4/15/2013 

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U.S. Department of Transportation] : .'.'; 1 3 ? 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 Court NE, Redmond, WA 98052 

Email pskirk@guietwing.com 

Exemption to: 14CFR section 25.981(b) 

Relief Requested: 

QUIET WING 

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 vapor temperature that is 

significantly different from that ofthe adjacent fuel. This renders the fuel temperature 

itself as an inaccurate indicator of the flammability condition of the tanks, as assumed 

when the rule was written. 14 CFR section 25.981(b) requires substantiation of 

compliance according to Appendix N, which in turn requires the use of the method "Fuel 

Tank Flammability Assessment Method User's Manual, dated May 2008, document 

number DOT/FAA/AR-05/8" (Monte Carlo method). This method uses "Bulk Average 

Fuel Temperature" as the primary indicator of tank 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 of the tank. 

Public Interest: 

The intention of Rule 14 CFR section 25.981 is to reduce the probability of fuel tank 

explosions. Aircraft operators are required to start to bring their fleets into compliance 

with the rule by September 2014. At this time, the only solution available 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 

modification that 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 8565 
level of safety as the modifications currently available. It is in the public interest to have 

available a lower cost and simpler solution that provides the same level of safety. This 

will reduce operator costs and ultimately reduce airfares for the public. 

Safety: 

The existing rule requires the use of fuel temperature as the primary indicator of fuel tank 

flammability. However, fuel tank flammability is determined not by fuel tank 

temperature, but by fuel vapor temperature in the tank, as well as fuel to air ratio of the 

vapor. The Quiet Wing modification reduces both the 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 of safety as if the fuel temperature were in fact equal to 

the fuel vapor temperature for an unmodified tank configuration. Furthermore, Quiet 

Wing has developed a method of analysis 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 of fuel vapor temperature instead of fuel 

temperature in the determination of tank flammability, as specified in Appendix N25.2 

paragraph (a), this being the method of determination of tank flammability required by 14 

CFR section 25.981(b). 

Contact: Philip S. Kirk 

Chief Engineer 

425 451 8565 Ext. 101 

pskirk@quietwing.com 
APPENDIX A 

EQUIVALENT FLAMMABILITY SUBSTANTIATION 
The 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 with the limits prescribed in Appendix M. This method provides a simplified 

way of determining the flammability of a fuel tank containing some amount of Jet A fuel, 

based only upon the temperature of the fuel and the pressure in the tank. However, in 

reality, the flammability of the tank relates not to the condition of the liquid fuel, but to 

the properties of the vapor in the tank, i.e. the temperature and pressure of the air/fuel 

vapor, and the ratio of fuel to air in the vapor. The simplified method in the Monte Carlo 

analysis was based on empirical data relating the condition of the liquid fuel to the 

measured or observed flammability of the tank. The method assumes a quasi steady state 

condition in the tank when the vapor pressure has increased to an equilibrium condition. 

As pointed out by the FAA, the vapor condition is affected not only by the liquid fuel 

vapor pressure, but also other related processes occurring in the tank such as 

condensation on the walls of the tank or other objects inside the tank. Therefore, the 

relation between the liquid fuel temperature and pressure, and the flammability of the 

vapor represents the combined result of various physical processes. 

The above scenario, and the validity of the simplified Monte Carlo analysis depends on 

the conditions inside the tank being in some form of equilibrium, otherwise there can be 

no definable relation between only the fuel temperature at a particular point in time, and 

the flammable condition of the vapor. The point becomes particularly important when 

dealing with tank configurations that include non-standard effects such as ullage purging. 

However, notwithstanding the breakdown of the simple fuel temperature relationship, the 

physics of flammability remain essentially unchanged for the vapor condition. In other 

words, if we measure the vapor temperature and the fuel to air ration ofthe vapor, then 

we will be able to define if the vapor is flammable or not without reference to any 

external effects such as fuel temperature or condensation processes occurring in the tank 

that affect the fuel to air ratio or the vapor temperature. The vapor will or will not be 

flammable based only on the measured condition of the vapor. 

Quiet Wing have proposed a method of developing a comparative analysis that results in 

the formulation of an equivalent fuel temperature that allows the continued use of the 

basic Monte Carlo method in predicting the flammability of the tank contents, consistent 

with the assumptions made when adopting the actual flammability limits specified in 

Appendix M. This method relates the fuel temperature in the modificed configuration to 

the fuel temperature in the basic tank configuration having the same, or lower, vapor 

flammability. The FAA have pointed out that the simplified method of relating liquid fuel 

temperature to flammability of the tank takes account the various other processes taking 

place in the tank, such as condensation. Simply using the measured properties of the 

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 in the tank that results in lower fuel 

to air ratios 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 tank 
flammability condition equivalent to a standard tank configuration with a lower fuel 

temperature. 

The fact that condensation or other factors affected the flammability of the standard 

configuration has no bearing on the validity of the proposed method, since those same 

conditions will be present in the modified configuration. Furthermore, it must be 

recognized that the Monte Carlo method itself is far from an exact predictor of 

flammability. Over a wide range of operating conditions, using only the two parameters 

of liquid fuel temperature and pressure cannot possibly provide an accurate determination 

of flammability in every situation.