FRM: Nitrogen System

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

• FAA Fire Safety Research developed prototype
  OBIGGS to demonstrate its 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

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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

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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)

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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

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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

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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)

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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

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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

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Comparison of System Performance During Takeoff
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Comparison of System Performance During Landing
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Comparison of System Performance During Takeoff
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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)

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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

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Block Diagrams of Continuous Gas Sampling Methods
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Vacuum Bottle Gas Sample Assembly for Flight Test
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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

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FAA Oxygen Concentration Measurement Method
Fuel Tank Vapor Fuel Tank Liquid Pressurized
Air Electrical Power Electronic Signals
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OBOAS Validation Data from Airbus Flight Test
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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

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Good and Poor Mixing Data from Ground Inerting
Test
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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