May 21, 2015 CP
-L4L-15-00191 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: 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 90740-1515**~f-)/-J- -(OJ~-**  L_U 

## C J

The Boeing Company 2401 E. Wardlow R oad Long Beach, C A 90807-5309

Sub j ect: 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 Center Wing Tank Fuel Quantity Indication System (FQIS) Fuselage Wiring Installation.


### Reference: a) NPRM 2011-NM-094-AD b) NTSB report TWA 800 AAR-00
-03 c) Service Bulletin 757-28
-0136 BITE check 

### Certification Item No: 48065 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 757-200 (Freighters Only) , 757-200PF Center Wing Tank (CWT) Fuel Quantity Indication System (FQIS) Fuselage Wiring Installation. The Extent of Relief Sought and Reason(s): 

### 14 CFR 25

.901(c) at Amendment 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 will jeopardize the safe operation ofthe airplane except that thePage 2 of 6 CP
-L4L-15-00191 failure of structural 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 of in-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-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 offuel or vapors. This must be shown by: (3) Demonstrating that an ignition source could not result from each sing

.le 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 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 has issued NPRM 2011-NM-094-AD 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) 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 using the FQIS Built in Test Equipment (BITE) check. The BITE service bulletin 757-28-0136 is already approved by the FAA

. The CWT FQIS wiring separation is being prepared with guidance from the FAA's approved standard for retrofit wi re 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. FAA's response to the reference a) NPRM comment suggest that FAA will accept petitions for exemption for the "latent-plus-one" requirements of sections 25

.901 (c) and 25.981 (a)(3) ofthe Federal Aviation Regulations.Page 3 of 6 CP-L4L-15-00191

This petition for an exemption is being made since full compliance to these rules for the 757 would require extensive system design/installation changes, but do not substantially improve the safety ofthe current airplane design, and are economically prohibitive given that the 757 freighter fleet is limited, and so are not in the public's interest. The FQIS system was previously shown compliant to §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. For systems that may cause a catastrophic hazard, §25.1309 requires no single failure and no combination offailures greater than 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 (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 ofthe type design ofthe airplane. Statement of Public Interest: Without exemption relief for the FQIS CWT wiring changes, the intent ofthe NPRM and the FAA's previous communication of intent for airplane design improvement for the CWT FQIS to address those 757 freighters that do not have Nitrogen Generating System (NGS) cannot be met. For compliance, airplane changes to the entire FQIS (all tanks) would be required for fleet retrofit such as: • New system architecture to 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 closer to the tank, but it would be a more extensive and new architecture

. • 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.

• 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 ci rcuit wi ring of the LRU (FQPU)

. The current FQIS system has been thoroughly evaluated for safety aspects as was required per SFAR88 for the Fuel Quantity Indicating System, howeverthe FAA determined there were safety issues identified with FQIS wiring connected into a high flammability tank, for the 757 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 757 aircraft. These new completePage 4 of 6 CP-L4L-15-00191 FQIS design changes would not provide any economic benefit to the operators and do not provide any significant benefit or increased level ofsafety. The Boeing Company considers that the granting ofthis 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: I. Enhance the safety of the 757 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 757 fleet, by also including FQIS BITE interval checks for the CWT 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 NTSB Statement of No Adverse Effect on Safety. The addition ofthe 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 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 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 ofthe FQIS BITE interval checks for the CWT the probability is better since the exposure ofwiring failures will be further reduced. For the CWT FQIS, where the changes to the FQIS are being done instead ofthe installation of NGS as determined by the FAA to be an acceptable substitute safety improvement, the combination ofthe separation, the ICA, and the BITE addresses no single failures in combination with latent failures with an acceptable level ofsafety by ensuring the continued integrity of protection 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 low failure probability which is improved by the lower exposure. • B1
- The FQPU is a HSP up to 28V

. If a low probability higher voltage power source short were to occur on the airplane side ofthe FQPU or within the FQPU, it most likely would, but for specific failures 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 wi ring has low probability failure modes which also have reduced exposure provided by the 750 hour interval BITE check.Page 5 of 6 CP-L4L-15-00191 

### Whether the failure is A, or B1, or B2 ,there is no single failure

. For the combination offailures, A and B1, 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 757 FQIS was found to be compliant to §25.901 (c) during airplane type certification for the FQIS change in 1992 and 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

. Following the SFAR88 system safety assessments, Boeing assessment was that the risks for the 757 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 ofthe airplane, and the densitometer tank circuit HSP provided by Boeing SB 757-28A0085, will provide for an enhanced safety margin over the existing , approved FQIS on the inservice fleet. Request to Waive Publication and Comment: 

### Since the NPRM 2011-NM-094-AD is directly connected to this change , makes mention ofthis 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

. Additionally, given that the FAA and Boeing have made a rapid schedule for this project, and so the schedule-sensitive nature ofthis request for exemption , 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 ofthe SFAR88 project. Privileges of the Exemption Outside the United States: 

### Per 14 CFR 11.81 (h), Boeing requests that the privileges ofthis 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 requirements for most ofthe wire runs. For the areas that minimum separation are not achievable due to space limitation , other positive separation means, such as clamps ,Page 6 of 6 CP-L4L-15
-00191 spacers and sleeveing 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 757-200 (freighters), 757-200PF 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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