Lauda Air Flight 004 (NG004/LDA004) was a regularly scheduled international passenger flight from Bangkok, Thailand to Vienna, Austria. On May 26, 1991, the Boeing 767-300ER crashed following an uncommanded midair deployment of the thrust reverser on the No. 1 engine, causing the aircraft to enter an aerodynamic stall, uncontrolled dive and midair breakup, killing all 213 passengers and ten crew members on board. It is the deadliest aviation accident involving the Boeing 767, and the deadliest aviation incident in Thailand's history. The crash marked the 767's first fatal incident and third hull loss. Formula One world motor racing champion Niki Lauda, who founded and ran Lauda Air, was personally involved in the accident investigation.
Aircraft
The aircraft involved was a Boeing 767-300ER, the 283rd Boeing 767 built, that was powered by Pratt & Whitney PW4060 engines and was delivered new to Lauda Air on 16 October 1989. The aircraft was registered OE-LAV and named Mozart. At the time of the incident, the No. 2 engine had been on the airframe since assembly of the aircraft (7,444 hours and 1,133 cycles) whereas the No. 1 engine (with the faulty thrust reverser) had been on the aircraft since October 3, 1990 and had accumulated 2,904 hours and 456 cycles.
Accident
At the time of the accident, Lauda Air operated three weekly flights between Bangkok and Vienna. At 23:02 ICT on 26 May 1991, the Boeing 767-3Z9ER operating as Flight 4 (originating from Hong Kong's Kai Tak Airport) departed Don Mueang International Airport in Bangkok for its passenger service to Vienna International Airport with 213 passengers and ten crew under the command of American captain Thomas J. Welch (48) and Austrian first officer Josef Thurner (41). Both pilots were regarded as very competent. At 23:08, Welch and Thurner received a visual warning indication on the EICAS display that a possible system failure would cause the thrust reverser on the No. 1 engine to deploy in flight. After consulting the aircraft's Quick Reference Handbook, they determined that the alert was "coming on and off" and that it was "just an advisory thing." The pilots took no remedial action, possibly believing that the indication was false, but also with the knowledge that the 767 could stop with only one operational reverser.
At 23:17, the No. 1 engine reverser deployed while the plane was over mountainous jungle terrain in the border area between the Suphan Buri and Uthai Thani provinces in Thailand. Thurner's last recorded words were, "Oh, reverser's deployed." Moments later, the cockpit voice recorder (CVR) recorded a shuddering sound, followed closely by a snap. Because of the reverser design, the lift on the aircraft's left-wing leading edge became disrupted because of the aerodynamic plume formed during the engine's rundown to idle thrust and resulted in a 25% loss of lift and an aerodynamic stall.
The aircraft immediately began a diving left turn. The CVR recorded a second snapping sound, followed by various alerts such as overspeed and master caution, and Welch's last recorded words: "Jesus Christ" in response to the rapid rolling sensation, "here, wait a minute" as he shut down the engine and finally, "damn it." Following this, the CVR recorded an increase in background noise followed by several loud bangs. Maneuvering overloads produced by the pilots' attempts to regain pitch control, in combination with the velocity during the dive, had already exceeded the aircraft's structural limits and destroyed the weakened rear fuselage along with the rest of the damaged flight surfaces. The loss of the tail caused further negative loading of the wings as the airplane nosed over vertically, reaching a speed of at least Mach 0.99 (the highest value that the aircraft's sensors could record), breaking the sound barrier.
The wings then failed and separated at the trailing edges, engulfing the remains of the falling aircraft in flames before impacting mountainous wooded terrain and exploding. Most of the wreckage was scattered over a remote forest area roughly one square kilometre in size, at an elevation of , in what is now Phu Toei National Park, Suphan Buri. The wreckage site is about north-northeast of Phu Toey, Huay Kamin (), Dan Chang district, Suphan Buri province, about northwest of Bangkok, close to the Burma-Thailand border. Rescuers found Welch's body still in the pilot's seat.