UNITED STATES OF AMERICA 

DEPARTMENT OF TRANSPORTATION 

FEDERAL AVIATION ADMINISTRATION 

RENTON, WASHINGTON 98057-3356 

In the matter of the petition of 

Exemption No.15780 

The Boeing Company 

Regulatory Docket No. FAA-2015-3166 

for an exemption from§§ 25.901(c) and 

25.981(a)(3) ofTitle 14, Code of Federa1 

Regulations 

GRANT OF EXEMPTION 

By letter dated June 29, 2015, Mr. Tim Hendrix, Certification & Airworthiness Manager, The 

Boeing Company, 2401 E. Wardlow Road, Long Beach, California, 90807-5309, petitioned the 

Federal Aviation Administration (FAA) for an exemption from the requirements of§§ 25.901(c) 

and 25.981(a)(3) ofTitle 14, Code ofFederal Regulations (14 CFR). This exemption, if granted 

would allow planned type-design changes to the Boeing Model 747-400 (freighters only) and 

747-400BCF airplane center wing-tank (CWT) fuel-quantity indication system (FQIS) fuselage 

wiring installation. 

The petitioner requests relief from the following regulation(s): 

Section 25.90l(c), at Amendment 25-126, requires that for each powerplant and auxiliary 

power unit installation, it must be established that no single failure or malfunction or 

probable combination of failures will jeopardize the safe operation of the airplane except 

that the failure of structural elements need not be considered if the probability of such 

failure is extremely remote. 

Section 25.981(a)(3), at Amendment 25-125, requires that no ignition source may be 

present at each point in the fuel tank or fuel tank system where catastrophic fai lure could 

occur due to ignition of fuel or vapors. This must be shown by demonstrating that an 

ignition source could not result from each single failure, from each single failure in 

combination with each latent fai lure 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 petitioner supports its request with the following information: 

This section quotes the relevant infonnation from the petitioner's request, with minor edits for 

clarity. The complete petition is available at the Department of Transportation's Federal Docket 

Management System, on the Internet at http://regulations.gov, in Docket No. F AA-20 15-3 166. 

The Extent of Relief Sought and Reasons 

14 CFR 25.90 1 (c) Amendment 25-1 26 Boeing relief is necessary because direct 

compliance would require excessive design changes and retrofit of in-service airplanes. 

Changes include the following: Further redesign of the Airplane Wiring System to ensure 

FQIS wire separation for numerous airplane configurations. Make new tank penetrations 

and procure new in-tank wiring harnesses. Modify the Fuel Quantity Processor Unit 

(FQPU) and associated connectors. The solution would require a FQIS architecture 

change to relocate the FQPU and consider Remote Data Connectors (RDCs). In-tank 

component changes (new brackets, potential need for shorter probes, revised in-tank wire 

harness and retention means). Software changes related to Fuel Tank Gauging Function 

(to account for capacitance changes if shorter in-tank probes are required). 

14 CFR 25.981 (a)(3) Amendment 25-125 Boeing relief is necessary because: Same as 

above. 

Background 

The FAA is planning to issue a notice of proposed rulemaking (NPRM) for the 747, 

similar to the 757 NPRM No. 2011-NM-094-AD, which proposes to require modifying 

the FQIS wiring or fuel tank systems to prevent development of an ignition SJurce inside 

the center fuel tank. Paragraph (h)(2) states, "Within 72 months after the effective date of 

this airworthiness directive (AD), modify the airplane by separating FQIS wiring that runs 

between the FQIS processor and the center fuel tank, including any circuits that might 

pass through a main fuel tank, from other airplane wiring that is not intrinsically safe." 

Boeing is proposing to accomplish the FAA's planned AD requirements by separating the 

FQIS CWT wiring between the FQIS processor and the center fuel tank, as well as using 

the FQIS built in test equipment (BITE) check. The BITE service bulletin (SB) 747-28-

2340, which includes the instructions to ensure tank circuit wiring integrity, is already 

approved by the FAA. The CWT FQIS wiring separation is being prepared with guidance 

from the FAA's approved standard for retrofit wire separation from FQIS wiring. But the 

combination of BITE and wire separation will not result in the FQIS system being 

compliant to§ 25.901(c) at Amendment 25-126 and§ 25.98 1(a)(3) at Amendment 25-

125. Since the wiring changes are a change to type design, but wi ll not be compliant, an 

exemption is being petitioned. Boeing states that the FAA's response to a 757 NPRM 

comment suggests that the FAA will accept petitions for exemption for the "latent-plusone" requirements of§§ 25.90 l(c) and 25.98l(a)(3), and there has also been FAA 

communication that this acceptance applies to the 747. 

