May 21, 2015 CP-L4L-15-00191

U.S. Department ofTransportation Docket Operations M-30 1200 NewJerseyAvenue, SE Room W12-140, West Bldg. Ground Floor Washington DC 20590-0001

Attention: Applicant: FAA TransportAirplane 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. ThomasA. Enyart Email: Thomas.a.Enyart@boeing.com Phone: (562) 797-8956 Seal Beach, CA 90740-1515 The BoeingCompany 2401 E. Wardlow Road Long Beach, CA 90807-5309

Subject: 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 757-200 (Freighters Only), 757-200PF CenterWing Tank Fuel Quantity Indication System (FQIS) FuselageWiring Installation.

Reference: a) NPRM 2011-NM-094-AD b) NTSB reportTWA800AAR-00-03 c) Service Bulletin 757-28-0136 BITE check



### Certification Item No: 48065

The Boeing Company respectfully submitsthe 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 forthe 757-200 (Freighters Only), 757-200PF CenterWing Tank (CWT) Fuel Quantity Indication System (FQIS) FuselageWiring Installation.

The Extent of Relief Sought and Reason(s):

14 CFR 25.901(c) atAmendment 25-126 requires:

(c) For each powerplant and auxiliary power unit installation, it must be established that no single failure or malfunction or probable combination of failures willjeopardize the safe operation ofthe airplane exceptthatthePage 2 of6 CP-L4L-15-00191



### failure ofstructural elements need not be considered ifthe 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 ofin-service airplanes. Changes include the following: Further redesign ofthe Airplane Wiring System to ensure FQIS wire separation for numerous airplane configurations. Make new tank penetrations and procure new in-tankwiring harnesses. Modifythe 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-tankwire harness and retention means). Software changes related to Fuel Tank Gauging Function (to accountfor capacitance changes ifshorter in-tank probes are required).

14 CFR 25.981 (a)(3) atAmendment 25-125 requires: (a) No ignition source may be present at each point in the fuel tank orfuel tank system where catastrophic failure could occurdue to ignition offuel or vapors. This must be shown by: , (3) Demonstrating that an ignition source could not resultfrom each single failure, from each single failure in combination with each latent failure condition not shown to be extremely remote, and from all combinations offailures not shown to be extremely improbable. The effects ofmanufacturing 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 has issued NPRM 2011-NM-094-AD which proposesto require modifying the FQIS wiring orfuel tank systems to prevent development ofan ignition source inside the centerfuel tank. Paragraph (h)(2) states : "Within 72 months afterthe effective date of this AD, modifythe airplane by separating FQIS wiring that runs between the FQIS processor and the centerfuel 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 CWTwiring between the FQIS processor and the centerfuel tank, as well as using the FQIS Built in Test Equipment (BITE) check. The BITE service bulletin 757-28-0136 is already approved bythe FAA. The CWT FQIS wiring separation is being prepared with guidance from the FAA's approved standard for retrofitwire separation from FQIS wiring. Butthe 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, butwill not be compliant, an exemption is being petitioned. FAA's response to the reference a) NPRM comment suggest that FAAwill accept petitions for exemption for the "latent-plus-one" requirements ofsections 25.901 (c) and 25.981(a)(3) ofthe Federal Aviation Regulations.Page 3 of6 CP-L4L-15-00191

This petition for an exemption is being made since full compliance to these rules forthe 757 would require extensive system design/installation changes, but do notsubstantially improve the safety ofthe current airplane design, and are economically prohibitive given thatthe 757 freighterfleet is limited, and so are not in the public's interest.

The FQIS system was previously shown compliantto §25.1309 and 25.901 (c) for TC and ATC; however, both §25.901(c) [25-126] and §25.981(a)(3) [25-125] have additional failure combination requirements. Forsystems that maycause a catastrophic hazard, §25.1309 requires no single failure and no combination offailures greaterthan extremely improbable; §25.901 (c) [25-126] requires no single failure or probable combination offailures; and §25.981(a)(3) [25-125] requires no single failure, no single plus latent (greaterthan extremely remote), and no multiple combination greaterthan extremely improbable. This exemption is being soughtforthe no single plus latent (greaterthan extremely remote) as is stated in §25.981 (a)(3) and interpreted bythe FAAfor §25.901 (c). An exemption is also being soughtforthe 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 ofthe type design ofthe airplane.
Statement of Public Interest: Without exemption reliefforthe FQIS CWTwiring changes, the intent ofthe NPRM and the FAA's previous communication of intentfor airplane design improvement for the CWT FQIS to address those 757 freighters thatdo not have Nitrogen Generating System (NGS) cannot be met. For compliance, airplane changes to the entire FQIS (all tanks) would be required forfleet retrofit such as:

