David Fullbrook/SINGAPORE
Investigators are working to determine the ignition source of a probable centre wing fuel tank explosion in a Thai Airways International Boeing 737-400 at Bangkok airport on 3 March. Analysis so far shows no evidence of sabotage, despite early reports to the contrary (Flight International, 13-19 March).
US National Transportation Safety Board (NTSB) head investigator Robert Swaim says: "The CVR [cockpit voice recorder] and physical evidence indicates a CWT [centre wing tank] explosion took place and preliminary reports are that no evidence of an explosive device has been found."
A CVR analysis indicates that the noise signature of the initial explosion was similar to that of a blast in the centre fuel tank of a Philippine Airlines (PAL) 737-300 which was being pushed back from the stand at Manila airport on 11 May 1990.
As in the Thai case, it was a hot day and the air conditioning packs were running but the engines had not been started. The air conditioning packs, which create some heat, are near the centre wing fuel tank. The Thai 737-400 was preparing for a domestic flight, carrying prime minister Thaksin Shinawatra.
Investigators are checking for evidence of damaged wiring near the tank or a defective fuel quantity sensor, but say they are still "pursuing both sabotage and accident theories". Chemical samples from the destroyed aircraft have been sent to the US Federal Bureau of Investigation, but no explosive traces have been detected.
Apart from the PAL explosion, NTSB quotes three other Boeing centre wing-tank explosions to back its recommendation to fill empty fuel tanks with an inert gas: an Iranian air force 747-100 which exploded on approach to Madrid airport in May 1976; an Avianca 727 near Bogota, Colombia, in which a very small bomb under a seat caused the centre fuel tank to explode in November 1989; and the Trans World Airlines flight 800 Boeing 747-100 disaster off Long Island, USA, in July 1996.
Source: Flight International