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This petition for an exemption is being made because full compliance to these rules for 

the 747-400 freighters would require extensive system design/installation changes, but do 

not substantially improve the safety of the current airplane design, and are economically 

prohibitive given that the retrofit 747-400 freighter fleet is small, and so extensive system 

changes are not in the public's interest. The FQIS system was previously shown 

compliant to§§ 25.1309 and 25.901(c) for type certification (TC); however, both 

§ 25.901(c) at Amendment 25-126 and§ 25.981(a)(3) at Amendment 25-1 25 have 

additional failure combination requirements. For systems that may cause a catastrophic 

hazard, § 25. 1309 requires no single failure and no combination of failures greater than 

extremely improbable;§ 25.901(c) at Amendment 25-126 requires no single failure or 

probable combination of failures; and§ 25.981(a)(3) at Amendment 25-125 requires no 

single failure, no single-plus-latent (greater than extremely remote), and no multiple 

combination greater than extremely improbable. This exemption is being sought for the 

no single-plus-latent (greater than extremely remote) as is stated in§ 25.981(a)(3) and 

interpreted by the FAA for§ 25.901(c). An exemption is also being sought for the nonenvironmental aspects of§ 25.90 I (c) provided by FAA interpretation, and the explicit 

§ 25.981(a)(3) requirements ofmanufacturing variability, aging, wear, corrosion, and 

likely damage, as these considerations are already covered by airline maintenance of the 

type design of the airplane. 

Statement of Public Interest 

Without exemption relief for the FQIS CWT wiring changes, the intent ofthe NPRM and 

the FAA's previous cornmunicatio11 of intent for airplane design improvement for the 

CWT FQIS to address those 747-400 freighters that do not have a nitrogen generating 

system (NOS) cannot be met. For compliance, airplane changes to the entire FQIS (all 

tanks) would be required for fleet retrofit such as: 

• Extensive airplane wiring changes to ensure adequate wire separation throughout 

the fuel quantity indication system. 

• In-tank component changes (new brackets] potential need for shorter probes] 

revised in-tank wire harness and retention means). 

• Software changes related to Fuel Tank Gauging Function and Safety function if 

shorter probes are required. 

The current FQIS system has been thoroughly evaluated for safety aspects as was required 

per Special Federal Aviation Regulation No. 88 (SFAR 88) for the FQIS; however, the 

FAA determined there were safety issues identified with FQIS wiring c01mected into a 

high flammability tank. For the 747-400, it is the CWT. The time needed to successfully 

design and implement system changes beyond the proposed CWT FQIS wiring separation 

design changes to comply with the rules would further impact the availability of the 

wiring change on this out-of-production fleet of747 airplanes. These new complete FQIS 

design changes would not provide any economic benefit to the operators and do not 

provide any significant benefit or increased level of safety. The Boeing Company 

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considers that the granting of this exemption would negate the need for the FAA to 

evaluate the additional data required to support compliance with these regulations. 

Therefore, granting this exemption would reduce the burden on FAA resources and 

consequently public expenditure. 

Granting this exemption is in the public interest for the aforementioned reasons and will: 

1. Enhance the safety of the 747-400 freighter fleet (as has been determined by the 

FAA) for those airplanes not installing NOS by adding wiring changes for the 

CWTFQIS. 

2. Enhance the safety of the 747-400 freighter fleet by also including FQIS BITE for 

the CWT circuit interval tests, so that when done in addition to the CWT FQIS 

wiring separation, the airplane addresses the single-plus-latent failure modes for 

the FQIS, which was the probable fai lure combination indicated by the National 

Transportation Safety Board (NTSB) Report AAR-00-03 for the Trans World 

Airlines (TWA) flight 800 accident. 

Statement of No Adverse Effect on Safety 

The addition of the FQIS CWT wiring changes with a BITE interval check, in lieu of a 

fully compliant (all tanks) FQIS (which would have a hot short protector (HSP1

) FQPU 

line replacement unit (LRU), wire separation for the tank (total of 7) circuit wiring (both 

outside and inside the tank), and a fault detecting BITE that is annunciated); does not 

have an adverse effect on safety. 

In the proposed CWT FQIS wire separation design change, the ignition source threat from 

other airplane systems is reduced, since the wiring will have increased separation. There 

are also airworthiness limitations in the instructions for continued airworthiness (ICA) 

that ensure that any other system change will continue to mainta in the separation 

provided by this proposed design change which will now clearly be visually identified as 

FQIS wiring. Together, this improves the wiring failure modes and improves the system 

probability relative to fuel tank ignition. With the addition of the FQIS BITE interval 

checks for the CWT, the probability is better since the exposure of wiring failures will be 

further reduced. 

