# Presentation: 818191

## Fuel Tank Flammability Modeling

- Ivor Thomas
- Chief Scientific and Technical Advisor
- to the FAA,
- Fuel System Design
- _[ivor.thomas@faa.gov](mailto:Ivor.thomas@faa.gov)_
- 425 227 1132

## Agenda

- Concept
- Approach
- Assumptions
- Results
- Questions

## 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 an 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 overall average but doesn’t look at specific risk on any one case.

## Airplane Performance

- A simple 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

## Slide 8

## Slide 9

## 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**

## Slide 11

## 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

## Slide 13

## 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.

## Slide 15

## Slide 16

## 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

## Slide 18

## Slide 19

## Flammability Assessment

- An 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.

## Slide 21

## Slide 22

## Slide 23

## 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.  Low nitrogen flow in Descent)

## Slide 25

## Conclusions

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

## Open Issues

- Flammability limits with altitude and with various levels of Nitrogen in the ullage
- FAA testing in progress
- Flammability Limits with very low fuel quantities and time dependent effects of non-equilibrium conditions
- FAA /Rutgers University work in progress

## Questions??