1. An Update on FAA Fuel Tank Ullage Modeling
William Cavage, Steven Summer, and Robert Ochs AAR-440 Fire Safety Branch Wm. J. Hughes Technical Center Federal Aviation Administration
International Systems Fire Protection Working Group CAA London, England UK April 19-20, 2006

2. Outline Background Models Employed Flight Test Results Comparison
Representative Flight Modeling
AAR-440 Fire Safety R&D

3. Background
FAA developed a proof of concept inerting system to inert the CWT of classic style Boeing model 747
FAA intends to make a rule requiring flammability control of some or all CWTS with an emphasis on inerting system technologies
Fire Safety research has been working to expand modeling capabilities to allow for systems analysis, trade studies, etc.
Used lab research and flight test data to develop, expand, and validate a multiple bay fuel tank inerting model and an ullage flammability model
Performed analysis of a single fictitious flight to examine the SOA of our modeling capability
Developed a more typical 747 flight profile with a bigger OBIGGS than the NASA flight test system
AAR-440 Fire Safety R&D

4. Models Employed
Multiple bay inerting model same as model discussed in AIAA paper (see web site) with some small improvements
Tracks mass of oxygen in and out of each bay knowing NEA flow and given a set of generic flow splits between bays with no back pressure
Tracks mass of ullage knowing the change in density (given altitude changes) with no mass storage in tank
Model does these calculations in a time step process
Ullage flammability model is the evaporation/condensation model by Polymeropoulos & Ochs and discussed in Lisbon
Uses a generic fuel species model based on flash point developed by Sagebiel
Calculates the equilibrium state of the ullage given fuel temperature and then determines condensation effect from walls/ceiling in time
AAR-440 Fire Safety R&D

5. Results from Flight Test Comparison
Inerting model illustrated excellent agreement with measured flight test data with some exception
Good agreement is expected since a similar test was used to develop the bay to bay flow split ratios
Bay 4 trends look good but value is considerably off (the problem bay)
Resulting ullage average time values very consistent
Flammability model data trend results very good
Magnitude of data peaks and troughs illustrate significant deviation although measured data has some uncertainty so it is unclear how well the model duplicates flight test results
Models uses no empirical relationships to obtain results
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6. Comparison of Flight Test Results with FAA Model
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7. Comparison of Flight Test Results with FAA Model
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8. Results – Representative Flight
Developed flight profile, OBIGGS performance, and temperature profile from existing flight test data taking into account a typical 747 flight profile
Difficult to determine precise temperature peaks and troughs but did best to approximate
OBIGGS was assumed to 33% bigger (1 more ASM)
Data trends as expected but very little we can do to validate the magnitude of the results
Regardless of peak oxygen concentration in single bay, tank remained nonflammable during entire flight
Tank flammability dropped below LEL during peak oxygen concentration rise in bay 1
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9. Input Data for 747 Modeling Exercise
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10. Results of Modeling Exercise
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11. Summary
Analysis of FAA Fire Safety model capabilities illustrates good fidelity for this single aircraft type
Remains to be seen how this work could translate to other multiple bay CWT aircraft types
Need additional work to be able to simulate different deposit and venting schemes
No additional modeling work planned to date
AAR-440 Fire Safety R&D