Accident Overview

Photo of China Airlines Boeing 737-800 B18616
Photo of China Airlines Boeing 737-800 B18616
Photo copyright Sung-Yang Tong - used with permission
Illustration of Left and Right fuel tank levels during fuel leak - landing and taxi
Illustration of Left and Right fuel tank levels
during fuel leak - landing and taxi
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History of Flight

China Airlines Flight 120, a Boeing 737-800, departed on a regularly scheduled flight from Taiwan Taoyuan International Airport to Naha Airport, Okinawa, Japan. A normal approach and landing at Naha Airport was conducted, including deployment of the high lift system (leading edge slats and trailing edge flaps). The airplane landed uneventfully and began taxiing toward the designated gate. During taxi the slats were retracted. When the slats had fully retracted, fuel started leaking from the right main tank located at the No. 5 slat track housing (slat can). Leaking fuel initially flowed away from the airplane, carried by the airplane movement and engine exhaust. Once the plane stopped at Spot 41, fuel stopped flowing away from the wing and began dripping underneath the airplane in the vicinity of the right engine and brakes. Fuel was also pooling on the tarmac underneath the airplane.

Between the time of touchdown and parking, a time span of approximately five minutes, approximately 60 pounds of fuel leaked from the breached fuel tank. After stopping at Spot 41 (the designated gate), a significant quantity of fuel pooled beneath the wing. Approximately one minute after coming to a stop at Spot 41, fire broke out on the right side of the airplane. The ground crew alerted the flight crew to the fire. Approximately 35 seconds later, the captain notified the cabin crew of the fire, and evacuation was initiated approximately 30 seconds later.

An animation of the progression of the fire is available at the following link: (Fire animation).

Boeing 737 Leading Edge Slats

The Boeing 737-800 employs leading edge slats, in combination with trailing edge flaps, to modify the lift and drag characteristics of the wing. The high lift system allows the wing to produce more lift, and allows operation at slower speeds for takeoff and landing or to take off and land in shorter distances.

The 737-800 has four slats installed on the leading edge of each wing. They are identified sequentially starting at the left outboard slat, and increasing in number across the airplane to the right outboard slat. The No. 5 slat is the most inboard slat on the right wing.

Illustration of lower right wing and slats when extended
Illustration of lower right wing and slats when extended
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Illustration of action of leading edge slats and retraction into slat can
Illustration of action of leading edge slats
and retraction into slat can

Each slat extends on two main and two auxiliary tracks. The two main tracks are guided between two sets of guide rollers installed in the leading edge of the wing in front of the forward spar. Each main track retracts through a hole in the wing forward spar into the slat can, which is formed from thin-walled aluminum and extends into the fuel tank. When the slats are fully retracted, the track assembly is stored inside the slat can. The slat can is inside the fuel tank and is normally immersed in fuel.

Illustration of Downstop Assembly
Illustration of Downstop Assembly

There is a downstop assembly at the aft end of each main track. The downstop assembly comes in contact with another downstop, installed internally on the wing leading edge. When the two downstops contact each other, the slat is fully extended.

The design of the bolt assembly in the downstop requires a washer. The pass through hole in the downstop is larger than the bolt assembly. If the washer is missing, or if the nut is missing, the entire bolt assembly can pass through the downstop. The track can, which houses the retracted slat, is only large enough to accommodate the retracted slat track. Loose material, such as a disassembled downstop, can potentially be contacted by the slat track and pushed through the wall of the track can, causing a fuel tank breach.

Illustrations of exploded view of downstop assembly (left) and Buildup of downstop assembly and dimensions of components (right)
Illustrations of exploded view of downstop assembly (left)
Buildup of downstop assembly and dimensions of components (right)
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Maintenance History

Photo of recovered track can penetration and retrieved unbroken washer loose from the downstop assembly
Photo of recovered track can penetration and retrieved
unbroken washer loose from the downstop assembly
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During the investigation, it was learned that, prior to December 2005, there had been two reported cases of detachment of the downstop assembly nut. In one of these cases, fuel was reported to have leaked from the track can for the number 5 slat. The cause of the fuel leak was determined to have been a hole punctured in the slat can by the nut, that had fallen off the track assembly into the track can, and then pushed through the track can by the slat track. As a result of these two reported incidents, Boeing issued Service Letter 737-SL-57-084, dated December 15, 2005, which recommended removal, inspection, and reinstallation of the bolt assembly, including application of thread locking compound, and tightening the nut with a torque of 50 to 80 inch-pounds.

