Date:	Sunday 26 May 1991
Time:	23:17
Type:	Boeing 767-3Z9ER
Owner/operator:	Lauda Air
Registration:	OE-LAV
MSN:	24628/283
Year of manufacture:	1989
Total airframe hrs:	7429 hours
Cycles:	1132 flights
Engine model:	Pratt & Whitney PW4060
Fatalities:	Fatalities: 223 / Occupants: 223
Aircraft damage:	Destroyed, written off
Category:	Accident
Location:	5,6 km NNE of Phu Toey -   Thailand
Phase:	En route
Nature:	Passenger - Scheduled
Departure airport:	Bangkok-Don Muang International Airport (BKK/VTBD)
Destination airport:	Wien-Schwechat International Airport (VIE/LOWW)
Confidence Rating:	Accident investigation report completed and information captured

Narrative:
Lauda Air Flight 004 was a scheduled service from Hong Kong (HKG) back to Vienna (VIE), Austria. An intermediate stop was made in Bangkok (BKK), Thailand. The flight departed Bangkok at 23:02 hours. Some five minutes after takeoff the pilot-in-command stated "that keeps coming on," referring to a REV ISLN advisory warning. This indication appears when a fault has been detected in the thrust reverser system. The crew discussed the REV ISLN indication for about four and one-half minutes. The co-pilot read information from the Airplane Quick Reference Handbook as follows: "Additional systems failures may cause in- flight deployment" and "Expect normal reverser operation after landing." The pilot-in-command remarked "....its not just on, its coming on and off," he said, "...its just an advisory thing...," and shortly thereafter stated, "could be some moisture in there or something." At 23:12, the co-pilot advised the pilot-in-command that there was need for, "a little bit of rudder trim to the left." Fifteen minutes and one second into the flight the co-pilot exclaimed, "ah reverser's deployed," accompanied by sound similar to airframe shuddering, sounds of metallic snaps and the pilot-in-command stating "here wait a minute." With the deployment of the no. 1 engine thrust reverser, engine thrust was reduced to idle. Aerodynamic effects of the reverser plume in-flight during the engine run down to idle resulted in a 25 percent lift loss across the wing. The airplane stalled and entered an uncontrolled descent. Buffeting, maneuvering overload, and excessive speed caused pieces of the rudder and the left elevator to separate. This was followed by the down-and-aft separation of most of the right horizontal stabilizer from maneuvering overloads, as the crew attempted to control the airplane and arrest the high-speed descent. A torsional overload then caused the separation of the vertical and left horizontal stabilizers. The loss of the tail resulted in a sharp nose-over of the airplane, producing excessive negative loading of the wing. A downward wing failure was probably followed by the breakup of the fuselage. The complete breakup of the tail, wing, and fuselage occurred in a matter of seconds. The wreckage fell in mountainous jungle terrain.

PROBALE CAUSE: "The Accident Investigation Committee of the Government of Thailand determines the probable cause of this accident to be uncommanded in-flight deployment of the left engine thrust reverser, which resulted in loss of flight path control. The specific cause of the thrust reverser deployment has not been positively identified."

SKYbrary 
Aviation Week & Space Technology 03.06.1991 (32), 10.06.91 (28-30)

photo (c) Werner Fischdick; Las Palmas-Airport De Gran Canaria (LPA); 19 July 1990

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