Att: Exemption Requests 

U.S. Department of Transportation 

Docket Operations 

West Building Ground Floor 

Room W12-140 

1200 New Jersey Ave., SE 

Washington DC 20590 

SUBJECT: Petition for exemption to 14CFR section 25.981(b) 

REF: ST11785SE-T 

DEPARTMENT Oil 

TRANSPORTATION 

Z811 fEB 23 A CJ: 31 

DOCKET OPERATIONS 

12/21/16 

Please find enclosed our request for the proposed exemption. Should any questions or 

concerns arise you can reach me directly at: (858) 922-8705. 

Best Regards. 

Sincerely, 

Philip S. Kirk 

tl~·!7=-:- p ~·~cr- 12324 1341h Court NE, Redmond 

W A 98052 USA. 

Ph: (425) 451-8565 www.quietwing.com 1 of 1 
4/15/2013 

U.S. Department of Transportation 

Docket Operations 

West Building Ground Floor 

Room W12-140 

1200 New Jersey Ave., SE 

Washington DC 20590 

Petition for Exemption 

Applicant: Quiet Wing Aerospace 

12324 134th Court NE, Redmond, W A 98052 

Email pskirk@guietwing.com 

Exemption to: 14CFR section 25.981(b) 

Relief Requested: 

UL:--

QUIETWING 

The addition of wording in CFR 25 Appendix N, paragraph N.2(c) addressing the effects 

of modifications that influence ullage properties: 

(c) Flammable. With respect to a fluid or gas, flammable means susceptible to igniting readily or to 

exploding (14 CFR Part 1, Definitions). A non-flammable ullage is one where the fuel-air vapor is too 

lean or too rich to burn or is inert as defined below. For the purposes of this appendix, a fuel tank that is 

not inert is considered flammable when the bulk average fuel temperature within the tank is within the 

flammable range for the fuel type being used, and the corresponding vapor properties. For any fuel tank that is 

subdivided into sections by baffles or compartments, the tank is considered flammable when the bulk average fuel 

temperature and corresponding vapor properties within any section of the tank, that is not inert, is within the 

flammable range for the fuel type being used 

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, and a corresponding vapor content 

that does not follow the relationship of fuel temperature and vapor constituency and 

flammability for a closed tank system. 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 tum requires the use of the method "Fuel Tank Flammability 

Assessment Method User's Manual, dated May 2008, document number DOT/FAA/AR05/8" (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. 

Quiet Wing Aerospace, LLC 

12324 !34th Court NE 

Redmond, WA 98052 

425 451 8565 
Public Interest: 

The intention of Rule 14 CFR section 25.981 is to reduce 

the probability of fuel tank explosions. Aircraft operators QUIET WING 

are required to bring their fleets into compliance with the rule by December 2017. 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. These systems require 

regular and expensive maintenance. Quiet Wing has developed a modification that will 

be significantly lower cost and simpler, and will provide the same 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 alone, 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 or higher 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) and Appendix N, subparagraph 

N.2(a). 

The exemption requested would allow the consideration of external effects on vapor 

flammability instead of fuel temperature alone 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 

Quiet Wing Aerospace, LLC 

12324 134th Court NE 

Redmond, WA 98052 

425 451 8565 
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 closed 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 or cooling. 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 of the 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 modified 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. This would be established by tests of the basic and modified configurations 

under various atmospheric conditions. 

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 the proposed 

method establishes a flammability equivalency by direct measurement of the actual vapor 

conditions for both the basic and modified configurations. 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, particularly if ullage purging is occurring. The 

proposed method establishes flammability equivalency by direct measurement of the 

vapor properties.