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U.S. Department ofTransportation] :.'.'; 

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## ? I: S 0 Docket Operations West Building Ground Floor Room W12-140 1200 New Jersey Ave., SE Washington DC 20590

Petition for Exemption
Applicant: Quiet**Win~**Aerospace 12324 134 CourtNE, Redmond, WA 98052 Email pskirk@guietwing.com Exemption to: 14CFR section 25.981(b)



### ReliefRequested: 

# QUIETWING

Substitution of"Bulk average fuel temperature" with "Bulk average fuel vapor temperature" when assessing fuel tank flammability.



### Reason for Request:

The modification proposed by Quiet Wing for bringing the**B737~400** aircraft into compliance with 14 CFR section 25.981 results in a vaportemperature that is significantly different from that ofthe adjacent fuel. This renders the fuel temperature itselfas an inaccurate indicator ofthe flammability condition ofthe tanks, as assumed when the rule was written. 14 CFR section 25.981(b) requires substantiation of compliance according to Appendix N, which inturn requires the use ofthe method "Fuel TankFlammability AssessmentMethod User's Manual, dated May 2008, document number DOT/FAA/AR-05/8" (Monte Carlo method). This method uses "Bulk Average Fuel Temperature" asthe primary indicator oftank flammability. Quiet Wing argues that this simple relationship is not applicable to the proposed modification, and suggests an alternative method that establishes a modified relationship between fuel temperature and flammability that properly represents the flammability ofthe tank.


### Public Interest:

The intention ofRule 14 CFR section 25.981 is to reduce the probability offuel tank explosions. Aircraft operators are required to start to bring their fleets into compliance with the rule by September 2014. Atthis time, the only solutionavailable to the operators is an expensive, complicated, and high maintenance modification available from the OEM costing between$ 250,000 and$ 500,000 per aircraft. Quiet Wing has developed a modificationthat will be significantly lower cost and simpler, and will provide the same Quiet Wing Aerospace, LLC

12324 134th Court NE Redmond, WA 98052 425 451 8565level ofsafety as the modifications currently available. Itis in the public interest to have available a lower cost and simpler solution that provides the same level ofsafety. This will reduce operator costs and ultimately reduce airfares for the public. 

### Safety:

The existing rule requires the use offuel temperature as the primary indicator offuel tank flammability. However, fuel tank flammability is determined notby fuel tank temperature, butby fuel vapor temperature inthe tank, as well as fuel to air ratio ofthe vapor. The Quiet Wing modification reduces boththe fuel vapor temperature and fuel to air ratio, bringing the tank flammability into compliance with 14 CFR section 25.981. As such, it results in the same level ofsafety as ifthe fuel temperature were in fact equal to the fuel vapor temperature for an unmodified tank configuration. Furthermore, Quiet Wing has developed a method ofanalysis that ensures that the resulting modification has equal or lower flammability than a corresponding configuration having equal fuel and vapor temperatures, as assumed by the Monte Carlo method, see Appendix A to this request. 

### Summary:
Exemption is requested from 14 CFR section 25.981(b)

The exemption requested would allow the use offuel vapor temperature instead offuel temperature in the determination oftank flammability, as specified in Appendix N25.2 paragraph (a), this being the method ofdetermination oftank flammability required by 14 CFR section 25.981(b).

Contact: Philip S. Kirk ChiefEngineer 425 451 8565 Ext. 101 pskirk@quietwing.com

### APPENDIX A
EQUIVALENT FLAMMABILITY SUBSTANTIATIONThe Monte Carlo analysis is the substantiation tool defined by Appendix N to part 25 as the method for calculating the flammability exposure in fuel tanks, and for demonstrating compliance withthe limits prescribed in Appendix M. This method provides a simplified way ofdetermining the flammability ofa fuel tank containing some amount ofJet A fuel, based only upon the temperature ofthe fuel and the pressure in the tank. However, in reality, the flammability ofthe tank relates not to the condition ofthe liquid fuel, but to the properties ofthe vapor inthe tank, i.e. the temperature and pressure ofthe air/fuel vapor, andthe ratio offuel to air inthe vapor. The simplified method inthe Monte Carlo analysis was based on empirical data relating the condition ofthe liquid fuel to the measured or observed flammability ofthe tank. The method assumes a quasi steady state condition in the tank whenthe vapor pressure has increased to an equilibrium condition. As pointed out bythe FAA, the vapor condition is affected not only by the liquid fuel vapor pressure, but also otherrelated processes occurring inthe tank such as condensation on the walls ofthe tank or other objects inside the tank. Therefore, the relation betweenthe liquid fuel temperature and pressure, and the flammability ofthe vapor represents the combined result ofvarious physical processes.
The above scenario, and the validity ofthe simplified Monte Carlo analysis depends on the conditions inside the tankbeing in some form ofequilibrium, otherwise there canbe
no definable relation between only the fuel temperature at a particular point in time, and the flammable condition ofthe vapor. The point becomes particularly important when dealing withtank configurations that include non-standard effects such as ullage purging. However, notwithstanding the breakdown ofthe simple fuel temperature relationship, the physics offlammability remain essentially unchanged for the vapor condition. 

### Inother words, ifwe measure the vapor temperature and the fuel to air ration ofthe vapor, then
we will be able to define ifthe vapor is flammable ornot withoutreference to any external effects such as fuel temperature or condensationprocesses occurring in the tank that affect the fuel to air ratio orthe vapor temperature. The vapor will or will not be flammable based only onthe measured condition ofthe vapor. Quiet Wing have proposed a method ofdeveloping a comparative analysis that results in the formulation ofan equivalent fuel temperature that allows the continued use ofthe basic Monte Carlo method inpredicting the flammability ofthe tank contents, consistent with the assumptions made when adopting the actual flammability limits specified in Appendix M. This method relates the fuel temperature inthe modificed configuration to the fuel temperature inthe basic tank configurationhaving the same, or lower, vapor flammability. The FAAhave pointed outthat the simplified method ofrelating liquid fuel temperature to flammability ofthe tank takes account the various otherprocesses taking place inthe tank, such as condensation. Simply using the measured properties ofthe vapor to determine flammability circumvents the fact that the regulatory limits established in Appendix M took into account the fact that these other effects were taking place, and in themselves affecting the fuel to air ratio inthe tankthat results in lower fuel to airratios than one would otherwise expect. Quiet Wing agrees with this statement. However, Quiet Wing argues that the comparative method being suggested does not violate the assumptions made in establishing the Appendix M limits, but results in a tankflammability condition equivalent to a standard 

### tank configuration with a lower fuel temperature.

The fact that condensation or other factors affected the flammability ofthe standard configuration has no bearing onthe validity ofthe proposed method, since those same conditions will be present in the modified configuration. Furthermore, it must be recognized that the Monte Carlo method itselfis far from an exact predictor of flammability. Overa wide range ofoperating conditions, using only the two parameters ofliquid fuel temperature and pressure cannotpossibly provide an accurate determination offlammability 

### in every situation.