Development of a Model to Calculate Fuel Tank Flammability

International Fire and Cabin Safety Conference, Nov 2004

Development of a Model to Calculate Fuel Tank Flammability

Ivor ThomasConsultant to FAA

1-425 455 [email protected]


Presentation Agenda

• Concept

• Approach

• Assumptions

• Results

• Conclusions


Initial Problem

• Need: To be able to assess flammability in airplane fuel tanks so that safety enhancements could be assessed against each other.

• Problem: Flights all over the world create widely varying conditions and times when a tank may be flammable, but FAA needed to assess the overall safety benefits of any enhancement


Concept

• Create a computer model to:– Assess tank flammability for a large number

of flights throughout the world, – Assess the impact of any enhancements on

reducing overall flammability, – look at risks in specific conditions, – (and make it simple enough to run quickly).


Approach

• Monte Carlo technique to create several thousand flights in worldwide atmospheric conditions with critical variables such as flash point of the fuel also varying to represent the real world.

• This approach required several sub-models– The airplane performance– The tank thermal response– The atmosphere– The fuel– The system enhancement proposed


Monte Carlo Analysis

• Technique to allow a statistical analysis of a problem with a number of independent variables

• Technique uses known distribution probabilities for variables and runs 1000’s of cases with randomly selected values for each variable in each case.

• End result is a large amount of data, but typically the fleet average exposure is used as a reference for the tank in question.

• Specific high risk areas can be examined, such as days above 80 deg F


Airplane Performance

• An airplane performance model was developed to allow various airplanes to be studied, which included– Time on the ground– Fuel load– Climb: Time/Speed schedule– Cruise: Alt. And Mn. including step climbs on longer

flights– Descent and Landing: Time/Speed schedule in

Descent– Mission Length Distribution


Flammability studies

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Time- minutes

Alti

tude

100

0's

ft

z


Flight Length Distribution

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200

250

300

Mission Length (nm)

# of

Flig

hts

in 2

00 n

m

Incr

emen

ts


Tank Thermal Response

• Tank treated as simple object with thermal response characteristics determined from separate thermal modeling or flight test.– Characteristics defined by:

• Exponential time constant for full and empty conditions both ground and flight

• Equilibrium temperature the tank would reach (given enough time) relative to total air temperature for both ground and flight


TankTemperatures vs TimePacks On,

-40

-20

0

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120

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160

180

0 100 200 300 400 500 600 700

Time -Minutes

Tem

pe

ratu

re -

De

g F

Outbd Main

Inbd Main

Center Wing Tank

TAT


Atmospheric model

• Any flight uses two inputs, – Ground ambient, and – Ambient temperature above the Tropopause.

• For any given flight, the two values are picked randomly to match the known world temperature distribution


Probability Distribution

-100

-50

0

50

100

150

200

0 20 40 60 80 100

Cummulative percentage

Tem

pera

ture

(de

g F) Ground Amb.

Cruise amb.


Atmospheric model

• The Temperature Profile versus Altitude is the determined, using a standard lapse rate to the tropopause, and constant above, with a temperature inversion effect if the ground ambient is below 00 F.


Ambient and TAT Temperatures vs Altitude

0

5000

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15000

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50000

-60 -40 -20 0 20 40 60 80 100 120 140 160 180 200

Temperature Deg F

Alt

itu

de

1000

's f

t

TAT

AmbientTemp



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10000

15000

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Temperature Deg F

Alt

itu

de

1000

's f

t

TAT

AmbientTemp


Fuel Variability and Flammability

• The FAA has surveyed the fuels being used by the fleet and determined the flash point range and distribution,

• Fuel Air Ratio at the Flash Point has been measured for a number of these fuels, and this has been used to correlate Flammability range to Flash Point


FAA Jet Fuel Flash Point Survey

80.0

90.0

100.0

110.0

120.0

130.0

140.0

0 0.5 1

Percent of samples

Fla

sh P

oin

t d

eg F

SurveyResults

GaussianMean=120F, 1 sigma= 8F


Flammablity Range relative to Flash Point

0

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30000

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60000

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Temperature relative to Flash Point

Alt

itu

de

Ft.

LFL

UFL

Too Rich

Too Lean


Flammability Assessment

• A Computer model was created to integrate all the factors discussed to predict fuel tank flammability– Model can run one flight to look at specific

risk, or– Model can run several thousand flights to

determine fleet average fuel tank flammability exposure.

– Model can determine flammability for specific ranges of conditions


Flammability studies

-40-20

020406080

100120140160180200

0 500 1000

Time- minutes

Tem

pe

ratu

re-

De

g F

0

20

40

60

80

100

LFL

UFL

TFuel

TAT

z

Flammable Time

Flammability %



0

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10000

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-60 -40 -20 0 20 40 60 80 100 120 140 160 180 200

Temperature Deg F

Alt

itu

de

1000

's f

t

LFL

UFL

TFuel

TAT

AmbientTemp


Distribution of Flammability Exposure for All Flights

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100

0 200 400 600 800 1000

Flight number

Fla

mm

abili

ty E

xpo

sure

, % o

f m

issi

on

ti

me


Potential Mitigating Effects

• The model can be used to assess mitigating systems– Reduced heat flow to the tank– Increased heat flow out of the tank– Fuel Tank Inerting

• Ground Only• In-flight• In-Flight with limitations (e.g. High nitrogen flow in

Descent)

– Other Flammability Reduction methods


Potential Mitigating Effects (Continued)

• The model must be modified by the user to represent the reduction methodology.– For any system the model computes if the

tank flammable every one minute. The user can add instructions to overlay this and change a “flammable’ count to a “non-flammable” count if the system is effective at that point in the flight.


Potential Mitigating Effects (Continued)

• For Example, – A ground based inerting system is effective

once the tank is flooded with N2, and will stay that way until the tank breathes in sufficient air to dilute the N2.

• IF the tank is empty, the tank remaining inert until the end of cruise,

• IF the tank is full, the tank will become un-inert early in cruise

– Algorithms added to the model can compute this and reduce flammability appropriately.


Distribution of Flammability Exposure for All Flightswith potential Inerting System

0

20

40

60

80

100

0 200 400 600 800 1000

Flight number

Fla

mm

abili

ty E

xpo

sure

, % o

f m

issi

on

tim

e


Current Status

• The Flammability Exposure model has given the FAA and industry a common tool to assess fuel tank flammability and to evaluate potential mitigating actions.

• The 747 proposed Special Condition uses the model to define the current and expected flammability exposure when an NGS or other flammability reduction method is added.


Current Status

• FAA is drafting a revised AC25.981-2 with the assistance of AIA to reflect this approach.


Model Enhancements

• FAA model is generic and must be made specific for a given airplane.– Thermal models– Fleet mission distribution– Daily flight distribution effects– Mitigation System effects


Cautions

• The original model was developed by ARAC I to assess the correlation of flammability to the fleet accident history

• ARAC I concluded that low flammability tanks, (fleet average exposure below 7%) could be considered acceptable but high flammability tanks needed t o be addressed

• As the model is refined and modified the 7% may need to be examined for it’s validity.


Conclusions

• FAA has provided a tool to allow the industry to assess flammability in fuel tanks and to evaluate the effectiveness of mitigating actions.

• Enhancements could be incorporated but we must recognize the model is a comparative tool not an absolute predictor of flammability.

• The baseline “Low Flammability” level of 7% needs to be examined carefully if too many enhancements to the model are made.


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Development of a Model to Calculate Fuel Tank Flammability