Boeing is developing an onboard inerting system for fuel tanks using a simplified approach demonstrated by the US Federal Aviation Administration. The system is aimed at meeting new regulations intended to reduce the risk of fuel tank explosions like that thought to have caused the TWA 800 Boeing 747 crash in 1996.
Confounding earlier studies that concluded onboard inerting would be too complex and costly to be practical, the FAA Technical Center has demonstrated a simplified system it believes is inexpensive and lightweight enough to be viable. A prototype built by Shaw Aerospace, Parker and Air Liquide and installed in a Boeing 747SP for ground testing weighs 73kg (160lb), the FAA says.
An inerting system replaces oxygen in the fuel tank with nitrogen, preventing the ignition of fuel vapour. Combined with work under way to eliminate ignition sources, nitrogen inerting would further reduce the risk of explosions, particularly in heated centre-wing fuel tanks, which are more likely to explode than unheated tanks, the FAA says.
Several features reduce the complexity and increase the practicality of the FAA's design. The system uses engine bleed air, eliminating the need for compressors and motors. The maximum safe level of oxygen is eased to 12% from the 10% used in previous studies. A dual-flow mode provides a low flow of high-purity nitrogen on the ground and in the climb and cruise, and higher flow at lower purity in descent.
Hot, pressurised bleed air is ducted via a heat exchanger and filter to air separation modules, which remove oxygen using membrane filters. Nitrogen-enriched air is ducted to the centre wing tank via the dual-flow valve. The tank is inert on touchdown and remains so during subsequent ground operations, take-off and climb, says the FAA.
Flight tests are planned in the second quarter of 2003. Boeing, meanwhile, is pursuing "fast-track" development of the system, the agency says. Under the SFAR 88 special airworthiness rule issued in June 2001, aircraft manufacturers were to submit plans by 6 December to redesign fuel systems to preclude ignition sources. The FAA has given manufacturers a further three months to look at hybrid solutions incorporating its inerting system.
Source: Flight International
