China Airlines Flight 120

Investigation

The cause of the accident was investigated by the Aircraft and Railway Accidents Investigation Commission of Japan. The crew, as confirmed by the recording in the cockpit voice recorder, did not observe any abnormalities before the accident.

The investigating team confirmed that the aircraft caught fire in the gate area and the sign of fuel leakage onto the tarmac when taxi to the gate. The investigation focused on the possibility that a fuel leak led to the fire. At a news conference on August 24, investigators revealed that a bolt, which had come loose from the slat track, had punctured the right wing fuel tank, creating a hole 2-3 cm in diameter.

On 28 August 2009, the Japan Transport Safety Board (JTSB) published the results of the investigation. It is considered highly probable that this accident occurred through the following causal chain: When the aircraft retracted the slats after landing at Naha Airport, the track can that housed the inboard main track of the No. 5 slat on the right wing was punctured, creating a hole. Fuel leaked out through the hole, reaching the outside of the wing. A fire started when the leaked fuel came into contact with high-temperature areas on the right engine after the aircraft stopped in its assigned spot, and the aircraft burned out after several explosions.

With regard to the cause of the puncture in the track can, it is certain that the downstop assembly having detached from the aft end of the above-mentioned inboard main track fell off into the track can, and when the slat was retracted, the assembly was pressed by the track against the track can and punctured it.

With regard to the cause of the detachment of the downstop assembly, it is considered highly probable that during maintenance to prevent the nut from loosening, which the company carried out on the downstop assembly about one and a half months prior to the accident based on the service letter from the manufacturer of the aircraft, the washer on the nut side of the assembly fell off, following which the downstop on the nut side of the assembly fell off and then the downstop assembly eventually fell off the track. It is considered highly probable that the key factor contributing to detachment of the downstop assembly was its design, which relied on a washer to prevent the assembly from falling off.

With regard to the detachment of the washer, it is considered probable that the following factors contributed to this: Despite the fact that the nut was in a location difficult to access or inspect during maintenance, neither the manufacturer nor the airline paid sufficient attention to this when preparing the service letter and engineering order job card, respectively. Also, neither the maintenance operator nor the job supervisor reported the difficulty of the job to superiors.

The Japan Transport Safety Board recommended the Civil Aeronautics Administration of Taiwan to supervise China Airlines to take the following actions: when planning and implementing maintenance jobs, the scope of jobs should be fully ascertained and the working conditions and environments should be appropriately evaluated, and the countermeasures to prevent maintenance errors including the actions taken in 2009 against the recurrence of this accident should be steadfastly implemented and enhanced.