As an aside, the Flight 004 accident aircraft, was the 20 month old 767-3Z9ER OE-LAV named Wolfgang Amadeus Mozart.
This loss was quickly replaced by an almost new 767-3Q8ER F-GHGF leased from Aeromaritime, then a month later another brand new, and as yet undelivered Aeromaritime order was leased to Lauda as F-GHGG until 04/1992, and painted by Boeing in full Lauda Air livery.
Another new 767 that had been not yet delivered to Martinair in October 1991 (767-31AER, PH-MCK), was shortly afterwards in December of the same year, passed on to Lauda Air and received the registration OE-LAT named Enzo Ferrari.
This aircraft also flew for Lauda Air Italy and returned to the parent company before being passed on to Austrian Airlines in 2007 and stayed with them until March 2021, when it was sent to the Arizona desert for parting out.
With 133,000 hours on the clock, flown over 30 years, it became the first Boeing 767 to be withdrawn from Austrians' fleet.
The Accident Investigation Report findings of OE-LAV were:
Investigation of the accident disclosed that certain "hot-short" conditions involving the electrical system occurring during an auto-restow command, could potentially cause the DCV (Directional Control Valve) to momentarily move to the deploy position. However, no specific wire or component malfunction was physically identified that caused an uncommanded thrust reverser deployment on the accident airplane.
Testing identified hypothetical hydraulic system failures that could cause the thrust reverser to deploy. However, no specific component malfunction was identified that caused an uncommanded thrust reverser deployment on the accident airplane.
No specific Lauda Air maintenance action was identified that caused uncommanded thrust reverser deployment on the accident airplane.
The design changes recommended by Boeing and thereafter mandated by U.S. Federal Aviation Administration Airworthiness Directive 91-22-09 for the B767/PW4000 thrust reverser system should effectively prevent in-flight deployment even after multiple failures.
Probable Cause (As DaveReidUK highlighted above)
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.