Japan Airlines Flight 123 - Cause

Cause

The official cause of the crash according to the report published by Japan's then Aircraft Accidents Investigation Commission is as follows:

1. The aircraft was involved in a tailstrike incident at Osaka International Airport on 2 June 1978, which damaged the aircraft's rear pressure bulkhead.
2. The subsequent repair of the bulkhead did not conform to Boeing's approved repair methods. The Boeing technicians fixing the aircraft used two separate doubler plates, one with two rows of rivets and one with only one row when the procedure called for one continuous doubler plate with three rows of rivets to reinforce the damaged bulkhead. This reduced the part's resistance to metal fatigue by 70%. According to the Federal Aviation Administration, the one "doubler plate" which was specified for the job (the Federal Aviation Administration calls it a "splice plate" - essentially a patch) was cut into two pieces parallel to the stress crack it was intended to reinforce, "to make it fit". This negated the effectiveness of two of the rows of rivets. During the investigation Boeing calculated that this incorrect installation would fail after approximately 10,000 pressurizations; the aircraft accomplished 12,318 successful flights from the time that the faulty repair was made to when the crash happened.
3. When the bulkhead gave way, the resulting explosive decompression ruptured the lines of all four hydraulic systems and blew off the vertical stabilizer. With the aircraft's flight controls disabled, the aircraft became uncontrollable.