For the CWT FQIS, where the changes to the FQIS are being done instead of the 

installation of NOS as determined by the FAA to be an acceptable substitute safety 

improvement, the combination of the separation, the ICA, and the BITE/circuit test 

addresses no single failures in combination with latent failures with an acceptable level of 

safety by ensuring the continued integrity of protection features from threats to the tank 

circuit wiring. 

1 A HSP is an LRU that is also designed to have the function to protect the tank circuit wiring from Electrical Power 

Source threats that could affect the airplane circuit wring of the LRU (FQPU). 

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• A - The fault modes of the in-tank wiring have reduced exposure to 750 hours due 

to the BITE check and FLSS circuit test. Wiring inside the tank is well protected 

and has a very low failure probability which is improved by the lower exposure. 

• B 1 - The FQPU is an HSP up to 115V AC. If a low-probability higher voltage 

power source short were to occur on the airplane side of the FQPU, it most likely 

would, but for very high voltage power sources may not stop it from propagating 

through to the tank circuits, but the failure would at the least be detectable when 

the BITE interval check was done, and in most cases sooner, as it would most 

likely affect indication. 

• B2- The wiring from the FQPU to the tank will have separation provided by the 

AD-mandated SB. This wiring has low probability failure modes which also have 

reduced exposure provided by the 750-hour interval BITE check. 

Whether the failure is A, orB 1, or B2, there is no single failure. For the 

combination of failures, A and B 1, or A and B2, the BITE has reduced the 

exposure and the latency interval for each protection design feature (wiring has 

less than extremely remote failure rates) and so the probability for each failure 

mode is better than remote. This also addresses the single-plus-latent failure 

requirement with an acceptable level of safety as the probability of either failure 

on the flight before the next interval check would both be better than remote. 

The 747-400 FQIS was found to be compliant to§ 25.90 l(c) during airplane type 

certification in 1989. Since then, Boeing has completed the SF AR 88 safety assessment 

and has increased the airplane fuel system safety with installation of an HSP LRU for the 

densitometer. For the 747-8 project, the FQPU, which has the same architecture as the 

747-400 FQPU, was also shown to be an HSP LRU and so the fuel level sensing 

probes/compensator have that same protection. Following the SF AR 88 system safety 

assessments, Boeing's assessment was that the risks for the 747-400 FQIS installation 

were extremely improbable, which is the typical industry and§ 25.1309 standard for an 

acceptable safety system. The Boeing approach to incorporate design changes to add 

CWT FQIS wire separation with visual identification, a CWT FQIS 750 hour BITE 

check, along with ICA that ensures that added systems wiring will maintain the separation 

for the life of the airplane, the HSP FQPU LRU, and the densitometer tank circuit HSP 

provided by Boeing SB 747-28A2266, will provide for an enhanced safety margin over 

the existing approved FQIS on the in-service fleet. 

Request to Waive Publication and Comment: 

Since the reference a) NPRM for the 757 is closely related to this change as the 747- 400 

issue is the same, and the NPRM makes mention of a 757 petition for exemption, and has 

already gone through public conm1enting process, Boeing respectfully requests that the 

FAA waive the "publication and comment" step in the process for making a final decision 

on this exemption. 

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In addition to the timeliness concern, it is the Boeing position that the safety-associated 

aspects related to this system have been fully vetted in the public forum as part of the 

SF AR 88 project. 

Privileges of the Exemption Outside of the United States 

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

outside of 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. 

Justification for Exemption 

In accordance with the requirements of§ 11.81(e), The Boeing Company has provided a 

means of compliance to the FAA to establish an acceptable level of safety to that 

provided by §§ 25.901(c) and 25.981(a)(3) by separating the FQIS wiring from other 

airplane power wires that will meet the minimum separation requirements for most of the 

wire runs. For the areas that minimum separation is not achievable due to space 

limitation, other positive separation means such as clamps, spacers, and sleeving will be 

used for the CWT FQIS which also includes FQIS BITE interval checks for the CWr. 

These design changes address the single-plus-latent failure modes for the FQIS which 

was the probable failure combination indicated by the NTSB TWA flight 800 accident 

report. Therefore, granting this exemption would, in turn, reduce the burden on FAA 

resources and consequently public expenditure. 

Conclusion: 

Boeing is petitioning for an exemption from the provisions of 14 CFR 25.90l (c)[25-126] 

and 14 CFR 25.981 (a)(3)[25-1 25] at the system level as they apply to the FQIS installed 

on the 747-400 (Freighters Only) and 747-400 BCF airplanes for the FQIS wiring 

changes described above. 

Federal Register publication 

Although the petitioner requested that action on its petition not be delayed for publication in the 

Federal Register, the FAA found that the petition, if granted, would set a precedent. Therefore, 

to allow an opportunity for the public to comment on the petition, a summary of it was published 

in the Federal Register on September 2, 2015 (80 FR 53223). No comments were received. 

