The aircraft was a Boeing 767-300ER registered OE-LAV and named Mozart, the 283rd Boeing 767 built. It was delivered to Lauda Air on 16 October 1989 and was powered by Pratt & Whitney PW4060 engines. At the time of the crash the No. 1 engine, which housed the faulty thrust reverser, had accumulated 2,904 hours and 456 cycles since being fitted to the airframe in October 1990.
At 23:02 local time on 26 May 1991, Flight 004 departed Don Mueang International Airport in Bangkok under the command of American captain Thomas John Welch, 48, and Austrian first officer Josef Thurner, 41. Both were regarded as highly competent. Shortly after departure, the crew received a cockpit warning that a system failure might cause the No. 1 thrust reverser to deploy in flight. After consulting the Quick Reference Handbook, they concluded the alert was intermittent and advisory, and took no remedial action.
At 23:17, while climbing over mountainous jungle on the Thai-Burmese border, the No. 1 engine thrust reverser deployed. Thurner's last recorded words were "Oh, reverser's deployed." The deployment created an aerodynamic plume that disrupted airflow over the leading edge of the left wing, causing a 25 percent loss of lift and an aerodynamic stall. The aircraft immediately entered a diving left turn. The crew's attempts to regain pitch control generated manoeuvring overloads that exceeded the aircraft's structural limits, destroying the aft fuselage and flight surfaces. The aircraft accelerated to at least Mach 0.99 — the maximum the sensors could record — and the wings separated at the trailing edges before the wreckage impacted forested terrain at around 600 m elevation in what is now Phu Toei National Park, Suphan Buri province, approximately 100 km northwest of Bangkok.
Captain Welch's body was recovered still in the pilot's seat. Of the 223 people aboard, 27 were never identified.
The flight data recorder was completely destroyed, leaving investigators only the cockpit voice recorder as direct evidence. Thailand's Aircraft Accident Investigation Committee concluded after eight months that the probable cause was an uncommanded in-flight deployment of the left engine thrust reverser resulting in loss of flight path control, while the specific trigger for the deployment could not be positively determined.
Niki Lauda flew to Thailand immediately after the crash and later conducted simulator flights at Gatwick Airport that appeared to suggest a thrust reverser deployment was a survivable event. The official investigation rejected this finding, concluding that recovery was uncontrollable for an unsuspecting flight crew.
In parallel, the Vienna Public Prosecutor's Office conducted its own inquiry. Documents seized from Lauda Air showed dozens of pages missing from the aircraft's technical logbooks. A former head of maintenance, Hanns Pekarek, testified that he had repeatedly warned Niki Lauda of unsafe maintenance practices caused by management pressure and had left the company eight months before the crash, banning his family from flying on Lauda Air. An expert report commissioned by the prosecutor, prepared by Professor Ernst Zeibig, established that a fault message related to the left thrust reverser was first documented on 28 December 1990. Boeing maintenance guidelines required the fault to be rectified within 500 flight hours, after which the aircraft should have been grounded. That deadline expired on 25 January 1991, but the aircraft continued flying for over 2,000 further hours. The prosecutor stated that had the damaged electrical cables in the engine strut area survived the crash, Niki Lauda himself would have been the main defendant.
Boeing subsequently issued alerts to operators of more than 1,600 late-model 737, 747, 757, and 767 aircraft about similar thrust reverser systems, and directed replacement of potentially faulty valves. The accident prompted design changes including sync locks that prevent thrust reversers from deploying when the main landing gear truck-tilt angle is not in the ground position.
Lauda Air bore the name and personal involvement of Niki Lauda, two-time Formula One world champion and one of the sport's most celebrated figures. Lauda described the aftermath of the crash as the worst period of his life, surpassing even his recovery from severe burns sustained in the 1976 German Grand Prix. His personal investigation of the wreckage and public engagement with the Boeing inquiry made the disaster unusually prominent in motorsport history, linking his legacy as a racing icon with a defining moment in aviation safety.
A memorial shrine stands at the crash site within Phu Toei National Park, and another memorial and cemetery is located at Wat Sa Kaeo Srisanpetch approximately 90 km away in Mueang Suphan Buri district.
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