Lauda Air Flight 004 was a scheduled international passenger flight from Kai Tak, Hong Kong, China to Wien-Schwechat International Airport, Vienna, Austria via Don Mueang International Airport, Bangkok, Thailand. On 26 May 1991, the Boeing 767-3Z9ER operating the flight under the callsign NG004 (LAUDA 4, LDA004) suffered an uncommanded thrust reverser deployment midair while flying over Burma-Thai border and crashed into wooded terrain about 94 nautical miles northwest of Bangkok, killing all 213 passengers and 10 crew members on board. It is the first accident and the deadliest loss involving the Boeing 767, and as of 2022, Thailand's worst aviation accident.
The positions of the left engine thrust reverser actuators along with data from the electronic engine control (EEC) and the cockpit voice recorder (CVR) indicate that the left engine thrust reverse system deployed while the airplane was at approximately .78 Mach (478 kn, 550 mph, 885 km/h), climbing through 24,700 feet to flight level 310. The preliminary evidence suggests that the reverse event was recognized by the flight crew, but that the airplane departed controlled flight, accelerated past the maximum operating velocity, and experienced an in-flight structural breakup at a low altitude. Indications of an in-flight fire prior to the breakup have not been found. However, during the breakup, a large explosion was witnessed and burning debris fell to the ground. The explosion was most likely the ignition of discharged fuel from the aircraft's wings during the breakup.
The true cause of the reverser deployment was never found because all of the wires and the Digital Flight Data Recorder were destroyed in the crash, but it was thought to be caused by a short circuit that opened both valves for a split second, allowing the reverser to activate.