What happened
On May 26, 1991, Lauda Air Flight 004, a Boeing 767-300ER, departed Bangkok, Thailand, at 16:02 for its scheduled flight to Vienna, Austria. Shortly after takeoff, the crew engaged in a discussion lasting approximately four and a half minutes regarding a cockpit alert related to a thrust reverser isolation valve. While the crew consulted the Quick Reference Handbook (QRH) to address the alert, no corrective actions were implemented.
During the climb, the co-pilot noted a requirement for left rudder trim. Approximately fifteen minutes into the flight, the co-pilot observed that a reverser had deployed. This was immediately followed by sounds of airframe vibration. Less than thirty seconds later, the cockpit voice recorder captured sounds consistent with a structural breakup, and the recording ended. The aircraft subsequently crashed into jungle-covered mountains at 14 degrees 44 minutes North and 99 degrees 27 minutes East at roughly 16:17. The accident occurred at night under visual meteorological conditions, and all 223 fatalities were recorded.
Findings
Data recovered from the left engine electronic engine control (EEC) indicated that at the time of the suspected deployment, the aircraft was climbing at a speed of Mach 0.78 and an altitude of approximately 24,700 feet. The investigation identified the unintended deployment of a thrust reverser as the critical event preceding the structural failure of the aircraft.