China Airlines Flight 120
On August 20th, 2007, the Boeing 737-800 flying from Taiwan to Naha Airport in Okinawa was just landed with engines turned off, before the ground crew noticed the fire on its number 2 engine. Captain You Chien-kou informed the air traffic controller, then followed through the check list required ordering an emergency evacuation. With 157 passengers and 8 crew on board, all the passengers and flight attendants managed to leave the aircraft safely through the four hatches using slides. First Officer Tseng Ta-wei and the captain left the aircraft through the cockpit windows when the last flight attendant had fled from the right aft hatch. Immediately after the evacuation of the last person (the Taiwanese captain), the number 2 engine and its right wing fuel tanks exploded and burst violently into flames, igniting a blaze that destroyed the aircraft. This incident has received much attention since such fire appeared after the engine had shut down, while the majority of the accidents happened during landing, some during taxing, but rarely after engines turned off, and also due to the fact that B737 is nearly the most popular airplane in the world.
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 incident. When the investigators were collecting testament from witnesses, it was observed that the fire started at the right side of the wing. The cause was first suspected to be the wheel fire, but as the investigation carried on going through the component examination, such possibility was eliminated soon. The electrical fire cause was also ruled out by the information provided in the Black Box recording all the airplane status and that every circuit data turned out to be fine. Then the investigation focused on the aircraft right engine, especially the engine core at the back where most of the accidents usually occur, while the results showed that such fire started externally rather than internally. The testament from a ramp worker had caused attention of this accident investigation, stating that there was the liquid dripping from the leading edge of the right wing.
Such stated observation could only lead to one possible object: jet fuel. The next question became what had caused fuel leak. The first direct suspect was the fuel line, which was viewed as the most vulnerable part in the fuel system and that most of the tanks are located inside the two massive wings. However, as the entire fuel system was traced, the investigation results came out as that nothing was found ruptured in fuel line. Such source of accident was also eliminated. The remaining reason was the fuel tank with no doubts, but the fuel tanks were designed to withstand the rigors of flight for years and that any cracks or leaks should not happen. A borescope, a small camera useful in tight space inspection was taken advantage by the investigators and a bolt sticking out of the fuel tank was discovered with the help. The bolt was first doubted to be originate externally since there has been accidents caused by a piece a metal debris on the runway. After the schematics of the 737 wing structure, the mysterious bolt was determined to be from a downstop assembly of the slat mechanism on the wing’s leading edge. The downstop assembly was used to prevent the slats from reaching too far forward from the wing when flaps and slats are extended during every takoff and landing. And this particular bolt was not a runway debris but was fixed at the end of the track constraining the movement of slats from reading its maximum deployment length. The investigation moved on as they are pouring over the Boeing service document and learned that two previous instances happened before with this part failing and causing fuel leak on a minor scale and in both cases, the fuel tanks were punctured. And Boeing was aware and subsequently and issued a special work order to secure the downstop assembly on all 737s. The solution document issued by Boeing suggested to remove the nut and stall a thread hardening material before reinstalling the nut, in order to harden it in place. Nonetheless, the work order of this airplane indicated that the very assembly has just manipulated a few weeks before the accident. The investigated carried further in checking all the component parts when as missing washer, which was supposed to position behind the nut, was found out missing. The searching for the washer in the rest of the wing remains ended as it was lying in the leading-edge assembly. Bearing the question that, what could cause the in-between washer to fall off while with the nut still staying the on the bolt, in mind, the investigators ask the workers at China Airlines to demonstrate the procedure of their work since the report does not tell the details. The possibility of washer falling off during maintenance procedure due to restricted visual area in performing, and further causing the whole assembly falling apart, had come into the picture. Since such particular washer was required, the whole assembly could fall apart without it attaching.
The first plausible explanation was landed for China Airlines Flight 120 by suggesting that as the jolt offered by touchdown enough to knock off the bolt from the downstop, it landed in the slat can, which is a tight, confined area that houses the moving track of the slat, that any foreign object could cause accident. As the hydraulic force retracted the slat track back into the can with downstop bolt in the way, it was punctured through the right wing fuel tank creating a hole, thus resulting in the fuel leak. Normally, the engine exhaust can disperse the leaking fuel, but as the airplane was parked and engines shut down, fuel started dripping on the scorching-hot tail pipe, and consequently ignited it on contact.
Later, Boeing redesigned the downstop mechanism and made sure improved part is installed on each and every plane. 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.
The United States Federal Aviation Administration (FAA) ordered emergency inspections of wing leading-edge slat tracks on all Next Generation Boeing 737 aircraft. The Emergency Airworthiness Directive (EAD) from the FAA requires operators to inspect the slat track downstop to check for missing parts, ensure proper installation, and check the inside of the slat can for foreign object debris and damage. The directive requires operators to inspect within 24 days and every 3000 flight cycles thereafter. Following feedback from completed inspections revealing loose parts in several other aircraft and one with a damaged slat can, the FAA issued a new emergency airworthiness directive on August 28. Airlines were then required to perform the inspection on Next Generation Boeing 737 aircraft within 10 days instead of 24 days. In addition, the AD required a one-time torquing of the nut and bolt in the downstop assembly for the slat track within 24 days.
If I were to suggest in addressing the cause, I would paid more attention in improvement regarding first, the downstop mechanism or adopt other system of assembly so that a washer would not cause such huge influence, and second, improvement on the slate can to have more space or safely increasing the space between the slat can and the fuel tank.
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