1Federal AviationAdministration

FAA Fire Safety BranchSeptember 9-11

Background: Inerting History

Federal AviationAdministration

AAR-440 Fire Safety BranchWm. J. Hughes Technical CenterFederal Aviation Administration

2Federal AviationAdministration

FAA Fire Safety BranchSeptember 9-11

Inerting Background• Inerting refers to rendering the ullage (air above fuel)

unable to propagate a reaction given flammable conditions and ignition source− In this case Refers specifically to reducing tank oxygen

concentration− Other methods of compliance possible

• Fire Triangle must be satisfied to have a reaction (explosion) in the ullage of a fuel tank− Ignition Source− Correct ratio of fuel and air

3Federal AviationAdministration

FAA Fire Safety BranchSeptember 9-11

Fuel Tank Inerting History

• Inerting has been studied since 1950s

• Stored gas inerting used by military in 1970s

− FAA built and tested demo cryogenic nitrogen system on DC-9

− FAA demo Post Crash effectiveness

• DOD did OBIGGS research using PSA and ASMs

• C-5, C-17, C-130, Fighter Aircraft

− Found HFM technology made ASMs cost effective for OBIGGS

4Federal AviationAdministration

FAA Fire Safety BranchSeptember 9-11

Fuel Tank Inerting History

• FAA research illustrated fuel tank inerting could be practical if applied in a cost effective manner− Initially focused on ASM performance for fire suppression

capabilities− After second ARAC, focused on using ASMs to generate inert gas

on an aircraft from bleed air during the flight cycle− FAA experiments agree with previous experiments and indicated

that a tank oxygen concentration below 12% will render tank inert

5Federal AviationAdministration

FAA Fire Safety BranchSeptember 9-11

• Hollow fiber membrane (HFM) technology uses the selective permeation properties of certain materials to separate air into two streams, one nitrogen enriched air (NEA) and the other oxygen rich (relative to air)− As air is forced through the fibers, fast gases escape through the

fiber wall and the nitrogen rich stream to pass through the fiber core

− The separation is not direct as some oxygen passes through the core and some nitrogen ventilates from the fiber

− As flow through the core is slowed (back pressure), more fast gases escape (and N2) making the core stream (product) more pure (less O2)

Hollow Fiber Membrane for Air Separation

6Federal AviationAdministration

FAA Fire Safety BranchSeptember 9-11

• HFM materials are woven into hair-sized fibers and bundled by the thousands into a canister called an air separation module (ASM)− Fibers are generally potted into an epoxy sheet at each end with

some structure for strength− The epoxy sheet is machined flat, to expose the fibers to the

pressurized air source− The ASM is mounted in a canister to facilitate pressurized air feed,

gather the product (NEA), and collect the ventilated waste gas (OEA)

HFM Air Separation Module Construction

Air InNEA Out

OEA Vent

7Federal AviationAdministration

FAA Fire Safety BranchSeptember 9-11

• Primary Parameters used to describe ASM performance are permeability and selectivity− Permeability describes how much air flows into the ASM

− Selectivity describes what percentage of that air becomes NEA

• These performance parameters are a function of ASM feed pressure, OEA vent pressure, and system back pressure− Given these three parameters the ASM will flow a specific NEA

volume and purity (residual O2 concentration)

− This is only valid at a given operational temperature

− Specific flow and purity is described by the manufacturer in terms of the above two stated parameters

ASM Performance

8Federal AviationAdministration

FAA Fire Safety BranchSeptember 9-11

Simplified Inert Gas Generation System Concept

• Concept utilizes ASMs in a two flow methodology− Uses low flow mode during taxi, takeoff, ascent, and cruise to

deplete CWT of oxygen almost completely− Uses high flow mode during descent to offset (but not eliminate) the

air entering the fuel tank vent system resulting in a net inert fuel tank oxygen concentration

• Does not need to run on ground or store NEA, eliminates need for compressors or ground service equipment

• System only needing to reduce the oxygen concentration below 12% (by FAA research) makes sizing more realistic

9Federal AviationAdministration

FAA Fire Safety BranchSeptember 9-11

FAA Simplified Inerting System Block Diagram

HeatExchanger

Cool Air Source

Exi

stin

g A

ircr

aft B

leed

air

Supp

ly

Filter

Heater

Shut-OffValve

Variable/High Flow Valve

Fuel Tank

ThrottlingValve

AS

M

System Controller

TemperatureSensor

AS

M

AS

M

Air

craf

tP

ower

Oxygen RichWaste Gas

Waste StreamDiscarded

Nitrogen RichProduct Gas

Low Flow Orifice

10Federal AviationAdministration

FAA Fire Safety BranchSeptember 9-11

FAA Inerting System Construction• Uses 3 ASMs based on HFM technology

− Excepts 350 degree F air from aircraft bleed system through an SOV

− Uses a H/x to cool air to 180˚F +/- 10˚F and a filter to condition air− Air is separated by ASMs and NEA is plumbed to output valves to

control flow, OEA is dumped overboard with H/x cooling air− System flow control is accomplished with low flow orifice and high

flow control valve

• System controlled by control box in cabin that is connected to system with cable

• Prototype built on aluminum pallet for ease of construction and to support a wide variety of installation methods

11Federal AviationAdministration

FAA Fire Safety BranchSeptember 9-11

CAD Rendering of FAA Inerting System Prototype