After years of deliberation about the best way of making fuel tanks safer, Boeing and the FAA are testing available solutions

Boeing appears to have developed a cost-effective fuel tank inerting system, but at this prototype stage there is no guarantee that it will be adopted by the company or mandated by the US Federal Aviation Administration. It looks hopeful, however, and Boeing, although not willing to talk openly about its plans yet, seems to be enthusiastic.

The National Transportation Safety Board will definitely be hopeful, because it has been calling for fuel tank inerting for more than a decade, and the FAA had resisted the call until an explosion in the centre wing fuel tank was given as the probable cause for the Trans World Airlines Boeing 747-100 crash in 1996. The fuel tank was empty except for unusable fuel, air, and fuel vapour - a highly flammable mixture - all at a high temperature because the aircraft had, 20min earlier, left from a hot New York Kennedy airport. Boeing's new device would reduce the flammability by halving the oxygen content in the air in the empty fuel tank.

The FAA's original argument was that fuel tank explosions should be prevented by eliminating potential ignition sources. Of course they were right - if success in eliminating them is guaranteed. It also maintained that because fuel tank explosions are rare and nitrogen-injection and other traditional inerting systems are so expensive and heavy, they could not meet the cost/benefit requirement. This was the conclusion reached in 1998 by the Fuel Tank Inerting Harmonisation Working Group of the Aviation Rulemaking Advisory Committee.

It is true that, across the world airline fleet as a whole, fuel tank explosions are rare. But the evidence of three fatal centre-wing fuel tank explosions in FAA-certificated types since 1990 proves that the agency's aim of eliminating ignition sources has not been achieved. And the fact that TWA 800 has generated about 40 FAA airworthiness directives concerning wiring insulation, wiring routeing, inspection regimes and the positioning of heat-producing equipment near the centre tank suggests that the agency, having rejected the NTSB's inerting idea, had not been applying its own solution effectively.

After TWA 800, the NTSB did not have to throw down the gauntlet again; it just reminded the FAA that it was still there to be picked up.

Instead, Boeing has now done so, and wisely. The three fatal centre-wing fuel tank explosions since 1990 have all involved Boeing types: two 737s and the TWA 747-100. The actual ignition source will never be proven, but in all cases there was little or no fuel in the tanks concerned and in two cases the weather was hot. In a Philippine Airlines 737-300 in 1990 the explosion took place during pushback and eight passengers died. In the Thai Airways International 737-400 in 2001 the explosion occurred before pushback and fortunately before passenger embarkation, but a cabin attendent died. In both cases the aircraft were gutted by fire.

In the case of the Thai explosion, Boeing has pointed out that fuel pumps were being run in empty tanks, which is forbidden by the operations manual. But to have an aircraft system which is vulnerable to explosion because the crew fail to follow a drill is, in today's world, unnecessary and unsustainable. If the pumps must not operate for safety reasons when there is no fuel in a tank, they should shut down automatically under those conditions, not wait for a circuit breaker to trip them out when they get hot from dry running. Airbus fuel systems have pumps that automatically shut down when fuel gets low.

Boeing has to continue with its prototype Flammability Reduction System (FRS), at least for retrofit to its more vulnerable types and as standard equipment on new-build aircraft that have similar fuel systems to the types in which the accidents occurred. There is an indication that Boeing plans to fit the FRS to the planned 7E7. Maybe so - that is a risk and cost-benefit analysis decision for Boeing and the FAA. But the fundamental wisdom of the FAA's original contention - that the safest fuel tank is one in or near which there are no ignition or heat sources - should be the guiding philosophy in designing an aircraft of the future.

If any other aircraft manufacturers think that this is a Boeing problem alone, they need to think again. They have had the opportunity to learn from another's mistakes, and they must do just that. It is good to be assured Airbus is taking part in the FAA research programme, but the certificating agencies have to review the situation realistically as it applies to all the others as well, including manufacturers of regional jets.

Source: Flight International