RA-14-05714 Enclosure

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Petition for Exemption from 14 CFR 25.981 (a)(3) Amendment 25-125

Name and Address of the Petitioner

Douglas M. Lane, Director, Commercial Airplanes

The Boeing Company

PO Box 3707

Seattle, Washington, 98124

Section(s) of the Regulations from which Relief is Sought:

Boeing is petitioning for an exemption to 14 CFR 25.981(a)(3) for fuel tank structural lightning

protection for the 737-8 (Project Numbers PS12-0038). The petition is also requested for the 

737-7 (Project Number PS12-0037) and 737-9 (Project Number PS12-0039). The references to 

the 737 MAX in this document refer to the 737-7, 737-8 and 737-9. The subject regulation 

section is unchanged between Amendment level 102 and 125. The regulation as currently

written:

(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.

The Extent of Relief Sought and Reason(s)

Regulation Requires: Relief is Necessary because:

14 CFR

25.981(a)(3)

Amendment

25-125

Compliance to 14 CFR

25.981(a)(3) for fuel 

tank structural lightning 

protection would require

dual fault tolerant or 

three independent layers 

of protection.

Boeing has concluded that for fuel tank fasteners

and structural joints, there are no practical means 

in current technology to provide three means of

protection in all locations.

Boeing has considered changes which might 

provide additional mitigation, but found these 

means:

a) Still do not provide a means of direct 

compliance to 25.981(a)(3),

b) Are unnecessary to ensure safety of the fleet 

c) Would incur a significant cost (e.g. fuel burn 

due to increased weight) to the public.

d) Do not provide a commensurate safety benefit
Petition for Exemption from § 25.981(a)(3)

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Description of Issue

Boeing is petitioning for an exemption to 14 CFR 25.981(a)(3) for fuel tank structural 

lightning protection for the 737 MAX. The PS-ANM-25.981-02 FAA policy 

memorandum provides the following alternate requirements:

(1) The fuel tank structure and systems must be designed and installed to prevent catastrophic 

fuel vapor ignition due to lightning. 

(2) The fuel tank structure and systems lightning protection design must be fault tolerant for 

failures that result in lightning-related ignition sources. 

(3) Fault tolerance is not required for any specific design feature if: 

(a) Fault tolerance is shown to be impractical for that feature, and 

(b) Fuel tank vapor ignition because of that feature and all other non-fault-tolerant 

features, when their fuel tank vapor ignition event probabilities are combined, is 

shown to be extremely improbable. 

Note: FAA Policy Memo PS-ANM-25.981-02 defines “Practicality” is a balance of 

available means, economic viability, and proportional benefit to safety.

(4) Inspections or other procedures must be established to prevent development of lightningrelated ignition sources within the fuel tank structure and systems, for example: 

(a) Identifying as airworthiness limitations, mandatory maintenance actions (i.e., 

inspections), or CDCCLs, necessary to preclude the development of unsafe 

conditions due to non-fault-tolerant lightning protection features;

(b) Including sampling programs, maintenance, and/or inspections for fault-tolerant 

lightning protection features in the manufacturer’s recommended airplane 

maintenance program; 

Note: If inspections from non-mandatory programs such as Baseline Zonal inspection 

program, Corrosion Prevention and Control Program (CPCP), etc., are going to be 

used to support the robustness of the overall inspection program, these programs must 

become mandatory and must be included in the Airworthiness Limitations section of 

the airplane’s Instructions for Continued Airworthiness. 

(c) Incorporating into applicable airplane maintenance documents, including the 

manufacturer’s structural repair manual, caution information that identifies the 

lightning protection features of the fuel system design to minimize the potential for 

inadvertent damage or disruption of these features. 

(5) An analysis must be performed to show that the airplane’s design, its manufacturing 

processes, and the Airworthiness Limitations section of its Instructions for Continued 

Airworthiness include all practical measures to prevent, and detect and correct, failures of 

fuel tank structure and systems lightning protection features because of manufacturing 

variability, aging, wear, corrosion, and likely damage. 
Petition for Exemption from § 25.981(a)(3)

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Discussion

Boeing has concluded that there are no practical means in current technology to obtain 

triple redundant means of fuel tank structural fastener lightning protection in all locations. 

For example, one impractical option would be to apply inspection interval which 

monitors the health of each fastener that penetrates the fuel tanks. With thousands of

fasteners on the wing of the 737 MAX, and the inability to visually inspect most means 

of protection, this option is impractical. Increasing the number of in-tank inspection 

intervals could also have a potential unintended consequence of damaging the ignition

protective features or other components inside the fuel tanks.

