Petition to Amend Exemption No. 10174 from 14 CFR 25.981 (a)(3) Amendment 25-102

Name and Address of the Petitioner

Doug Lane, Director, Regulatory Administration Deputy Lead Administrator 

The Boeing Company 

PO Box 3707 

Seattle, Washington, 98124 

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

Boeing is petitioning to amend Exemption Number 10174 to 14 CFR 25.981(a)(3) at Amendment 

102 for fuel tank structural lightning protection for the 747-8/-8F through Certification Project 

Number PS14-1034 (High Performance Program (OZARK) – Wingbox – Revision) based on 

recently released FAA policy PS-ANM-25.981-02. Exemption Number 10174 was applicable 

to the wing tanks on both the 747-8 and 747-8F (PS05-0211 and PS05-0212 respectively). The 

regulation (unchanged between Amendment level 102 and 120) as currently written requires: 

(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 25-102 

Note: Boeing is 

requesting 

exemption at this 

amendment levels to 

cover changes that 

affect lightning 

protection, which 

might require 

compliance to either 

amendment level. 

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. 

As previously stated in Exemption 10174, 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

Enclosure to RA-15-01067 

Page 1 of 8
Description of Issue

Boeing is petitioning to amend Exemption No. 10174 to 14 CFR 25.981(a)(3) for fuel 

tank structural lightning protection for the 747-8 and 747-8F to remove the requirement 

of cap sealing wing tank rivets in Lightning Zone 2 (1/4” diameter and larger) based on 

new FAA Policy. 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” as 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. 

Enclosure to RA-15-01067 

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The 2009 Policy Memo AMM-112-08-002 did not provide a specific definition of 

“practical” or “impractical”, however the policy stated: “Determinations of practicality 

are somewhat subjective and can be dependent on the proposed design and the “state of 

the art” at the time.” The 2014 PS-ANM-25.981-02 policy statement provides a specific 

statement that defines “practicality” as a balance of available means, economic viability, 

and proportional benefit to safety. This definition is consistent with the May 2011 

recommendations of the Aviation Rulemaking Committee that the FAA formed after 

release of the initial policy memo release. 

Discussion

As stated in Exemption 10174, Boeing concluded and the FAA concurred 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 option deemed 

to be impractical was to apply an inspection interval which monitors the health of each 

fastener that penetrates the fuel tanks. This was due to the thousands of fasteners on the 

wing of the 747-8/-8F and the inability to visually inspect most means of lightning 

protection. 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. As discussed in Exemption 10174, examples of 

additional design considerations found to be impractical were: conductive steel fasteners, 

additional sealant on locations already sealed, composite skin protection methods, fuel 

tank bladders or double walled fuel tanks, spot bonds, and fastener free wing structure. 

During the initial certification of the 747-8/8F, while it was recognized that cap sealing of 

wing tank rivets (1/4” diameter) may have been unnecessary to provide a fault tolerant 

design, the 2008 policy statement required use of “all practical measures” and equated 

practicality with “state of the art”. Because cap sealing was within the state of the art, it 

was implemented for rivets in Zone 2 areas without consideration of the safety benefit to 

the airplanes, whether it was necessary for fault tolerance, and the impact upon aircraft 

customers and the flying public of the significant weight added to each aircraft. 

Boeing has since done testing to evaluate the safety benefit of sealing 1⁄4” rivets in fuel 

tank Zone 2 areas. It has been determined by analysis and testing of all anticipated faults 

that cap sealing 1⁄4” rivets (and larger) in skin thicknesses used on the 747-8/8F is 

unnecessary to ensure safe operation during lightning strike conditions, even when faults 

are present. The use of sealant on rivets in Zone 2 of the 747-8/-8F wing, results in 

additional weight to the aircraft. This in turn increases fuel burn, carbon emissions, and 

reduces inspectability during maintenance. Further, 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, testing (will repeat for compliance) demonstrates that even with 

faults present, the 1⁄4” (and larger) 747-8/8F rivet 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 

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

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. 

Enclosure to RA-15-01067 

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Note that cap sealing of bolt installations in Zone 2 areas is practical and used for sealing 

bolts to ensure fault tolerant protection. 

