The Effect of Fuel on an Inert Ullage in Commercial Airplane Fuel Tanks

1. The Effect of Fuel on an Inert Ullage in Commercial Airplane Fuel Tanks William CavageAAR-440 Fire Safety BranchWm. J. Hughes Technical CenterFederal Aviation Administration International Systems Fire Protection Working Group Tropicana Casino and Resort Atlantic City, NJ November 1-2, 2005

2. Outline • Background • Test Article • Test Methods • Calculations • Results • Summary AAR-440 Fire Safety R&D

3. Background • FAA developed a proof of concept inerting system to illustrate the feasibility of fuel tank inerting • FAA intends to make a rule requiring flammability control of some or all CWTS with an emphasis on inerting system technologies • The effect of adjacent fuel loads on an inert ullage has not been studied thoroughly • Air in fuel can evolve and spoil the inert atmosphere in the ullage • Military work indicates “fuel scrubbing” is necessary to prevent large increases in ullage oxygen concentrations with high fuel loads • Need to know what considerations need be made to account for adjacent fuel loads (more NEA required?) • Commercial airlines have no intention of scrubbing fuel • Fuel tanks effected by rule tend to have low fuel loads AAR-440 Fire Safety R&D

4. Test Article • Used a 3x3x2 ft rectangular tank made for fuel tank flammability and inerting research • Instrumentation panel installed to allow for gas samples, thermocouples, and inerting agent to pass through • Used lab oxygen analyer for sea level work and single channel altitude analyzer (similar to OBOAS) for altitude work • Could deposit nitrogen, air, or NEA into the tank depending upon the needs of the experiment • Had manifold installed in the bottom of tank to allow for gases to be passed through the fuel • Selector valve allowed for air, ullage gas, or NEA to be passed through the manifold to scrub fuel, revive fuel, or equalize the ullage gases with the fuel gases AAR-440 Fire Safety R&D

5. Block Diagram of Experiment Configuration AAR-440 Fire Safety R&D

6. Test Methods – 2 Primary Areas • Sea level testing focused on the change in [O2] due to fuel load when tank is brought to equilibrium • Looked at how to bring fuel/ullage to stable state • Quantified the change in oxygen concentration due to fuel load • Examined the benefit of inerting the ullage through the fuel (rudimentary scrubbing) • Altitude testing focused on quantifying the altitude effects for both equilibrium state and potentially what would be seen in a commercial transport fuel tank • Validated measured sea level changes and quantified altitude effects • Examined what stimulates oxygen evolution from fuel • Simulated two flight tests to determine modeling capability AAR-440 Fire Safety R&D

7. Calculations – Two Ways Ullage [O2] Increases • Tank air entry due to fuel consumption • Tanks normally vented to atmospheric pressure • Use inerting equation with fuel consumption is VTE and inerting gas is air (20.9% oxygen concentration) • Change in [O2] due to air evolving from fuel • Solve a series of equations that equalize the partial pressure of oxygen and nitrogen across the fuel given the Ostwald Coefficient Mass of O2 in system is constant and partial pressure of O2 in Ullage and fuel equal at state 2 calculate mass of oxygen at state 2 given conditions at state 1 calculate partial pressure O2 with equation of state AAR-440 Fire Safety R&D

8. Results – Sea Level Testing • Stimulation Methods Studied • Best method by far was ullage recirculation which highlights the fact that “oxygen evolution” is a misnomer, the process is an exchange of gases to bring partial pressures of fuel/ullage gases to equilibrium • Resulting increase in oxygen concentration due to adjacent fuel (maximum increase) was measured/calculated • Calculations match measured numbers fairly well • The benefits of inerting through fuel (rudimentary scrubbing) • Illustrated some benefit by depositing inert gas at the bottom of a fuel tank, allowing the inert gas to displace some O2 • Requires more inert gas per volume of ullage to inert in this manner, but still less inert gas then required to inert empty tank AAR-440 Fire Safety R&D

9. Increase in [O2] Over Time with Different Stimulations AAR-440 Fire Safety R&D

10. Resulting Maximum Increase in Ullage [O2] due to Fuel AAR-440 Fire Safety R&D

11. Benefit of Inerting Through Fuel Note: Inerting through manifold required more 5% NEA for the same ullage volume AAR-440 Fire Safety R&D

12. Results – Altitude Testing • Effect of altitude on ullage oxygen concentration quantified • After ullage is at equilibrium with fuel at sea level, increase in altitude (decrease in ullage pressure) causes partial pressure imbalance • Used ullage recirc at three altitudes and illustrated consistent results with poor agreement to calculations • Altitude stimulation work increase examined qualitatively • Besides ullage recirc, examined fuel pumping and altitude change only as potential methods of balancing the ullage/fuel gas partial pressures • Simulation of Boeing GBI flight tests compared fair • Simulation was a performed with no fuel/ullage stimulation (altitude only) and compared with GBI flight tests (not the descent portion) • Results illustrated qualitatively that altitude stimulation was closest studied to flight test data but more work is needed to optimize results AAR-440 Fire Safety R&D

13. Change in [O2] Increase due to Altitude AAR-440 Fire Safety R&D

14. Resulting Max Increase in Ullage [O2] due to Altitude Initial Oxygen Concentration 8% Stimulated with Ullage Recirc AAR-440 Fire Safety R&D

15. Comparison of Stimulation Methods at Altitude AAR-440 Fire Safety R&D

16. Comparison of Lab Simulation with Flight Test Data AAR-440 Fire Safety R&D

17. Summary • Oxygen evolving from fuel is a misnomer, changes in ullage oxygen concentration due to adjacent fuel are a result of the equalization of the partial pressures of gases at the fuel/ullage interface and is difficult to get without mixing fuel/ullage together • Measured sea level increases in ullage oxygen concentration match well with calculations • Some benefit can be garnered from remedial fuel scrubbing by inerting through fuel but more NEA / ullage volume is required • Changes in ullage altitude cause additional increases in ullage oxygen concentration from fuel with calculations agreeing poorly • Lab experiments can simulate flight test results with some accuracy with very little stimulation needed to match results AAR-440 Fire Safety R&D