The FAA's analysis 

We agree with the petitioner's justification that granting the petition is in the public interest, with 

two exceptions. 

For clarity, we do not agree with the petitioner's public-interest statements that "The time needed 

to successfully design and implement system changes beyond the proposed CWT FQIS wiring 

separation design changes, to comply with the rules, would further impact the availability of the 

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wiring change on this out-of-production fleet of 747 aircraft." The AD worksheet for correcting 

this unsafe condition was issued in 2004. Boeing has not provided service instructions since that 

time. The time needed to develop a compliant design is minimal when compared to the 

petitioner's delay in providing the service instructions. We did not consider this argument in our 

determination that granting the exemption is in the public interest. 

We also do not agree with the petitioner's public interest statement that " ... the granting of this 

exemption would negate the need for [the] FAA to evaluate the additional data required to 

support compliance with these regulations. Therefore, granting this exemption would reduce the 

burden on FAA resources and consequently public expenditure." FAA resources needed to 

review design changes that are needed for full compliance would be minimal, and these resources 

are available to address unsafe conditions. There would be no savings in public expenditure and 

no public interest by not providing a compliant design, and we did not consider this argument in 

our determination that granting the exemption is in the public interest. 

We agree with the petitioner's justification that granting the petition is in the public interest. 

Modification of existing in-service 747 airplanes to provide a compliant design would be costly 

because changing the fuel-quantity processor, separating airplane wiring from other high-power 

wiring, or providing electrical isolating devices to prevent electrical energy from entering the fuel 

tanks for the small number of cargo airplanes is cost prohibitive. This exemption will allow 

approval of service instructions needed to address an unsafe condition on the center wing-tank 

FQIS wiring. 

The FAA has detem1ined that the proposed modifications represent a reasonable, cost-effective 

method to achieve a meaningful reduction in the risk due to potential FQIS fuel-tank-ignition 

sources. The FAA's current fuel-tank-safety airworthiness standards rely upon a balanced 

approach of limiting fuel-tank-flammability exposure time and precluding ignition sources that 

could form in the fuel tanks. For this reason,§ 25.981 includes separate and distinct requirements 

for limiting fuel-tank flammability and preventing ignition sources in the fuel tanks. The wing 

and center wing-tank safety are addressed by considering flammability and ignition source 

mitigations. The wing tanks are exposed to outside air, are unheated, naturally cooled, and are 

considered low flammability. We do not consider it to be an unsafe condition for the FQIS wiring 

entering these wing tanks to be co-routed in wire bundles with wires carrying high electrical 

energies. Because the center wing tank is heated and is considered a high-flammability tank, 

additional ignition-source mitigations are provided. The modifications to separate a portion of 

the wiring for the center wing-tank FQIS significantly reduces the likelihood of a failure that 

could introduce high power onto the FQIS wiring. In addition, mandatory maintenance checks for 

the center tank FQIS wiring will be required as part of the petitioner's design approval. This will 

significantly reduce the likelihood of a latent failure in the FQIS wiring resulting in an ignition 

source in the fuel tanks. Granting this exemption will allow FAA approval of service instructions 

needed to address the unsafe condition and therefore result in a safety improvement that is in the 

public interest. 

SF AR 88 requires the type-design-approval holder to develop all design changes to the fuel-tank 

system that are necessary to meet the requirements of§§ 25.901 and 25.981. The petitioner did 

not specifically request exemption from this requirement. However, it did request an exemption 

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from§§ 25.901 and 25.981. Granting the exemption to these sections results in granting an 

exemption to the same ignition-source prevention requirements of SF AR 88. Therefore, we have 

added clarification in the exemption that states that exemption from SF AR 88 for the service 

instruction is also included in this exemption. 

The FAA's decision 

In consideration of the foregoing, I find that a grant of exemption is in the public interest. 

Therefore, pursuant to the authority contained in 49 U.S.C. 40113 and 44701 delegated to me by 

the Administrator, The Boeing Company, is hereby granted an exemption from SFAR 88, and 

§§ 25.90 l(c) and 25.98l(a)(3), as they pertain to fuel-tank-ignition prevention associated with 

the center wing-tank FQIS wiring installation, to allow for FA A-approval of Boeing Service 

Bulletins to address the unsafe condition in the FQIS that was identified through the SF AR 88 

safety analysis. The applicability of any service bulletin approved using this exemption must be 

limited to airplanes with an all-cargo configuration. 

Issued in Renton Washington, on MAR 2 9 20E 

Victor Wicklund 

Acting Manager, Transport Airplane Directorate 

Aircraft Certification Service 

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