Japan Airlines Flight 123 (日本航空123便 Nihonkōkū 123 Bin) was a scheduled domestic Japan Airlines passenger flight from Tokyo's Haneda Airport to Osaka International Airport, Japan. On Monday, August 12, 1985, a Boeing 747SR operating this route suffered a sudden decompression twelve minutes into the flight and crashed in the area of Mount Takamagahara, Ueno, Gunma Prefecture, 100 kilometres (62 miles) from Tokyo thirty-two minutes later. The crash site was on Osutaka Ridge (御巣鷹の尾根 Osutaka-no-One), near Mount Osutaka.
Japan's Aircraft Accident Investigation Commission officially concluded that the rapid decompression was caused by a faulty repair after a tailstrike incident during a landing at Osaka Airport seven years earlier. A doubler plate on the rear bulkhead of the plane was improperly repaired, compromising the plane's airworthiness. Cabin pressurization continued to expand and contract the improperly repaired bulkhead until the day of the accident, when the faulty repair finally failed, causing the rapid decompression that ripped off a large portion of the tail and caused the loss of hydraulic controls to the entire plane.
Casualties of the crash included all 15 crew members and 505 of the 509 passengers; some passengers survived the initial crash but subsequently died of their injuries hours later, mostly due to delays in the rescue operation. It remains the deadliest single-aircraft accident in aviation history, the second-deadliest Boeing 747 accident and the second-deadliest aviation accident after the 1977 Tenerife airport disaster.
The official cause of the crash according to the report published by Japan's Aircraft Accident Investigation Commission is as follows:
The aircraft was involved in a tailstrike incident at Osaka International Airport seven years earlier as JAL Flight 115, which damaged the aircraft's rear pressure bulkhead.
The subsequent repair of the bulkhead did not conform to Boeing's approved repair methods. For reinforcing a damaged bulkhead, Boeing's correct repair calls for one continuous splice plate with three rows of rivets. However, according to the Federal Aviation Administration, the Boeing technicians carrying out the repair had cut the plate specified for the job into two pieces parallel to the stress crack it was intended to reinforce, "to make it fit". Cutting the plate in this manner negated the effectiveness of one of the rows of rivets, reducing the part's resistance to fatigue cracking to about 70% of that for a correct repair. During the investigation, Boeing calculated that this incorrect installation would fail after approximately 10,000 pressurization cycles; the aircraft accomplished 12,318 successful flights from the time that the faulty repair was made to when the crash happened.
Consequently, after repeated pressurization cycles during normal flight, the bulkhead gradually started to crack near the single row of rivets holding it together.
