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FRM: Nitrogen System Validation

The Federal Aviation Administration (FAA) conducted ground and flight testing to validate the prototype OBIGGS system for fuel tank ullage flammability reduction. The system was installed and tested on a Boeing 747SP and an Airbus A320. The results showed the effectiveness of using a simplified inerting system as a flammability reduction method (FRM).

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FRM: Nitrogen System Validation

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  1. Federal Aviation Administration FRM: Nitrogen System Validation AAR-440 Fire Safety BranchWm. J. Hughes Technical CenterFederal Aviation Administration

  2. Background • FAA Fire Safety Research developed prototype OBIGGS to demonstrate it’s potential for fuel tank ullage flammability reduction to support FRM rulemaking • Built and tested inerting system based on the two flow concept • Validated system operation during ground and flight testing • Developed instrumentation and methods to validate both inerting system operation and FRM effectiveness • Measured inerting system parameters during aircraft operation and compared data with existing static performance data • Developed instrumentation system to measure oxygen concentration during flight testing

  3. Prototype Installation – Ground Testing • System installed in a Boeing 747SP with fully functioning systems and ground service equipment • Decommissioned from airline service and purchased by the FAA for Ground Testing Only • All major systems fully operational • Has independent power for test equipment and instrumentation • System installed between two structural members in empty pack bay as designed • Completely Instrumented • Oxygen sampling, pressure taps, and thermocouples on FRM to measure performance • Thermocouples in Pack Bay • Some Weather Data Available

  4. Measured Results – Ground Testing • Ground tested the inerting system on the 747SP test article • Operated inerting system with static conditions (best as possible) for the purposes of validating the system performance • Focused on the volume flow of 5% and 11% NEA that could be generated with varying ASM pressure at sea level • Testing Illustrated • Stable system performance is difficult to achieve in a dynamic aircraft environment • Inerting system has potential of being used as a flammability reduction method (FRM)

  5. Prototype Installation – In-Flight Demonstration • Installed FAA prototype inerting system on Airbus • System was installed on a cargo pallet and mounted in the aft cargo bay of an Airbus A320 used for the purpose of flight test • System and aircraft fully instrumented with extensive DAS capabilities • System modified to use only 1 ASM to match size to aircraft • Operated out of Airbus, France Flight Test Toulouse • Measured both system performance and ullage [O2] • Testing validated concept of using a simplified inerting system as an FRM

  6. Measured Results – In-Flight Demonstration • Compared dynamic ASM performance with static measurements • Operated on A320 flight test and compared to static lab data • Data comparison generally good with some big discrepancies Single ASM Test • Illustrates the difficulty in obtaining stable conditions during flight test • Obtaining 180 degree F air temperature inlet problematic • This makes model validation difficult

  7. Measured Results – In-Flight Demonstration • Observed oxygen concentration progression and distribution for typical short flight with a rapid descent • Need to show ullage oxygen concentration stays below 12% under flight cycle scenarios deemed important by engineering analysis • Data illustrated the effectiveness of NEA and reducing ullage flammability • Inert gas distributed easily with no mixing devices • Ullage behavior consistent with perfect mixing (easy to model)

  8. Prototype Installation – Total System Installation • Installed system in highly modified Boeing 747-100 • Modified and operated by NASA for the purposes of carrying a Space Shuttle Orbiter for operations and maintenance • Fully operational, standard, fuel system with unmodified pack bay • Standard 747 Multiple-bay / compartmentalized center-wing tank • Operated by excellent test pilots/crew and dedicated maintenance group • System Installed in pack bay as would be in fleet aircraft • Instrumentation • Aircraft is Fully Instrumented • Oxygen sampling, pressure taps, and thermocouples on system for OBIGGS performance • Thermocouples in Pack Bay Area • Pressure altitude measured

