Ramon Lopez/WASHINGTON DC
The US National Transportation Safety Board (NTSB) has closed the final chapter in the "TWA 800" investigation, the longest and costliest airline crash inquiry in its 33-year history. Its verdict is that the 17 July 1996 mid-air explosion of a Trans World Airlines Boeing 747-100 was caused by an explosion of the centre wing tank (CWT) resulting from ignition of the flammable fuel/air mixture in the tank.
Bernard Loeb, director of the NTSB's office of aviation safety, speaking at a public hearing in Washington on 23 August, blamed faulty electrical wiring. "Although the voltage in the fuel quantity indication system (FQIS)wiring is limited by design to a very low level, a short circuit from high-voltage wires could allow excessive voltage to be transferred to fuel quantity indication system wires and enter the tank.
"We cannot be certain that this occurred, but of all of the ignition scenarios that we considered, this scenario is the most likely," Loeb says. Dispelling theories of a government cover-up of a bombing or missile attack, Loeb has reached "the inescapable conclusion" that a fuel/air explosion inside the CWT caused the loss of TWA Flight 800.
The Board said contributing factors to the fatal accident were: design and certification criteria which assumed that fuel tank explosions could be prevented solely by precluding ignition sources; and the existence of heat sources beneath the CWT, with no means to block heat transfer into the tank or to render the fuel vapours in the tank non-flammable.
In voting unanimously to adopt the NTSB staff's report, the Board members urged the US Federal Aviation Administration to review the design specifications for aircraft wiring systems and require better maintenance training.
The NTSB reiterated previous recommendations, particularly one pressing the FAA to give "significant consideration" to nitrogen-inerting systems for fuel tanks in existing and newly certificated civil transports.
In the wake of the crash, the FAA has issued nearly 40 airworthiness directives regarding fuel system safety on a range of civil transport aircraft, mostly concerning wiring, FQIS and fuel pumps, while inerting is under serious consideration. The FAA is forming an Aviation Rulemaking Advisory Committee (ARAC) to advise whether a "practical" means of inerting fuel tanks can be found. o
Source: Flight International
