China Airlines Downstop Failure Animation Audio Transcript

On August 20, 2007, a Boeing 737-800 operated by China Airlines departed from Taiwan and had an uneventful flight to Naha Airport, Okinawa, Japan. The 737-800 employs a high-lift system that includes leading-edge slats which enhance takeoff and landing performance.

These slats move on tracks which retract into the wing-leading edge until contact with a downstop assembly. When retracted, the slat tracks are stored in housings which are only large enough to accommodate the slat track. The track housing itself is an integrated part of the wing fuel tank and extends through the front spar into the main fuel tank.

Approximately a month prior to the accident, the No. 5 slat-can had been inspected and reworked in accordance with a fleet-wide Boeing service letter. It was believed by investigators that a washer had been omitted during reassembly, following this maintenance activity. The bolt assembly can be seen here as part of the downstop. The washer was intended to prevent the bolt assembly from passing through the downstop, since the nut is smaller than the hole in the downstop assembly.

Investigators believed that at sometime between the earlier maintenance action and this flight, the bolt assembly migrated out of the downstop assembly and fell into the slat track housing. With the bolt assembly lying inside the track housing, upon slat retraction, the track pushed the bolt assembly through the bottom of the housing, penetrating the main fuel tank, which resulted in a fuel tank breach. Leaking fuel migrated to the leading edge of the wing and was carried aft by airflow during airplane taxi. This photo was taken from the penetrated fuel tank after the accident occurred. As shown, the entire bolt assembly was driven into the fuel tank.

After arriving at the designated parking position, leaking fuel began pooling beneath the aircraft. Ground personnel observed this leaking fuel coming from the right side of the aircraft and immediately contacted the flight crew via the cockpit interphone system advising them of the situation.

During taxi, and while the engine was still running at idle, leaking fuel was not able to reach the hot engine tailpipe or brakes as the cool engine fan exhaust prevented fuel migration to these hot areas. Upon engine shutdown, the fuel was able to drip onto the tailpipe of the engine and onto the hot brakes, resulting in ignition. Flames then quickly spread back to the aircraft, which was eventually completely destroyed by the fire. 

The airplane was successfully evacuated at the first indication of fire. There were no fatalities or serious injuries. The first picture shows the aircraft during the fire. The second picture shows the No. 5 slat track housing following the fire.

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