Boeing says a spark from wiring in the pumps could ignite vapour in a near-empty tank. The Seattle-based company first learned about the problem in August, when it said only 118 jets might be affected. But last week the company said 35,000 fuel pumps could malfunction and that newer 737s, plus all 747s and 757s, need to be checked.
The US Federal Aviation Administration last week ordered inspections of relevant planes owned by US airlines, and said that in the meantime commercial airliners should carry enough fuel to keep the pumps covered at all times. That will prevent a vapour explosion even if a pump is faulty. Regulators in other countries normally follow the FAA's lead.
But consumer pressure groups say that isn't enough. "It's the usual Band-Aid approach. Eventually, we'll have a problem that we don't know about, and another plane will blow up," says William Kauffman, an aeronautical engineer at the University of Michigan in Ann Arbor. Kauffman and other aviation experts want the FAA to order the installation of fuel tank "inerting" systems that would prevent sparks causing an explosion. One way to do this is to pump nitrogen into the fuel tank, smothering any spark.
The Boeing fuel pumps in question are not known to have caused any explosions. But in 1996, TWA Flight 800, a Boeing 747, exploded in mid-air shortly after taking off from New York, killing all 230 people aboard. Investigators decided a spark in the fuel tank was the most likely cause.
The centre fuel tank, one of three, is giving the most cause for concern on the Boeing aircraft under scrutiny. This tank is often empty or nearly empty on flights where extra fuel is not needed. Vapour in this tank can be heated by waste heat from air-conditioning units, raising the risk of explosion.
In 1996, the US National Transportation Safety Board recommended that the FAA amend its regulations to require fuel-tank inerting systems on all new commercial jets. Similar systems are already in use on some military planes. But an FAA advisory committee last year concluded that the benefit wouldn't be worth the estimated cost to the airlines of $35 billion over 15 years. It said more rigorous inspections of wiring was sufficient. But consumer advocates who sat on the advisory panel complain that it was stacked with industry representatives, and that the cost estimates are inflated.
Gail Dunham, president of the Philadelphia-based National Air Disaster Alliance, which campaigns for improvements to airline safety, pointed out that an earlier advisory committee estimated the cost could be as low as $5 billion.
Despite the industry view that inerting is too expensive, FAA engineers are pressing ahead with tests on two types of system. One would inject nitrogen into the fuel tank on the ground. Another would pick up nitrogen during the flight, using a membrane to filter it out of air passing through the jet engine, and funnel it into the fuel tank. Flight tests of the in-air system begin next month.
Author: Kurt Kleiner
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New Scientist issue: 5 October 2002
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