In an apparent change of heart, the US Federal Aviation Administration is considering the case for changing the way it certificates commercial-aviation fuel tanks, say senior officials close to the year-long investigations into the mid-air explosion of a Trans World Airlines Boeing 747-100.

Accident investigators have yet to pinpoint the source of the ignition which caused an explosion in the aircraft's fuel tank in July 1996, but the National Transportation Safety Board (NTSB) has already recommended action to reduce the risk of future explosions.

These include potential installation of nitrogen-inerting systems, more insulation between heat-generating components and tanks, and revised fuelling procedures.

The proposals, however, provoked immediate complaints from the airline industry over the potential massive cost and safety implications. One US airline executive claims that requiring nitrogen-inserting would cost US airlines alone $19 billion, as well as introducing new risks and being operationally impractical.

Long-standing FAA certification standards are designed to ensure that the ignition of fuel vapour by lightning strike, hot components, or component/systems failure is precluded. The administration initially appeared to join with airlines and aircraft manufacturers in maintaining that this remained the better solution, saying that it was not possible to keep potentially explosive fuel vapour from the tanks.

While the airline industry stands firm, Ronald Wojnar, manager of the FAA's Transport Aircraft Directorate, now says that the NTSB may have a point. "Our minds are much more open now," says Wojnar, who chaired a three-day FAA and Society of Automotive Engineers Transport Fuel Flammability Conference.

"There is no way we can assure ourselves that all ignition sources will ever be eliminated. What I don't want to see is a reflexive action by industry that leads to our recommendations being rejected out of hand," says NTSB chairman Jim Hall.

The safety board, due to hold a public hearing into the TWA Flight 800 crash on 8 December, is still pressing to see its proposals adopted. "Regardless of what we find to be the ignition source of that blast, we must do all we can to render these fuel tanks non-explosive," says Hall.

The airline industry, meanwhile, has launched the Aircraft Fuel Systems Safety Programme, a project designed to head off the NTSB proposals. Some 2,000 airliners will be inspected over the next three years for fuel system defects. The hope is to confirm the safety of both the fuel systems and the design philosophy which created them.

Wojnar calls the initiative "a step in the right direction", but warns that the FAA may have to check more aircraft. About 12,000 Western-built transport aircraft are in service, including 8,000 aircraft built by Boeing and Airbus. He says that a sampling of 2,000 aircraft may not be adequate, and all Boeing 747-100s may need to be inspected.

The FAA has yet to decide on its next step, but Wojnar says that airworthiness directives or rulemaking are still possibilities, and a public meeting may be conducted to get specific comments on proposals. "Anything we do must be universally applied by make and model It appears we will go beyond the airline initiative," he says.

The FAA says that a requirement for nitrogen-inerting systems on civil transports is not out of the question, "but there is no off-the-shelf inerting system available for immediate installation on commercial aircraft," it adds.

Source: Flight International