• Newsystem architectureto redesign the FQPU to function as a Hot Short Protector (HSP*) Line Replaceable Unit (LRU) and relocate the FQPU to enable wire separation. Other architectures could be used such as providing a HSP LRU closerto the tank, but itwould be a more extensive and new architecture.
• Extensive airplanewiring changes to ensure adequatewire 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 Safetyfunction if shorter probes are required.

• A HSP is an LRU that is also designed to have the function to protectthe tank circuit wiring from Electrical Power Sourcethreats that could affect the airplane circuitwiring ofthe LRU (FQPU).
The current FQIS system has been thoroughly evaluated for safety aspects as was required perSFAR88 for the Fuel Quantity Indicating System, howeverthe FAA determined there were safety issues identified with FQIS wiring connected into a high flammability tank, forthe 757 it is the CWT. The time needed to successfully design and implement system changes beyond the proposed CWT FQISwiring separation design changes to comply with the rules would further impactthe availability ofthe wiring change on this out ofproduction fleet of757 aircraft. These new completePage 4 of6 CP-L4L-15-00191

FQIS design changeswould not provide any economic benefitto the operators and do not provide any significant benefit or increased level ofsafety. The Boeing Company considers thatthe granting ofthis exemption would negatethe need for FAAto evaluate the additional data required to support compliancewith these regulations. Therefore, granting this exemption would reduce the burden on FAA resources and consequently public expenditure. Granting this exemption is in the public interestforthe aforementioned reasons and will: 1. Enhance the safety ofthe 757 fleet (as has been determined by the FAA) forthose airplanes not installing NGS by adding wiring changesforthe CWT FQIS. 2. Enhance the safety ofthe 757fleet, by also including FQIS BITE interval checks for the CWT so thatwhen done in addition with the CWT FQIS wiring separation, the airplane addresses the single plus latentfailure modes forthe FQIS which was the probable failure combination indicated by the NTSB
Statement of NoAdverse Effect on Safety. The addition ofthe FQIS CWTwiring changes along with a BITE interval check, in lieu ofa fully compliant (all tanks) FQIS (which would have a HSP FQPU LRU, wire separation forthe tank 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 is also Instructions for Continued Airworthiness (ICA) that ensures that any other system changes will continue to maintain the separation provided bythis proposed design change which will be now clearly be visually identified as FQIS wiring. Together, this improvesthe wiring failure modes, and improvesthe system probability relative to fuel tank ignition. With the addition ofthe FQIS BITE interval checks forthe CWTthe probability is better since the exposure ofwiring failures will be further reduced. Forthe CWT FQIS, where the changes to the FQIS are being done instead ofthe installation of NGS as determined bythe FAA to be an acceptable substitute safety improvement, the combination ofthe separation, the ICA, and the BITE addresses no single failures in combination with latentfailures with an acceptable level ofsafety by ensuring the continued integrity ofprotection features from threats to the tank circuit wiring.

$ A- The fault modes ofthe in tank wiring have reduced exposure to 750 hours due to the BITE check. Wiring inside the tank is well protected and has a very lowfailure probabilitywhich is improved bythe lower exposure.

• B1 -The FQPU is a HSP up to 28V. Ifa low probability highervoltage power source shortwere to occuron the airplane side ofthe FQPU orwithin the FQPU, it most likelywould, butfor specific failures may not stop it from propagating through to the tank circuits, butthe failure would atthe least be detectablewhen the BITE interval check was done, and in most cases sooner, as it would most likely affect indication

..
• B2- Thewiring from the FQPU to the tankwill have separation provided bythe AD mandated SB. This wiring has low probabilityfailure modeswhich also have reduced exposure provided bythe 750 hour interval BITE check.