Japan Airlines Flight 123 was a scheduled domestic flight from Tokyo's Haneda Airport (also known as Tokyo International Airport) to Osaka International Airport (Also known as Osaka Itami Airport). On August 12, 1985, the Boeing 747-146SR developed mechanical problems 12 minutes after taking off. The plane then crashed into Mount Takamagahara in Gunma Prefecture 32 minutes later, after the pilots tried to control the plane, but failed. 520 people died, and only 4 survived. The crash was caused by incorrect repair after a tailstrike incident, which caused metal fatigue and eventually an in-flight structural failure, in which the whole tail of the plane separated from the plane.

The crash was the worst plane crash involving only one plane by number of fatalities.

Airplane involved 

The plane that crashed was a Boeing 747-146SR (SR stands for short range), with tail number JA8119. The plane was used to fly short, domestic routes within Japan. The plane first flew on January 28, 1974. It had 4 Pratt & Whitney JT9D-7A engines. Before it crashed, it had flown for 25030 hours, and took off and landed 18835 times.

On June 2, 1978, JA8119 was involved in a tailstrike accident at Osaka International Airport. When landing, the plane's tail struck the runway. The aft pressure bulkhead was damaged in the accident, as well as the rear of the plane. Later, during June 17 and July 11, repairman from Boeing repaired the plane, by replacing the lower part of the rear and a part of the bulkhead. The plane later returned to service.

While investigating the accident, the investigators realised that the repair done 7 years before the accident by Boeing was incorrect. While two rows of rivets were required for a splice plate in the bulkhead to be installed, the repairman only used one row. This increased the chance of metal fatigue by 70% and also caused the subsequent accident.

Passengers and crew 
On that day, the plane was flown by three crew. The 49-year-old captain, Masami Takahama, was a very experienced pilot. He has flown for 12423 hours, with 4842 hours on 747s. The 39-year-old co-pilot, Yutaka Sasaki, was training to be a captain. On that flight, he was seated on the left-seat (which is usually for captains). He has flown 3963 hours, with 2665 hours on 747s. The last crew, 46-years-old flight engineer, Hiroshi Fukuda, has flown 9831 hours, with 3846 hours on 747s. There were also 12 cabin crew.
The countries which the passengers and crew came from are listed below.

Accident 
On the afternoon of the accident day, JA8119 has already completed two round trip flights, JL503/504 to and from Sapporo, and JL363/366 to and from Fukuoka. At 5.17pm, the plane has just completed flight JL366, and was parked at Spot 18 at Tokyo International Airport, getting ready for its flight to Osaka, JL123. 

At 6.04pm, the plane started taxiing towards the runway. Takeoff was at 6.12pm, and the plane climbed to its cruising height of 24000ft (about 7300 metres). Till then, everything was fine.

At 6.24pm, there was suddenly a loud noise heard, which was actually caused by the pressure bulkhead's failure. This caused an explosive decompression. Air rushed out of the plane and blew off the tail of the plane. As all four hydraulic lines, which control the plane, are located in the tail, the plane became uncontrollable. 46 seconds later the pilots squawked 7700 (meaning that they are in an emergency) and requested to return to Tokyo International Airport. However, although the air traffic controller approved the plane to turn towards Tokyo, the plane strangely started heading towards the north-west direction. At 6.28pm, the Tokyo Air Traffic Controller asked the pilots to turn left, but received the message from the pilots, "now uncontrollable." By then, the plane was moving in a way called the phugoid motion, climbing until losing speed and stalling, then the nose goes down and gaining speed until the plane is able to climb again. The phugoid motion repeated itself until the end of the flight.

At 6.31pm, the air traffic controller asked the plane if it can land at Nagoya Airport, which was 72 miles (about 133 km) from the plane. However, the pilots wanted to return to Tokyo. Between 6.35pm and 6.56pm, the pilots struggled to control the plane without the tail. This failed, as the plane flew uncontrollably towards the mountains.

At 6.56pm, the plane's right wing hit a ridge. The plane then flew on, crashed into a second ridge, and exploded. 520 people died, while 4 people survived.

The time taken for the plane to crash after the bulkhead failure is 32 minutes. The accident was the worst in aviation history involving explosive decompression.

References 

1985 disasters
1980s in Japan
Aviation disasters in the 1980s
Disasters in Japan
Gunma Prefecture
August events