Inerting of Commercial Airplane Fuel Tanks

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Inerting Systems for Commercial Airplane Fuel
Tanks Alan Grim Boeing Commercial
Airplanes November 18, 2004
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Summary

• Brief History
• System Overview
• Airplane Safety Considerations
• Hot Day Operations
• Goal
• Boeing Philosophy

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Brief History

• Military use of fuel tank inerting
• Designed for military threats
• Primary protection for routine combat threats
• Full time inerting / all tanks
• 1950s visible light justified 9 or 9.8 O2
  inert definition
• Various implementations including liquid nitrogen
  storage, pressure swing absorption, halon, etc
• Typically low reliability
• Typically heavy
• Not included in non-front line military aircraft
• Not practical for commercial application

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Brief History

• FAA Inerting Study in 1970s Not practical
• 1996 NTSB Recommendation following Flight 800
  accident
• FAA initiated ARAC teams to study flammability
  reduction and inerting for commercial use
• 1998 ARAC Studied flammability reduction options
• Recommended rule for new design to reduce
  flammability
• 2001 ARAC focused on Inerting
• Ground based
• On board in flight
• Recommended further development of onboard
  generation
• System still not practical in 2001
• Cost, weight, reliability all issues

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Brief History

• Changes that enable a cost effective, practical
  FRS
• FAA Testing validated that an Inert Benchmark of
  12 O2 precludes significant pressure rise for
  vast majority of commercial conditions
• Use of Hollow Fiber Membranes
• Applying an average risk fleet wide safety
  assessment (Monte Carlo)
• Reducing flammability exposure to levels at least
  equivalent to wing tanks will provide an order of
  magnitude improvement
• Defining the system as non-critical to airplane
  operations
• Use of inerting as an additional level of
  protection to ignition protection
• Focus on high flammability exposure center wing
  tanks only

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System Overview
Nitrogen Generation System (NGS)
External Inputs
System Control
System Status / Indication
Float Valve
Center Fuel Tank
Ram Cooling Flow via Existing ECS Scoop
High Flow Descent Control Valve
NGS Shut-offvalve
T
Bleed Flow
NEA to Tank
Filter
Ozone Converter
Heat Exchanger
Waste OEA to Cooling Exhaust
Witness Drain / Test Port
Cooling flow and Oxygen Exhaust Overboard
NEA Nitrogen Enriched Air OEA Oxygen Enriched
Air
Simplified to protect Boeing Proprietary Data
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System Overview

• Airplane bleed flow/pressure source of air
• Bleed air typically up to 450F
• To hot for current fiber to handle
• ASM requires warm air with as much pressure as
  available
• Cooling of Bleed air required
• Utilize ECS ram air for cooling source
• Control temperature to ASM for optimum
  performance
• ASM separates O2 from air to generate NEA
• Purity dependant on pressure available
• OEA exhausted overboard
• NEA supplied to tank

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System Overview

• Multiple flow modes used to reduce bleed
  consumption
• Low flow used in climb and cruise
• Inerting performance good
• Bleed flow conserved directly related to fuel
  burn
• High flow used during descent
• Vent system modifications may be required
• Boeing Puget Sound airplanes vent to both wing
  tips
• Condition dubbed cross-venting results
• Design change required

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System Overview

• Simple distribution system required to remain
  practical
• System size dependent on even distribution of NEA
• Tank structure will have an effect on
  distribution
• Discrete vent points will affect design

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Safety Considerations

• Design Precautions that must be addressed to
  preclude creating additional hazards
• Prevent potential new ignition sources inside
  fuel tank
• Bond for electrostatics
• Prevent lightning energy entering tank
• 450F bleed system indirectly connected to fuel
  tank
• System must absolutely preclude 450F air from
  reaching tank
• Requires redundant independent shutoff methods
• Minimize Impact of Bleed air use on existing
  systems
• Cabin pressurization
• Ability to evacuate smoke from cabin
• Engine performance

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Safety Considerations

• Hazards to maintenance personnel
• Limit NEA concentration to protect maintenance
  personnel
• Fuel tank
• Confined spaces where NGS is installed or routed
• Modifications to fuel tank vent system must not
  result in tank over/under pressure conditions
• NGS failures
• Rapid climb/emergency descent
• Refueling failure cases

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Hot Day Operations

• Unexplained accidents occurred on 80F ambient
  temps and greater
• 2 ground incidents and 1 climb incident
• Analysis shows significant flammability exposure
  on 80 F days on ground and in climb
• FAA Proposed 747 Special Condition covers this
  scenario
• 3 Fleet Average
• 3 Ground 80F
• 3 Climb 80F
• Ground requirement will likely be system size
  driver

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Goal

• Practical System to provide order of magnitude
  improvement in Fuel Tank Safety
• Design and install a practical and effective
  system that protects the airplane
• Address ground and climb operations on warm days
• Designed to achieve 10 day MMEL Classification
• Minimal bleed air use impact on fuel burn
• Minimize weight impact
• Ensure Service Ready
• Do not introduce any new hazards
• No new ignition sources in fuel tank
• No hazards to people

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Boeing Philosophy

• Safe and Efficient Global Air Transportation
• Minimize potential for future accidents
• NGS is a safety enhancement
• Ignition protection alone has achieved its
  maturity limits
• NGS provides a secondary level of protection to
  mitigate human factors in design, manufacture,
  operation and maintenance
• Leading the Effort to Develop NGS
• Practical design
• Service Ready Systems available
• 747-400 4th Quarter 2005
• 737NG 2nd Quarter 2006
• 777, 737-3/4/500, 767 and 757 to follow
• NGS is standard for all tanks on 7E7
• NGS as an additional level of protection is the
  future for Boeing airplanes

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The Fourth Triennial International Aircraft Fire
and Cabin Safety Research Conference
The Fourth Triennial International Aircraft Fire
and Cabin Safety Research Conference