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Additional Information to Support Exemption Petition from 14 

CFR 25.981(a)(3) for Fuel Tank Structural Lightning 

Protection 

References: 

(a) ANM 112-08-002 Policy on Issuance of Special Condition and Exemptions Related to Lightning 

Protection of Fuel Tank Structure, 5-26-2009 

(b) FAA ANM-112 letter, dated February 26, 2010, concerning Boeing request for exemption from 14 

CFR 25.981(a)(3) 

(c) Boeing Letter BDCO-09-05261, “Petition for Exemption from 14 CFR 25.981(a)(3) Fuel Tank 

Structural Lightning Protection, dated November 4, 2009 

Discussion: 

Boeing is requesting an exemption from 14 CFR 25.981 for Structural Lightning Protection per the 

requirements in reference (a) FAA policy memo. Per reference (b) FAA letter, the FAA has requested 

additional information in order to substantiate Boeing’s request for exemption. 

FAA comment 1): 

1) For pre-Amendment 25-125 airplanes, the fuel tank flammability standards for the design change 

will be those required under §§ 26.33 and 26.35 as adopted in Amendment 26-2. No data was 

provided with your petition. 

Boeing response to 1): 

 

Boeing will comply with FAR 26.35 and FAR 26.33. A flammability exposure analysis will be provided 

similar to the analysis provided for the 747-400 showing that the main wing tanks of the 747-8 are 

either equivalent to the conventional unheated aluminum wing tank (reserve and surge tanks) or are 

below 7% fleetwide flammability exposure (inboard and outboard main tanks) per 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. Performance will be demonstrated and documentation of compliance will be submitted 

for approval per CP 4288. 

FAA Comment 2): 

2) 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. Please provide practicality analysis to support 

your exemption request. 

Boeing Response to 2): 

Boeing has utilized all practical measures to meet the requirements of 14CFR 25.981(a)(3) for fuel 

tank structural lightning protection. Boeing has designed the 747-8 fuel tank structure to be fault 

tolerant for all but two failure modes as discussed in FAA comment 4). A detailed FMEA will be 

provided as part of the compliance documentation. A test program to demonstrate fault tolerance has 
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been developed as a part of the compliance demonstration. The test plan has been submitted, and 

the test results will be documented for compliance. 

As described in reference (c), the 747-8 design utilizes properly installed structural fasteners and cap 

sealing to provide fault tolerant structural lightning protection. Boeing considered additional design 

features to the fuel tank structure to achieve dual fault tolerance for compliance to 14CFR 

25.981(a)(3), but these were found either impractical to implement or insufficient to provide any 

additional lightning protection. 

Examples of design considerations were: 

o 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. 

o Additional sealant – Reduces structural inspectability for cracks and corrosion, potentially 

reducing overall fleet safety. Would result in additional weight, fuel burn and emissions. 

o 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. 

o 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. 

o 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. 

o 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 747-8 requires and 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 due lack of safety benefit, increased weight, 

increased fuel burn, increased emissions, and reduced inspectability during maintenance. 

FAA Comment 3): 

3) The policy memo addresses both direct and indirect effects of lightning. The statements in the 

petition included discussion of features in lightning zone 2. Please provide discussion in your petition 

regarding lightning protection features in lightning zone 3. 

Boeing Response to 3): 

Boeing will demonstrate that the structure in Zone 3 (and Zone 2) is capable of providing fault tolerant 
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protection to conducted currents, which is the only Zone 3 requirement. 

FAA Comment 4): 

4) Instead 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 to prevent lightning related ignition sources 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 because of 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. 

Please provide the number and location of possible failures that could cause an ignition source. While 

submittal of the complete quantitative analysis to support the exemption is not required until the type 

certification date, qualitative review of these features should be provided with the practicality review, 

noted in #2 above, to support your petition. 

Boeing Response to 4) 

As discussed in response 2 above, Boeing will provide a System Safety Assessment (SSA) as part of 

compliance for the747-8/8F programs. A fault tree analysis will be included in the SSA which will 

quantify the single failures, and demonstrate the probability of a fuel vapor ignition in the life of the 

747-8 fleet is extremely improbable. 

The only faults identified for which the design is not fault tolerant, are the potential for a latent crack in 

structure or a bolt failure which also causes the fastener nut (or collar) to release with sufficient force 

to tear free the associated cap seal. Note that 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. Boeing will demonstrate sparking does not occur due 

to these failures during conducted current tests. For both of these failure modes, Boeing has been 

unable to identify a practical alternative to provide a protection to the failure mode for direct 

attachment. A numerical assessment will be provided demonstrating the probability of a fuel vapor 

ignition due to these failures is extremely improbable (and thus unlikely to occur in the life of the fleet). 

Service experience on the 747-400 shows each of these failures to be rare. The failure rate for 

fasteners will be established using in-service data for fasteners on Boeing airplanes. The expected 

rate of occurrence of structural cracking will be conservatively based on Boeing’s structural fatigue 

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. 

FAA Comment 5): 

5) 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 

because of manufacturing variability, aging, wear, corrosion, and likely damage. Your petition 

provides a short discussion of the subject but does not state that a complete analysis would be 

conducted as required by the policy. Please provide a statement to this effect. 
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Boeing response to 5) 

Boeing has implemented in the design all practical measures for structural lighting protection. Boeing 

is currently reviewing the manufacturing processes to ensure the lightning protection features are 

implemented as intended on the drawings and are maintained. The repair procedures will be 

reviewed to ensure the lightning protection features will be maintained throughout the life of the 

airplane. Instructions for Continued Airworthiness will be developed and included in the certification 

documentation. A complete analysis of the manufacturing process and repair procedure will be part 

of the System Safety Assessment and submitted for compliance per CP 3308 deliverable number 62.