for new type certificates. The same ignition source prevention
conditions described above will be applied. For pre-Amendment 25-125
airplanes, the fuel tank flammability standards for the design change
will be those required under the applicable provisions of §§ 26.33 and
26.35 at Amendment 26-2.

[Use of Exceptions Under § 21.101 for Type Design Change Programs on
Pre-Amendment 25-102 Airplanes]{.underline}

For type design changes on pre-Amendment 25-102 airplanes that are
classified as "significant product level changes" under § 21.101, the
FAA will consider allowing applicants to show compliance with an earlier
amendment level of § 25.981(a) for lightning protection of fuel tank
structure, under the provisions of § 21.101. The FAA determination will
be based on the potential impact on fuel system safety that would result
from application of Amendment 25-102. If the change would not result in
a significant reduction in the risk of a lightning related fuel tank
vapor ignition event were it required to comply with § 25.981(a)(3) at
Amendment 25-102, the "would not materially contribute to the level of
safety of the changed product" exception provision of § 21.101(b)(3)
will be used to allow application of an earlier amendment of
§ 25.981(a). For example, the FAA has found that compliance with an
earlier amendment was acceptable for projects involving the installation
of winglets, in which a limited number of stringers and fasteners were
added within the fuel tank and for which all new fasteners were
cap-sealed to provide fault tolerance. Any exception would be limited to
aspects of lightning protection of fuel tank structure where no material
benefit from compliance is shown.

[Exemptions for Type Design Change Programs on Pre-Amendment 25-102
Airplanes]{.underline}

For type design change programs that include § 25.981(a)(3) at Amendment
25-102 as a result of application of the certification basis upgrade
provisions of § 21.101, and for which an exception under § 21.101 is not
appropriate, the FAA will consider granting exemptions from §
25.981(a)(3). For example, for a design change that adds wing reserve
tanks to an airplane, or that redesigns a significant portion of the
wing structure, compliance with § 25.981(a)(3) at Amendment 25-102 would
have a material impact on fuel system safety. In such cases, the
exemption conditions described above will be applied to set appropriate
standards for that potential additional risk. For pre-Amendment 25-125
airplanes, the fuel tank flammability standards for the design change
will be those required under the applicable provisions of §§ 26.33 and
26.35 at Amendment 26-2.

[Methods of Compliance]{.underline}

Specific requirements and methods of compliance to address the above
considerations will be developed for each specific design via the
applicable special conditions or exemption and via the issue paper
process until the rulemaking discussed above is completed.

[Applicability of § 25.954]{.underline}

Applicants should note that the airplane design must comply with the
provisions of § 25.954. The policy discussed in this memorandum is not
intended to affect or eliminate the requirement to comply with that
regulation.

# Effect of Policy

The FAA individual implementing this policy should follow this policy
when it is applicable to a specific project. If the implementing office
becomes aware of reasons an applicant's use of this policy should not be
approved, the office must coordinate its response with the policy
issuing office.

Applicants should expect that certificating officials would consider
this information when developing certification options relevant to new
certificate actions.

# Implementation

This policy discusses administrative design approval options available
to applicants for type certificate, amended type certificate,
supplemental type certificate, and amended supplemental type
certification programs. These administrative compliance options are
currently available and will continue to be available until regulatory
changes preclude the need for these administrative design approval
options.

Attachment: Definition of Key Terms

**Distribution List**:

Manager, Los Angeles Aircraft Certification Office, ANM-100L

Manager, Denver Aircraft Certification Office, ANM-100D

Manager, Seattle Aircraft Certification Office, ANM-100S

Manager, Anchorage Aircraft Certification Office, ACE-115N

Manager, Wichita Aircraft Certification Office, ACE-115W

Manager, Chicago Aircraft Certification Office, ACE-115C

Manager, Atlanta Aircraft Certification Office, ACE-115A

Manager, Ft. Worth Airplane Certification Office, ASW-150

Manager, Ft. Worth Special Certification Office, ASW-190

Manager, New York Aircraft Certification Office, ANE-170

Manager, Boston Aircraft Certification Office, ANE-150

Manager, International Branch, ANM-116

Manager, Brussels Aircraft Certification Staff, AEU-100

Manager, Standardization Branch, ANM-113

Manager, Rotorcraft Directorate Standards Staff, ASW-110

Manager, Small Airplane Directorate Standards Office, ACE-110

Manager, Engine and Propeller Directorate Standards Staff, ANE-110

Manager, Military Certification Office, ACE-100M

> **Definition of Key Terms**
>
> Table A-1 defines the use of key terms in this policy statement. The
> table describes the intended functional impact, and the formatting
> used to highlight these items.

-   The term "must" refers to a regulatory requirement that is mandatory
    for design approval. Text communicating a requirement is in
    *italics*.

-   The term "should" refers to instructions for a particular method of
    compliance. If an applicant wants to deviate from these
    instructions, he has to coordinate the alternate method of
    compliance with the Transport Standards Staff using an issue paper.
    There is no special text formatting used for methods of compliance.

-   The term "recommend" refers to a recommended practice that is
    optional. Enclose recommendations in \[ \] brackets.

Table A-1 Definition of Key Terms

+----------+-----------------+----------------------+-----------------+
|          | **Regulatory    | **Acceptable Methods | **Re            |
|          | Requirements**  | of Compliance**      | commendations** |
+----------+-----------------+----------------------+-----------------+
| #        | Must            | Should               | Recommend       |
| Language |                 |                      |                 |
+----------+-----------------+----------------------+-----------------+
| **       | *Italics*       | Regular text (No     | \[Square        |
| Format** |                 | special formatting)  | brackets\]      |
+----------+-----------------+----------------------+-----------------+
| **Fu     | No Design       | Alternative has to   | None, because   |
| nctional | Approval if not | be approved by issue | it is optional  |
| Impact** | met             | paper.               |                 |
+----------+-----------------+----------------------+-----------------+

> Examples from policy on Power Supply Systems for Portable Electronic
> Devices (PSS for PED):

-   *Even though PSS for PED systems may use wiring that is produced for
    the consumer market, the wiring must meet the flammability
    requirements of § 25.869.*

-   Although multiple power control switches may be used (e.g., zonal
    control of system power), there should be a single master switch
    that allows for the immediate removal of power to the entire PSS for
    PED

-   \[We recommend that you provide a means of indication to enable the
    cabin crew to determine which outlets are in use or which outlets
    are available for use.\]