. rtJ-BOEING 

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The Boeing Company 

2401 E. Wardlow Road 

June 29, 2015 

CP-L4L-15-00278 

-; -. ::·. Long Beach, CA 90807-5309 

.. ·;.· --· 

U.S. Department of Transportation 

Docket Operations M-30 

1200 New Jersey Avenue, SE 

Room W12-140, West Bldg. Ground Floor 

Washington DC 20590-0001 

Attention: 

Applicant: 

Subject: 

References: 

FAA Transport Airplane Directorate 

The Boeing Company 

Boeing Commercial Airplanes 

Southern California Engineering Design Center 

2401 E. Wardlow Road, Mail Code D800-0022 

Long Beach, CA 90807-5309 

Contact: Mr. Thomas A. Enyart 

Email: Thomas.a.Enyart@boeing.com 

Phone: (562) 797-8956 

Seal Beach, CA 907 40-1515 

Petition for Exemption from 14 CFR 25.901(c) Amendment 25-126 and 14 

CFR 25.981 (a)(3) Amendment 25-125 pertaining to planned changes for 

the 747-400 (Freighters Only), 747-400 BCF Center Wing Tank (CWT) 

Fuel Quantity Indication System (FQIS) Fuselage Wiring Installation. 

a) NPRM 2011-NM-094-AD, The Boeing Company Model 757 

airplanes Center Fuel Tank- NPRM proposed to require modifying 

the fuel quantity indication system (FQIS) wiring 

b) NTSB report TWA 800 AAR-00-03 

c) Service Bulletin 747-28-2340, FUEL- Fuel Quantity Indicating 

System (FQIS) • FQIS Ground Test and Corrective Action, dated 

June 06, 2014 

Certification Item No: 48206 

The Boeing Company respectfully submits the enclosed Petition for Exemption from the 

Title 14 Code of Federal Regulations §14 CFR 25.901(c) Amendment 25-126 and 14 

CFR 25.981(a)(3) Amendment 25-125 pertaining to planned changes for the 747-400 

(Freighters Only), 747-400 BCF Center Wing Tank (CWT) Fuel Quantity Indication 

System (FQIS). 

The Extent of Relief Sought and Reason(s): 

14 CFR 25.901(c) at Amendment 25-126 requires: 

(c) For each power plant and auxiliary power unit installation, it must be 

established that no single failure or malfunction or probable combination of 
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CP-L4L-15-00278 

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. 

14 CFR 25.901(c) Amendment 25-126 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) at Amendment 25-125 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. 

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 NPRM for the 747 similar to the reference a) 757 NPRM 

which proposes to require modifying the FQIS wiring or fuel tank systems to prevent 

development of an ignition source inside the center fuel tank. Paragraph (h)(2) of the 

757 NPRM states: "Within 72 months after the effective date of this 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 mandated changes to 

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

well as performing repetitive system check using the FQIS Built in Test Equipment 

(BITE) check. The reference c) BITE service bulletin 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) [25-126] 

and 25.981 (a)(3) [25-125]. Since the wiring changes are a change to type design, but 

will not be compliant, an exemption is being petitioned. The FAA's response to a 757 

NPRM comment suggests that FAA will accept petitions for exemption for the "latentplus-one" requirements of sections 25.901 (c) and 25.981 (a)(3) of the Federal Aviation 
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CP-L4L-15-00278 

Regulations, and there has also been FAA communication that this acceptance applies 

to the 747. 

This petition for an exemption is being made since 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) [25-126] and §25.981(a)(3) [25-125] 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) [25-126] requires no single failure or probable combination of failures; and 

§25.981 (a)(3) [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 non-environmental aspects of §25.901 (c) 

provided by FAA interpretation, and the explicit §25.981 (a)(3) requirements of 

manufacturing 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 of the NPRM and 

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

CWT FQIS to address those 747-400 freighters that do not have Nitrogen Generating 

System (NGS) cannot be met. For compliance, airplane changes to the entire Fuel 

Quantity Indication System (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 SFAR88 for the Fuel Quantity Indicating System, however the FAA 

determined there were safety issues identified with FQIS wiring connected 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 of 747 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. 
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CP-l4L-15-00278 

The Boeing Company considers that the granting of this exemption would negate the 

need for 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 NGS by adding wiring changes for the 

CWT FQIS. 

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 with the CWT 

FQIS wiring separation, the airplane addresses the single plus latent failure 

modes for the FQIS which was the probable failure combination indicated by the 

reference b) NTSB report. 

Statement of No Adverse Effect on Safety. 

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

of a fully compliant (all tanks) FQIS (which would have a HSP* FQPU 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. 

* 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 wiring of the LRU 

(FQPU). 

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 is also Instructions for Continued Airworthiness (ICA) that ensures that any other 

system changes will continue to maintain the separation provided by this proposed 

design change which will be 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 NGS 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. 

• 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. 

• Bl- The FQPU is a HSP up to llSVAC. 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. 
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CP-L4L-15-00278 

o 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, or Bl, or 82, there is no single failure. For the combination of failures, A 

and Bl, 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 §25.901 (c) during airplane type 

certification in 1989. Since then Boeing has completed the SFAR88 safety assessment 

and has increased the airplane fuel system safety with installation of a Hot Short 

Protector (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 a HSP LRU and so 

the fuel level sensing probes/compensator have that same protection. 

Following the SFAR88 system safety assessments, Boeing assessment was that the 

risks for the 747-400 FQIS installation were extremely improbable, which is the typical 

industry and CFR §25.1309 standard for an acceptable safety for systems. 

The Boeing approach to incorporate design changes to add center wing tank 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 commenting process, Boeing respectfully requests 

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

decision on this exemption. 

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 

SFAR88 project. 

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. 

Justification for Exemption: 

In accordance to the requirements of 14 CFR §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 the rules 14 CFR §25.901(c) and 14 CFR §25.981(a)(3) by separating 

the FQIS wiring from other airplane power wires that will meet the minimum separation 
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CP-L4L-15-00278 

requirements for most of the wire runs. For the areas that minimum separation are not 

achievable due to space limitation, other positive separation means, such as clamps, 

spacers and sleeving will be used for the CWT FQIS and also including FQIS BITE 

interval checks for the CWT,. These design changes address the single plus latent 

failure modes for the FQIS which was the probable failure combination indicated by the 

NTSB report, reference b). 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 25.901(c)[25-126] 

and 14 CFR 25.981 (a)(3)[25-125] at the system level as they apply to the FQIS installed 

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

described above. 

Sincerely, 

Tim Hendrix 

Manager, Certification & Airworthiness 

Southern California Engineering Design Center 

Boeing Commercial Airplanes 

(562) 593-7074, MC 0800-0022 

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