Finally, this impractical conclusion is only with respect to providing a design that is dual

fault tolerant. The basic requirement to provide single fault tolerance as defined in Policy 

Statement PS-ANM-25.981-02, can be met without cap sealing rivets. As discussed in the 

initial release of Exemption 10174, the two faults which the 747-8/8F design is not single 

fault tolerant, remain; 1) attachment to or near a large crack in structure, and 2) 

attachment to or near a bolt failure that also caused the fastener nut (or collar) to release 

with sufficient force to tear free the associated cap seal. Boeing additionally identified 

another fault in support of Exemption 10477 which is non-fault tolerant; 3) Three Zone 2 

dome nuts for a single tank access door on the 747-8 only.

Proposed Risk Mitigation

To support granting of Exemption No 10174 per FAA Policy Memo ANM-112-08-002 

for the 747-8/-8F, Boeing submitted detailed proprietary information as compliance 

documents to the FAA about the measures taken in the 747-8/-8F design to provide 

practical structural lightning protection (FAA Project Numbers PS05-0211 and 

PS05-0212). In general, the 747-8/-8F 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. Boeing further enhanced the 

lightning protection by incorporating fault tolerant protection means where practical, 

thereby increasing the level of fuel tank safety. 

FAA Policy Memo ANM-112-08-002 discusses the need to demonstrate compliance to 

14 CFR 25.981 or Part 26 flammability requirements as applicable. For the 747-8/-8F, 

Boeing complied with FAR 26.35 and FAR 26.33, as well as voluntarily stepping up to 

the requirements of 14 CFR25.981 (b) at amendment 125 for the center tank. 26.33(b) 

and 26.35(b) require a flammability assessment be conducted for all fuel tanks. The 

flammability exposure analysis showed that the main wing tanks of the 747-8/-8F have a 

low fleetwide flammability exposure (inboard and outboard main tanks) as required by 

26.33(c). The Center tank of the 747-8 utilizes an inerting system to meet the 

flammability exposure criteria of Part 25 Appendix M, for compliance to 14 CFR25.981 

(b). 

To demonstrate compliance of the 747-8 without cap sealing of 1⁄4” rivets in Zone 2 of 

wing fuel tank areas, Boeing proposes primarily to use the compliance documentation 

originally submitted to fulfill the requirements of Exemption No 10174. In addition, 

Boeing will provide analysis and testing that demonstrates that 747-8/8F rivet 

installations are fault tolerant without sealant. Boeing notes that the alternate 

requirements that were the basis of Exemption 10174 were based upon FAA Policy 

memo ANM-112-08-002, released in 2009. The requirements of the new policy 

statement PS-ANM-25.981-02 released in 2014 are similar to the 2009 requirements, and 

differ primarily only in that the new statements allows for the determination of 

practicality to be based upon commensurate safety benefit, and allows exemptions for 

systems installations (which Boeing is not requesting for the 747-8/-8F.) Compliance to 

each of the Policy ANM-25.981-002 requirements will be as summarized below: 

Enclosure to RA-15-01067 

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Policy Requirement 1 - Prevent catastrophic fuel vapor ignition due to 

lightning: 

The 747-8/8F utilizes all practical measures for fault tolerant structural lightning 

protection. Boeing utilizes manufacturing processes to minimize the risk of errors 

in aircraft manufacturing which could affect the lightning protection features. 

The Structural Repair Manual procedures are provided to retain the lightning 

protection features throughout the life of the airplane. Instructions for Continued 

Airworthiness including CDCCLs are included in the Special Compliance Items 

Document. A complete analysis of the manufacturing process and repair 

procedures were provided as part of the original Fuel System Lightning Protection 

System Safety Assessment. 

Policy Requirement 2 – Provide fault tolerant lightning protection: 

The basic requirement to provide single fault tolerance will be met without cap 

sealing rivets. Boeing will provide analysis and/or testing that demonstrates that 

wing fuel tank rivet installations are fault tolerant without sealant. The requested 

Exemption amendment does not change any other data previously provided to 

demonstrate fault tolerant lighting protection. The 747-8/-8F provides fault 

tolerant lightning protection, except for 3 improbable combinations discussed in 

Requirement 3 below. 