Examples of additional design considerations, found to be impractical were:

• Conductive steel fasteners: Widespread use results in higher weight, fuel burn and 

associated emissions, and higher risk of corrosion. Design with all steel fasteners is 

still not dual fault tolerant.

• Additional sealant on bolt locations already sealed: Reduces structural inspectability

for cracks and corrosion, potentially reducing overall fleet safety. Would result in 

additional weight, fuel burn and emissions.

• Composite skin protection methods: Not applicable to aluminum design. Note the

majority of these methods are intended to increase conductivity of the outer tank 

surface in the area local to the fastener heads, which aluminum structure provides 

inherently.

• Fuel tank bladders or double walled fuel tanks: Would result in a new enclosed fuel 

vapor space, which would pose an additional vapor ignition threat during a lightning

attachment and thus does not increase safety. This would also increase weight which 

increases fuel burn and emissions, plus reduces usable fuel tank volume which 

reduces range. The bladders also inhibit structural inspectability inside the tank.

• Spot Bonds: Spot bond is a local bond in which the primer is removed at the internal 

fastener interface with structure. Removing primer increases corrosion risk, 

detracting from any benefit of increased conductivity offered by the spot bond. The

spot bonds would still be subject to faults and does not result in a dual fault tolerant 

design.

• Fastener Free Wing Structure: Structure that does not have any fasteners would not 

be susceptible to lightning protection faults associated with fasteners but is not 

necessarily dual fault tolerant. It is not possible to produce raw materials large

enough to create monolithic wing panels or spars in dimensions that the 737 MAX

requires. Also, monolithic designs would reduce structural crack tolerance. Using

adhesives instead of fasteners to bond major components represents a radical 

departure from established metallic wing primary structure experience.

These types of designs were determined to be impractical because some are not possible 

within the current state of the art, others introduce more risk than mitigated, and because

the design is safe without them. 
Petition for Exemption from § 25.981(a)(3)

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In addition, adding sealant over 1⁄4” and larger rivets was determined to be impractical

due to lack of proportional safety benefit, increased weight, increased fuel burn, increased 

emissions, and reduced inspectability during maintenance. Boeing has assessed the 

likelihood of lightning attachment to a faulted rivet during flammable tank conditions, 

and concluded this combination is not expected to occur in the life of the fleet. 

Regardless of this probabilistic conclusion, Boeing has performed development testing 

(will repeat for compliance) of rivet installations representative of the 737 design and 

demonstrated that even with faults present, the installations will not cause an ignition 

source during severe direct lightning attachments. Since the design is already safe during 

this combination of conditions that are unlikely to occur in the life of the fleet, Boeing 

concluded that further mitigation (i.e. covering 1⁄4” and larger rivets with sealant) would 

not provide a commensurate safety benefit to the public considering the significant 

weight and associated fuel costs plus carbon emissions, aircraft production costs and 

operator maintenance cost impacts. Additional detailed proprietary information has been 

provided to the FAA via Boeing letter RA-14-05275.

The only faults currently identified for which the design is not fault tolerant are:

1) The potential for a latent crack in structure, or

2) A bolt failure that also causes the fastener nut (or collar) to release with sufficient 

force to tear free the associated cap seal.

Both of these failure modes are identified in the FAA Policy Memo as typical situations 

where applicants have been unable to identify practical means for fault tolerance. In each 

of these cases, sparking due to these failures would only occur if there was a direct 

attachment to the local area of the failure. An assessment will be provided demonstrating

that the risk of a fuel vapor ignition due to the sum of these failures is not anticipated in 

the life of the fleet. 

Proposed Risk Mitigation

As discussed in the FAA Policy Memo, Boeing will submit detailed proprietary

information to the FAA about the measures taken in the 737 MAX design to provide

practical structural lightning protection. In general, the 737 MAX lightning protection 

features are based on state of the art industry design practices for aluminum wing

structure including inherent current paths that have been used in existing in service

designs. Further, Boeing proposes to enhance the lightning protection by incorporating

fault tolerant protection means where practical, thereby increasing the level of fuel tank 

safety. 

Boeing will comply with 14 CFR 25.981(b) at Amendment 25-125. A flammability 

exposure assessment will be provided showing that the 737 MAX main tanks are low

flammability exposure tanks equivalent to a conventional unheated aluminum main wing. 