  9. Measured Results – Total System Installation • Dynamic performance during takeoff/ascent and descent/landing portion of flight was analyzed • Each flight different, with a range of varying system performance • These variations in performance had a small but measurable effect on the resulting average oxygen concentration observed in the ullage at the end of the flight cycle (at touch down) • System undersized for this aircraft • Average ullage oxygen concentration behavior very typical of previous observations • Peak average ullage oxygen concentrations around 12% • Peak forward bay (vented bay) oxygen concentrations were well above 12% on descent

  10. Comparison of System Performance During Takeoff

  11. Comparison of System Performance During Landing

  12. Comparison of System Performance During Takeoff

  13. Measured Results – Multi-Bay NEA Distribution • Inert gas distribution with single point deposit more problematic during flight test than in scale tests in lab • Observations illustrate inert gas should be distributed differently on descent to keep peak oxygen concentrations below 12% in forward tank vent bays • Distribution models duplicate acquired data poorly • Inert gas flow did not split from bay to bay IAW bay wall areas • Some bays do not illustrate perfect mixing (hard to model)

  14. Background - Measuring Oxygen Concentration • To demonstrate the potential for flammability reduction, Fire Safety developed an in-flight oxygen concentration measurement system • FAA (OBOAS) used during several flight tests to validate that the OBIGGS did reduce the flammability of the fuel tank ullage • FAA has studied the different methods of measuring oxygen concentration and has garnered many lessons • There are many ways to measure oxygen concentration • Many different sensors available on the market • Reactive/soluable (Galvanic, Zirconium Oxide • Non-consumable (Paramagnetic, Light Absorption) • Many different ways to apply an oxygen sensor • Continuous gas sample (regulated or unregulated) • Discrete gas sample • In situ sensor placement

  15. Block Diagrams of Continuous Gas Sampling Methods

  16. Vacuum Bottle Gas Sample Assembly for Flight Test

  17. On-board Oxygen Analysis System Development • Wanted to apply a traditional oxygen sensor/analyzer used in lab for a flight test application • Must provide constant condition (pressure/temperature) gas sample • Developed a gas sample system that can do this from 0-40,000 feet for a wide range of ullage temperatures • Used powerful diaphragm pumps with active pressure controllers to provide a constant pressure and stable temperature gas sample • Integrated with remote flow through galvanic cell sensors • System used flash arrestors, liquid traps, desiccating filters, and a ventilated containment shroud to ensure safety of flight operations • Results from test series were published in DOT reports • Data lag significant, loud, heavy, sample train not reliable • With some refinement, was easy to calibrate and gave good repeatable, results and filled an otherwise empty niche

  18. FAA Oxygen Concentration Measurement Method Fuel Tank Vapor Fuel Tank Liquid Pressurized Air Electrical Power Electronic Signals

  19. OBOAS Validation Data from Airbus Flight Test

  20. The Trouble with Accurate Measurements . . . • Your measurement can be very accurate, but if your gas sample location is not representative of the volume you are measuring, all is for naught • Observed steep (1-3% [O2]) vertical gradients in fuel tank bay for extended periods of time (½ - 1 hour) • OBIGGS flow tends to take care of the horizontal mixing whether it be one large bay or multiple partitions in the tank • Only observed steep gradients in a few isolated ground inerting cases where the fuel tank was otherwise quiescent • Multiple-bay tanks pose a challenge to determine what ullage areas will act as one area and what areas will not • Can always add more sample locations, but where? • Ideally applicant will use a system of modeling in advance with flight test validation to illustrate inert gas distribution

  21. Good and Poor Mixing Data from Ground Inerting Test

  22. New Inovations – Light Absorption with TLD • Oxygen concentration can be measured by examining the amount of light absorbed in a photo cell filled with the gas • This methodology has been around for years but was made more practical with the advent of tunable laser diodes (TLDs) • Recently developed instrument for flight test • Unit acquires an unregulated gas sample and passes it through a cell • System has elaborate calibration to compensate for sample pressure and temperature • Technology has been applied in-situ

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