GUY NORRIS / LOS ANGELES & DAVID LEARMOUNT / LONDON
Honeywell system designed to reduce risk of explosion by preventing ignition of fuel vapour to be tested on 747
Boeing plans to begin flight tests of a fuel tank inerting system on a 747-400 by the end of July, almost two years after such a system was ruled out on cost-benefit grounds by the US Federal Aviation Administration. Airbus also says it is working with the FAA to test inerting equipment on one of its aircraft.
Flight testing to assess the feasibility of a lightweight inerting system is expected to last for just over a month. Tests will begin pending clearance from the FAA, which Boeing briefed on 8 July along with the US National Transportation Safety Board (NTSB).
The system is aimed at reducing the risk of fuel tank explosions like those suspected as the cause of the Trans World Airlines 747-100 crash in 1996 (TWA 800) and the destruction of a Thai Airways Inter-national 737-400 in Bangkok in 2001. An inerting system usually replaces air in part-empty fuel tanks with nitrogen, preventing the ignition of fuel vapour.
The Honeywell system will use bleed air from the aircraft's engines, rather than requiring compressors and a nitrogen supply. Bleed air will be ducted from the engines via a heat exchanger and filter to an air separation module which will remove a proportion of the oxygen. The nitrogen-richer air will then be fed to the centre wing tank.
Boeing says a system is planned for the 7E7 and design work has already started on an inerting system for the 737 family and is being planned for the 757, 767 and 777. Certification of the 747 system, which weighs around 115kg (250lb) in prototype form, is believed to be targeted for mid-2004, with installation in production aircraft by the end of that year. Retrofit kits are thought to be provisionally planned from 2006.
Tank inerting became an FAA objective after the TWA 800 investigation concluded in 2000 that the probable cause was a centre wing tank explosion. Efforts to mandate system testing stalled the following year, however, when the Fuel Tank Inerting Harmonisation Working Group of the Aviation Rulemaking Advisory Committee concluded that "fuel tank inerting will take many years to implement and have an enormous operational impact, with costs that far exceed the benefits". But subsequent studies by the FAA Technical Center showed the maximum safe level of oxygen in the tank to be closer to 11-12% rather than the original 10%, making the target much easier.
Airbus, working with the FAA and set to flight test an equipped A320, says it "is approaching this from a different position than Boeing", because its fuel systems are designed differently and have not suffered any explosions. On Airbus aircraft, the company says, fuel pumps automatically shut off when tanks become empty, and the space underneath the centre-wing fuel tank is vented to allow cooling air in and vapour out.
Source: Flight International