Policy Requirement 3 – Fault tolerance is not required for any specific design 

feature if fault tolerance is shown to be impractical for that feature

The requested Exemption amendment does not change this means of compliance. 

The only faults previously 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. 

3) Three Zone 2 dome nuts for a single tank access door on the 747-8 only. 

An assessment was provided to support the requirements of existing Exemption 

10174 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. 

Policy Requirement 4 – Provide lightning protection maintenance 

information: 

The requested Exemption amendment does not change this means of compliance. 

No revision to the existing ALI, mandatory inspections, or CDCCLs for structural 

lightning protection is anticipated. 

The Structural Repair Manual includes instructions to minimize the risk that 

alterations (repairs) will affect lightning protection. The SRM includes CDCCL 

references (equivalent to Cautions). Changes to the SRM will be made if required 

to clarify that production rivets are not sealed in Lightning Zone 2A. 

Enclosure to RA-15-01067 

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Policy Requirement 5 - Analysis of design, manufacturing and maintenance 

procedures: 

Boeing has implemented in the design all practical measures for structural 

lightning protection. As discussed above, it will be demonstrated by analysis and 

testing of all anticipated faults that cap sealing 1⁄4” rivets (and larger) is 

unnecessary to ensure safe operation during lightning strike conditions, even 

when faults are present. The use of sealant on rivets in Zone 2 of the 747-8/-8F 

wing results in additional weight to the aircraft. This in turn increases fuel burn, 

carbon emissions, and reduces inspectability during maintenance. Further, 

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. Since the design is already safe during 

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

Boeing has concluded that further mitigation (i.e. covering 1⁄4” and larger rivets 

with sealant) does 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 manufacturing processes include practical means to minimize the risk that 

lightning protection features are not implemented as intended on the drawings. 

CDCCL and the SRM repair procedures have been established to retain the 

lightning protection features throughout the life of the airplane. 

Statement of Public Interest

Boeing is working to improve the efficiency of the 747-8/-8F models by a redesign for 

weight improvement. For lightning protection, the 747-8/-8F has sufficient 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 (i.e. cap 

sealing of rivets) to directly meet the rule are impractical and result in significant cost, 

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

safety benefit. This renders the 747-8/-8F to be of lesser value to the public interest with 

no commensurate increase in safety. Further, this additional lightning protection reduces 

the ability to inspect the structure. Therefore, Boeing believes that it is in the public 

interest for the FAA to grant this revision to the 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 

Enclosure to RA-15-01067 

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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 747-8/-8F 

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

model 747-8 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 14 CFR 25.981(a)(3) issue on the 747-8/8F (Exemption No. 10174 was 

granted). Exemption 10477, which was requested to address the structural lightning 

protection on the Horizontal Stabilizer Tank (HST) of the 747-8 and was based on the 

same requirements of 10174, was issued without a public comment period. Additionally, 

Boeing has already submitted a Petition for Exemption based on the revised PS-ANM25.981-02 policy for the 737 MAX (Docket No. FAA-2014-1042). 

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 

747-8/-8F is intended for the global market place. 

Conclusion

Boeing is petitioning to amend Exemption No 10174 from 14 CFR 25.981(a)(3) at 

amendment 102 for fuel-tank structural lightning protection. Granting these revisions 

will allow Boeing to provide a competitive product without compromising safety. Per 

FAA Policy memo PS-ANM-25.981-02, Boeing has shown that certain existing design 

features that are considered impractical add significant cost, weight, increased emissions, 

and maintenance to the airplane without a proportional safety benefit. The proposed 

design for the 747-8/-8F will provide an appropriate level of safety against lightninginduced fuel-tank ignition and will be shown to comply with the alternative requirements 

provided in the FAA policy memo PS-ANM-25.981-02. 

 

Enclosure to RA-15-01067 

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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 747-8/8F model 

airplane. 

Enclosure to RA-15-01067 

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