Ground Based Fuel Tank Inerting

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Inerting of a Scale 747SP Center-Wing Fuel Tank
During a Typical Commercial Flight Profile
William CavageAAR-440 Fire Safety
ResearchFederal Aviation Administration
March 26-27, 2003International Aircraft Systems
FireProtection Working GroupPhoenix, Az
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Outline

• Background
• Model / Instrumentation
• Test Data
• Effect of Holds at 5K Feet
• Running System on Ground
• Effect of Deposit Schemes
• Starting Descent Altitude
• Blocking Vent System Dive Port
• Summary

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Background

• FAA is seeking to improve upon existing fuel tank
  safety in fleet in the wake of TWA800 air
  disaster
• Inerting of fuel tanks could provide significant
  fuel tank protection.
• Focus of the testing is to validate the ability
  of the FAA simplified fuel tank inerting system
  to inert the CWT of a 747SP during a typical
  commercial flight profile
• Use modeling results to validate modeling methods
  with full-scale data
• Study inert gas distribution during the
  commercial mission

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Description of Model

• Quarter-scale model of Boeing 747SP CWT was built
  from three-quarter inch plywood by scaling
  drawings from Shepherd report
• 24 length scale (1.4 Volume)
• Spars and spanwise beams simulated with
  quarter-inch plywood installed in slats with
  scaled penetration holes
• Vent system simulated with PVC tubing plumbed to
  an aluminum vent channel
• Removable lid to allow for model maintenance and
  modification
• Model in 6x6x7 altitude chamber
• Model inerted with manual NEA mixer

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Photo of Model
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747 SP Bay Diagram with Volume Data
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Instrumentation

• Onboard oxygen analysis system (OBOAS) acquired
  bay oxygen concentration data
• One sample port in each bay
• Sample returned to tank through manifold
• Thermocouple in chamber gave temperature
• Altitude measured by absolute pressure transducer
• NEA Flow metered/measured with mass flow
  controller and oxygen concentration determined
  with flow through type oxygen analyzer

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Onboard Oxygen Analysis System Block Diagram
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Scale Tank Testing Block Diagram
Altitude Chamber
NEA Generator
Flow Controller
Nitrogen
NEA Mixer
Oxygen Analyzer
Compressed Air
Scale Tank Model
DAS
OBOAS
Computer
Sample Return
T
Pressure Transducer
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Scope of Testing to Date

• All testing used same generic flight profile with
  different cruise times and different holds at 5K
  feet
• All testing uses same predicted system
  performance in terms of NEA flow and purity
  during above mentioned mission
• All tests had right side vent system blocked
• Some tests had aft port on open vent side also
  blocked

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OBIGG System Model
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Effect of Hold at 5K Feet

• Baseline case (no hold) repeated with two
  different hold times
• Hold at 5K feet
• 5 and 10 minute holds
• Results indicate that holds using high flow mode
  have little effect on both tank average oxygen
  concentrations and worst bay oxygen
  concentrations
• System is depositing NEA at approximate oxygen
  concentration as tank
• 5-10 minutes not that long to improve
  distribution for relatively small spread

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747SP Scale Fuel Tank Inerting Data
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747SP Scale Fuel Tank Inerting Data
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747SP Scale Fuel Tank Inerting Data
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Effect of Hold at 5K Feet (Contd)

• Ideally like to decrease tank average oxygen
  concentration before you use high flow mode to
  improve distribution
• Use low flow mode during a hold at 5K feet
  (lowers average tank oxygen concentration)
• Switch back to high flow for final descent
  (distributes)
• Results illustrate this flow methodology improves
  overall average tank oxygen concentration at
  touchdown but doesnt increase in the worst bay
  oxygen concentration
• Using high flow mode to distribute gas has
  diminishing returns
• Effect of sample system minimal when checked

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747SP Scale Fuel Tank Inerting Data
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747SP Scale Fuel Tank Inerting Data
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747SP Scale Fuel Tank Inerting Data
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Running System After Touchdown

• Repeated baseline case touchdown data with two
  tests that run system on ground after touchdown
• 15 and 30 minutes system run times
• one test only sampled beginning and end of ground
  sit time
• Running System in Low Flow Mode After Touchdown
  did little for worst bay oxygen concentration
• Gave consistent benefit over time for tank
  average oxygen
• Running System in High Flow Mode After Touchdown
  did decrease bay oxygen concentration spread
• Average oxygen concentration changed little
• Again, diminishing returns on reducing spread

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747SP Scale Fuel Tank Inerting Data
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747SP Scale Fuel Tank Inerting Data
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747SP Scale Fuel Tank Inerting Data
(High Flow Mode)
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Deposit Scheme Comparison

• Compared running system after touchdown with two
  different deposit scenarios
• Duel deposit method deposits low flow mode in bay
  6 and high flow mode in bays 1 and 3
• Multi-deposit method deposits all flow into bay 6
  and bay 2 (approximate equal split)
• Baseline used low flow mode during 10 minute hold
  pattern
• Fancy deposit schemes did little to improve
  touchdown average or worst bay oxygen
  concentration
• More work needed
• Repeatability check gave excellent results

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747SP Scale Fuel Tank Inerting Data
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747SP Scale Fuel Tank Inerting Data
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747SP Scale Fuel Tank Inerting Data
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Starting Altitude Comparison

• Compared original baseline descent case (from 39K
  feet) with two additional starting altitudes (32K
  and 25K)
• Used same altitude profile with different
  starting point (same descent rates) with no hold
• Approximated system performance best possible
• Results were as expected
• Starting altitude will have dramatic effect on
  resulting average oxygen concentration provided
  the tank is consistently inert for all cases

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747SP Scale Fuel Tank Inerting Data
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Effect of Blocking Dive Port

• Existing venting scheme does not give optimal
  inerting efficiency
• Illustrated in constant inerting tests
• Would improve by blocking dive port (aft vent
  port) on open side
• Repeated baseline test data (no hold) with this
  venting configuration
• No effect on average tank oxygen concentration
• Had adverse effect on distribution with 2 higher
  spike in worst bay and a full 3 greater
  resulting worst bay oxygen concentration
• Compared two deposit schemes with this venting
  config
• Use 10 min hold for comparison case
• Only small effect observed on bay 1 spike and on
  result

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747SP Scale Fuel Tank Inerting Data
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747SP Scale Fuel Tank Inerting Data
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747SP Scale Fuel Tank Inerting Data
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Summary

• Scale model data shows FAA inerting methodology
  sound given the predicted system performance
• System could be slightly undersized
• Using high flow mode only, decent hold buy you
  very little in terms of average of worst bay
  oxygen concentrations
• Running system on ground after touchdown in high
  flow mode will decrease oxygen concentration
  spread, but has diminishing returns (when average
  oxygen concentration is near 12)
• More elaborate deposit schemes and system
  methodologies give relatively small performance
  benefit, but could improve inerting capability
  significantly of a marginal system