Lauda Air Flight 004

Lauda Air Flight 004 was a regularly scheduled international passenger flight from Hong Kong, via Bangkok, Thailand, to Vienna, Austria. On 26 May 1991, the Boeing 767 operating the route crashed following an uncommanded deployment of the thrust reverser on the No. 1 engine during the climb phase, causing the aircraft to enter an aerodynamic stall, uncontrolled dive, and in-flight 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 accident in Thailand's history. The accident 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.

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 004 (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 10 crew under the command of American captain Thomas John 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 land safely 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. Due to the reverser design, an aerodynamic plume of air disrupted the airflow over the leading edge of the left wing during the engine's rundown to idle thrust, which resulted in a 25% loss of lift and an aerodynamic stall.

The aircraft immediately began a diving left turn. The CVR recorded master caution warning and a second snapping sound, followed by various alerts such as overspeed and a second master caution, and Welch's last recorded words: "Jesus Christ" in response to the rapid rolling sensation, "here, wait a minute" as he brought engine 1's thrust lever to idle and shut down the engine and finally, "damn it". Following this, the CVR recorded an increase in background wind noise followed by several loud bangs. Manoeuvring overloads produced by the pilots' sustained attempts to regain pitch control, in combination with the increasing velocity of the dive, had exceeded the aircraft's structural limits and destroyed the weakened aft 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 experienced Mach tuck and 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 experienced structural failure 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 1 sqkm in size, at an elevation of 600 m, in what is now Phu Toei National Park, Suphan Buri. The wreckage site is about 6 km north-northeast of Phu Toey, Huay Kamin (ห้วยขมิ้น), Dan Chang district, Suphan Buri province, about 100 km northwest of Bangkok, close to the Burma-Thailand border. Rescuers found Welch's body still in the pilot's seat.