In addition, the center wing tank of the 737 MAX will utilize an inerting system designed 

to meet the flammability exposure criteria of Part 25 Appendix M. The aspects of NGS 

Performance with respect to compliance to 14 CFR 25.981 are covered in the 737 MAX 

Fuel System Installation Certification Plan.
Petition for Exemption from § 25.981(a)(3)

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Boeing will provide the following with respect to each of the 5 policy requirements: 

Policy Requirement 1 - Prevent catastrophic fuel vapor ignition due to 

lightning:

In summary, Boeing will implement in the design all practical measures for fault 

tolerant structural lightning protection. Boeing will utilize manufacturing 

processes to minimize the risk of errors in aircraft manufacturing which could 

affect the lightning protection features. Structural Repair Manual procedures will 

be provided to retain the lightning protection features throughout the life of the

airplane. Instructions for Continued Airworthiness including CDCCLs will be

developed and included in the certification documentation. A complete analysis

of the manufacturing process and repair procedure will be part of the Fuel System 

Lightning Protection System Safety Assessment.

Policy Requirement 2 – Provide fault tolerant lightning protection:

Boeing will show that where practical the structural fastener and joint designs for 

the model 737 MAX wing will provide fault tolerant lightning protection. A 

detailed Failure Mode and Affects Analysis (FMEA) including a qualitative 

assessment of all known and hypothetical failure modes will be provided as part 

of the compliance documentation. Analysis, design review, and use of existing 

test data will be the primary means to demonstrate fault tolerance for both direct 

and conducted current lightning threats. Where analysis is insufficient or test data 

is not available, additional testing will be performed. The systems supporting 

structure will also be analyzed as part of the safety assessment to ensure that they 

meet the requirements of the policy statement. 

Testing will show that the fastener installations will not produce any sparks inside 

the fuel tanks during exposure to threats for the applicable lightning zone 

requirements. For example, during previous programs Boeing has shown by test 

that most structural joints and some fasteners provide inherent fault tolerant 

protection. In addition, Boeing has shown by test that a cap seal, as a second 

layer of protection, will contain sparking where a fastener or joint cannot 

demonstrate inherent fault tolerant protection. 

Policy Requirement 3 – Demonstrate safety of limited non fault tolerant 

conditions:

For the limited areas in which a single or cascading failure could result in loss of 

all protection features, a safety assessment will be done to show that the summed 

risk of all non fault tolerant lightning related fuel tank ignition threats is not 

anticipated to occur during the entire operational life of all 737 MAX airplanes. 

Service experience on the existing Boeing aircraft commercial fleet (including the 

737 fleet) shows each of these failures to be rare. This assessment is similar to 

the assessment for the 747-8 aircraft (ref: Exemption 10174), where analysis 

demonstrated that the probability of a lightning related fuel tank ignition is 

extremely improbable (and thus unlikely to occur in the life of the fleet). The 737 

MAX wing is a conventional aluminum wing, much smaller in size relative to 747-

8/8F and uses the same fastener types and installation procedures and design 

standards common to Boeing airplanes. A failure rate based on in-service data for 

fasteners on Boeing airplanes will be used. The expected rate of occurrence of 

structural cracking will be conservatively based on Boeing’s structural fatigue 
Petition for Exemption from § 25.981(a)(3)

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methodology. This methodology is based on test and service experience, and 

ensures that the presence of fatigue cracks is minimized during the life of the 

airplane. 

Currently Boeing has identified two non-fault tolerant failure modes (broken bolts 

and structural cracks), however analysis and/or testing is not complete thus 

additional non-fault tolerant installations may be indentified which may also be 

impractical to preclude. All areas where fault tolerance is impractical will be 

indentified in the Fuel System Lightning Protection System Safety Assessment.

Policy Requirement 4 – Provide lightning protection maintenance 

information:

Non fault tolerant features: No ALI or mandatory inspections for structural 

lightning protection related to non-fault tolerant features (e.g. broken bolts and 

cracks) are anticipated, however assessment is not complete; they will be included 

if necessary. Existing mandatory structural inspections are adequate to ensure 

related failures will not exceed failure rates used in risk analyses.

Fault tolerant features: As recently added to the 737 AWLs, a CDCCL will 

require an inspection of sealant per an Airplane Maintenance Manual Fuel Tank 

Closure procedure any time the tank area is exited after maintenance or alteration. 

No additional maintenance or inspections for structural lightning protection are 

anticipated, however assessment is not complete; Additional maintenance 

requirements will be included if necessary.

Repairs: The Structural Repair Manual will include instructions to minimize the 

risk that alterations (repairs) will affect lightning protection. The SRM will 

include CDCCL references (equivalent to Cautions). Also note that as discussed 

above, a CDCCL will identify an Airplane Maintenance Manual procedure to 

perform a visual sealant inspection before exiting any tank area where 

maintenance, inspection or repair has been performed.

