### Att: Exemption Requests
U.S. Department ofTransportation 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 1341 h Court NE, Redmond W A 98052 USA. Ph: (425) 451-8565 www.quietwing.com 

### 1 of 

### 14/15/2013

U.S. Department ofTransportation Docket Operations West Building Ground Floor Room W12-140 1200 NewJersey Ave., SE Washington DC 20590

Petition for Exemption Applicant: Quiet Wing Aerospace 12324 134th CourtNE, Redmond, W A 98052 Email pskirk@guietwing.com Exemptionto: 14CFRsection 25.981(b)


### ReliefRequested: 

# UL:--

QUIETWING

The addition ofwording in CFR 25 AppendixN, paragraphN.2(c) addressing the effects ofmodifications that influence ullage properties: (c) Flammable. With respect to a fluid or gas, flammable means susceptible to igniting readily orto exploding (14 CFR Part 1, Definitions). A non-flammable ullage is one where the fuel-air vapor is too lean ortoo rich to burn or is inert as defined below. Forthe purposes ofthis 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 vaporproperties. 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 vaporpropertieswithin any section ofthe 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 vaportemperature that is significantly different from that ofthe adjacent fuel, and a corresponding vapor content that does not follow the relationship offuel temperature and vapor constituency and flammability for a closed tank system. This renders the fuel temperature itselfas an inaccurate indicator ofthe flammability condition ofthe tanks, as assumed whenthe rule
was written. 14 CFRsection 25.981(b) requires substantiation ofcompliance according to AppendixN, which in tumrequires the use ofthe method "Fuel TankFlammability Assessment Method User's Manual, dated May 2008, documentnumberDOT/FAA/AR05/8" (Monte Carlo method). This method uses "BulkAverage Fuel Temperature" as the primary indicator oftank flammability. Quiet Wing argues thatthis 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 ofthetank.

QuietWingAerospace, LLC

12324 !34th Court NE Redmond, WA 98052 425 451 8565

### Public Interest:

The intention ofRule 14 CFR section 25.981 is to reduce the probability offuel tank explosions. Aircraft operators 

# QUIETWING

are required to bringtheir fleets into compliance withthe rule byDecember 2017. Atthis 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 ofsafety as the modifications currently available. It is in the public interestto have available a lower cost and simpler solutionthatprovides the same level ofsafety. This will reduce operator costs and ultimately reduce airfares for the public.


### Safety:

The existingrule requires the use offuel temperature as the primary indicator offuel tank flammability. However, fuel tankflammability is determined notby fuel tank temperature alone, butbyfuel vaportemperature in the tank, as well as fuel to airratio of the vapor. The Quiet Wing modificationreduces both the fuel vaportemperature and fuel to air ratio, bringingthe tankflammability into compliance with 14 CFR section 25.981. As such, it results in the same orhigher level ofsafety as ifthe fuel temperature were in fact equal to the fuel vaportemperature for an unmodified tank configuration. Furthermore, Quiet Wing has developed a method ofanalysis thatensures thatthe resulting modificationhas equal or lower flammability than a corresponding configuration having equal fuel and vaportemperatures, 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 AppendixN, subparagraph N.2(a).

The exemption requested would allow the consideration ofexternal effects onvapor flammability instead offuel temperature alone in the determination oftankflammability,
as specified in AppendixN25.2 paragraph (a), this being the method ofdetermination of tank flammability required by 14 CFR section 25.981(b).

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

12324 134th Court NE Redmond, WA 98052 425 451 8565

### APPENDIX A EQUIVALENT FLAMMABILITY SUBSTANTIATIONThe Monte Carlo analysis is the substantiation tool defined by AppendixN 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 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 notto the condition ofthe liquid fuel, butto the properties ofthe vapor in the tank, i.e. the temperature and pressure ofthe air/fuel vapor, and the ratio offuel to air in the vapor. The simplified method in the 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 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 vaporpressure, but also other related processes occurring in the tank such as condensation on the walls ofthe tank or other objects inside the tank. Therefore, the relation between the liquid fuel temperature and pressure, and the flammability ofthe vaporrepresents the combined result ofvarious physical processes.

The above scenario, and the validity ofthe simplified Monte Carlo analysis depends on the conditions inside the tank being in some form ofclosed equilibrium, otherwise there
can be no definable relation between only the fuel temperature at a particularpoint in time, and the flammable condition ofthe vapor. The point becomes particularly important when dealing with tank configurations that include non-standard effects such as ullage purging orcooling. However, notwithstanding the breakdown ofthe simple fuel temperature relationship, the physics offlammability remain essentially unchanged for the vapor condition. In other 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 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 orthe vaportemperature. The vapor will or will not be flammable based only on the 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 in predicting 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 in the modified configuration to the fuel temperature in the basic tank configuration having the same, or lower, vapor flammability. The FAA have pointed outthat the simplified method ofrelating liquid fuel temperature to flammability ofthe tank takes account the various otherprocesses taking place in the 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 thatthese 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 notviolate the assumptions made in establishing the Appendix M limits, but results in a tank flammability condition equivalentto a standard tank configuration with a lower fuel temperature. This would be established by tests ofthe basic and modified configurations under various atmospheric conditions.

The fact that condensation or other factors affected the flammability ofthe standard configuration has no bearing on the validity ofthe proposed method, since the proposed method establishes a flammability equivalency by direct measurement ofthe actual vapor conditions for both the basic and modified configurations. Furthermore, it mustbe recognized that the Monte Carlo method itselfis far from an exactpredictor of flammability. Over a wide range ofoperating conditions, using only the two parameters ofliquid fuel temperature and pressure cannot possibly provide an accurate determination offlammability in every situation, particularly ifullage purging is occurring. The proposed method establishes flammability equivalency by direct measurement ofthe vaporproperties.