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Lauda flight 004, service from Bangkok to Vienna, departed Don Muang International Airport at 11:02pm the night of May 26, 1991. Along with Captain Tom Welsh and First Officer Josef Thurner, eight cabin attendants and 213 passengers were on board the Boeing 767 that night. 004 climbed out normally from Bangkok, cleared to FL310. After being handed off to Bangkok Control, Thurner called the Lauda Air company base, reporting it's estimated arrival time in Vienna.

This would be 004's last transmission. Some twelve minutes later, while climbing through FL240, 004's target disappeared from Bangkok Control's radar screen. Further radio calls from Bangkok went unanswered. Shortly afterwards, Thailand's Department of Aviation's Rescue Co-ordination Centre received a call from from a remote police outpost reporting that people from a mountain village had reported hearing and seeing an aircraft explode in the air and fall into the jungle. The aircraft was less than two years old at the time and there was not forecast severe weather in the area. Clearly, whatever had caused the accident had struck swiftly, the crew having no time to report and signs of trouble.

......Investigators were unable to reach the wreckage until the following morning and it became immediately apparent that there had been a catastrophic in-flight failure. No impact crater was found and aircraft wreckage and bodies strewn at random across the jungle mountain slopes. Eyewitness reports of fire or explosion were substantiated by evidence of fire damage in the wreckage. Further investigation, however, showed that the aircraft was not on fire until after in-flight separation occurred. The reports of an explosion also prompted investigators to search for signs of sabotage, though no traces of explosives or shrapnel could be found.

Study of the engine cowlings began to reveal a picture of the accident. Inside the cowling of the Pratt & Whitney 4000 engines are rub strips which function as air seals for the fan blades and during takeoff, when maximum aerodynamic forces act on the cowling, the blades lightly touch the strip, creating a rub. Investigation of 004's engines showed that there was a much deeper than normal rub in the cowling and it was down from the top of the cowl, indicating a nosedown pitch moment sometime in flight. Most astonishing however was the finding that the port engine thrust reverser was in the deployed position.

After collection of the wreckage, it became clear that the aircraft had broken up due to excessive buffeting and excessive control forces. The FDR had been destroyed in ground fire, but the CVR was still readable and confirmed the investigator's findings. Just after calling Lauda's Bangkok facility, Welsh said "That keeps...that's come on again!" He then asks Thurner what the aircraft's handbook says about the indication that Welsh is seeing. Although it's still unclear at this point what the indication is, it quickly becomes clear. Thurner, then reading from the handbook, replied "Addition system failures may cause inflight deployment. Expect normal reverse operation after landing." Thurner then asks Welsh if he should call the ground facility to ask for advice.

Welsh replied " can tell 'm about's just...ah's probably...ah water or moisture or something because it's not just on, it's coming on and off." Nothing more was said about it for another five minutes when Thurner suddenly said "Reverser's deployed!" immediately after there are sounds of buffeting and metallic snapping until the CVR ends thirty seconds later. The investigators were not able to determine what caused the uncommanded deployment of the thrust reversers, but concluded that either the hydraulic or electrical systems could have been at fault. The destruction of the aircraft was so extensive that no determination could be made. Tunnel tests of the aircraft after the accident showed that the recovery window was as small as 4-6 seconds, the loss of lift on the effected wing causing roll rates of up to 28 degrees/second.