0. Background: Inerting History
Federal Aviation
Administration
Background:
Inerting History
AAR-440 Fire Safety Branch Wm. J. Hughes Technical Center Federal Aviation Administration

1. 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

2. 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

3. 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

4. Hollow Fiber Membrane for Air Separation
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)

5. HFM Air Separation Module Construction
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)
Air In
NEA
Out
OEA Vent

6. ASM Performance
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

7. 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

8. FAA Simplified Inerting System Block Diagram

9. 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

10. CAD Rendering of FAA Inerting System Prototype