Policy Requirement 5 - Analysis of design, manufacturing and maintenance 

procedures:

Boeing has implemented in the design all practical measures for structural 

lightning protection. The manufacturing processes will include practical means to 

minimize the risk that lightning protection features are not implemented as 

intended on the drawings. As noted for requirement 4, the continued airworthiness 

program will include appropriate CDCCL and the SRM repair procedures will be 

established to retain the lightning protection features throughout the life of the 

airplane. A complete analysis of the manufacturing process and repair procedure 

will be part of the Fuel System Lightning Protection System Safety Assessment. 

Additional detailed proprietary information has been provided to the FAA via 

Boeing letter RA-14-05275.
Petition for Exemption from § 25.981(a)(3)

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Statement of Public Interest

Boeing is working to improve the efficiency of the 737 model by a redesign. The 737 

MAX has an all new engine for improved fuel burn and reduced community noise over 

the 737-800 model. For lightning protection, the 737 MAX has additional features to 

enhance safety and protect the fuel tanks from potential ignition sources due to a 

lightning event. All practical measures have been utilized. Additional features to 

directly meet the rule are impractical and would potentially add significant cost, weight, 

increased emissions, and maintenance to the airplane without a proportional safety 

benefit. Further, additional lightning protection features could inhibit the ability to 

inspect the structure (reducing safety).

Requiring the 737 MAX to attempt to directly comply with this rule would significantly 

inhibit Boeing’s ability to design and certify the airplane on a competitive schedule. It 

would also render the 737 MAX to be of lesser value to the public interest with no 

commensurate increase in safety. Many Boeing customers are depending on the 737 

MAX to meet their business needs. The new 737 MAX airplane will typically replace

older, less fuel efficient models, reducing the public expense for fuel consumed in air 

travel and freight delivery. Therefore, Boeing believes that it is in the public interest for 

the FAA to grant this exemption.

Statement of No Adverse Effect on Safety

With current design practices for lightning protection, Boeing aircraft designs have

accumulated in excess of 800 million revenue service flight hours with no lightning

related events. Prior to these designs and service hours, there were 2 commercial jet 

transport hull loss events which are believed to have been caused by lightning strikes 

(reference ARAC Report: 1998 ARAC Fuel tank Harmonization Working Group report,

Task Group 1 Service History / Fuel Tank Safety Level Assessment). The first was in 

1963, which led to the development of lightning protection standards for fuel tanks. The

second and last event was in 1976 and also resulted in improvements in fuel tank 

lightning protection. The hours accumulated since the last event represent over 8 times 

the hours accumulated with earlier designs through 1976. The accident free fleet history

since 1976 shows current Boeing wing designs have sufficient protection against 

lightning induced fuel tank ignition events.

This safety record for the fuel system designs upon which the enhanced Boeing 737 

MAX design is based, indicates that the proposed fuel tank lightning protection design of

the model 737 MAX will be safe.

Request to ‘Waive Publication and Comment’

Boeing requests to waive the requirement for publication and comment. Boeing believes 

there is good cause to be granted a waiver due to the fact that the FAA previously found a

grant of exemption was in the public interest for the same fuel tank lightning protection 

compliance to CFR 25.981a(3) issue on the 747-8/8F (BDCO Project No PS05-0212 & 

PS05-0211) (Exemption No. 10174 was granted).
Petition for Exemption from § 25.981(a)(3)

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Privileges of the Exemption Outside the United States

Per 14 CFR 11.81(h), Boeing requests that the privileges of this exemption be extended 

outside the United States. This extension of privileges is necessary for operations based 

within foreign countries having bilateral agreements with the United States accepting 

FAA 14 CFR part 25 as their airworthiness standards for transport category aircraft. The

737 MAX is intended for the global market place.

Conclusion

Boeing is petitioning for an exemption from 14 CFR 25.981(a)(3) at amendment 125 for

fuel-tank structural lightning protection. Granting this exemption will allow Boeing to 

complete its design and certify the 737 MAX for delivery. The proposed design for the

737 MAX will provide an appropriate level of safety against lightning-induced fuel-tank 

ignition, and will be shown to comply with the alternative requirements provided in the

FAA policy memo PS-ANM-25.981-02.
Petition for Exemption from § 25.981(a)(3)

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Summary to be Published in the Federal Register

PETITIONER: The Boeing Company

SECTION OF 14 CFR AFFECTED: 14 CFR 25.981(a)(3)

DESCRIPTION OF RELIEF SOUGHT: The petitioner seeks relief from the

requirements of fuel tank structural lightning protection for the Boeing 737-7/-8/-9 model 

airplane.