The investigators determined that during maintenance activity carried out on July 6, 2007 to accomplish this service leter, the washer had not been re-installed on the downstop assembly. Without this washer, the downstop assembly was able to fall out of the slat track. The downstop assembly then fell into the slat can, where it was later pushed through the slat can wall, initiating the fuel leak.

Investigators examined the No. 5 slat track can in the wreckage of China Airlines Flight 120. This area was preserved due to the progression of the fire. A complete downstop assembly, minus one washer, was found to have punctured the No. 5 track can. The washer was found loose in the leading edge of the wing. The dimensions of the downstop bolt are smaller than the downstop assembly through which the bolt passes. Without the washer, the bolt can come free from the downstop assembly.

Based on the failure mechanism, the Boeing Company made a change to the design of the downstop assembly following this accident. This change was implemented on new production aircraft of the applicable models beginning in August 2008. Boeing also released a service bulletin that provided operators with instructions to replace the existing downstop hardware with the new design.

Fuel Auto-ignition

China Airlines Flt 120 following ignition of leaking fuel
China Airlines Flt 120 following ignition of leaking fuel

The auto-ignition temperature of a hydrocarbon fuel is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition or spark. This temperature for auto-ignition can vary depending on pressure and oxygen concentration.

Typically this material property is tested per the American Society for Testing and Materials (ASTM) test method E659. This test method covers the determination of hot- and cool-flame auto-ignition temperatures of a liquid chemical in air at atmospheric pressure in a uniformly heated vessel. Per the material safety data sheet (MSDS) and ASTM E659, the auto-ignition temperature is 410°F for Jet-A fuel.

Investigators conducted tests on another 737-800 in order to determine the surface temperatures that would have been present on the accident airplane as the fuel was leaking, leading up to the fire. The following table represents temperatures that were recorded just after engine shutdown, following taxiing operations similar to that of China Airlines Flight 120.

Temperatures Measured for the Boeing 737-800 Engine
AreaLow Temp. (°F)High Temp. (°F)
Exhaust Plug194374
Outlet Guide Vane374482
Exhaust Gas572644

It was believed by the investigators that fuel exposure to these hot areas started the fire.

Fire and Emergency Evacuation

Wreckage of China Airlines Flt 120 following fire
Wreckage of China Airlines Flt 120 following fire

The leaking fuel caught fire a little over one minute after the airplane came to a stop at Spot 41. During this time the ground crew was aware of the leak and of the fuel fire when it started. They immediately contacted the captain, reporting the fire before there was any indication of fire in the cockpit.

One minute after the fire started (two minutes after engine shutdown), the captain instructed the cabin crew to prepare the passengers for evacuation. The passengers were still unaware of the situation, but once the engines had stopped, they were already gathering their personal items and were preparing to exit the airplane. After receiving the evacuation order, the crew informed the passengers that there was a fire, and their exit would be via the aircraft emergency slides. The ground crew, realizing the situation, was at the bottom of the slides to aid in evacuation of passengers. See video of China Airlines Flight 120 emergency evacuation and fire fighting progress (Source: Sankei Newspaper, Japan).

All of the aircraft passengers exited the plane within two minutes following the evacuation order, and three minutes after the fire had started. There were no reported injuries or fatalities.

Fire fighting operations were stopped, and the fire was declared "out" one hour after it had started. The airplane was completely destroyed.

Photos of Aircraft wreckage seen from four different views
Photos of Aircraft wreckage seen from four different views

During the fire the right wing drooped and rested on the ground. This occurred approximately six minutes after the fire had started. As a result, the remaining fuel flowed to the wing tip, stopping the leak from the inboard No. 5 slat can area. The extinguishment by ground fire and rescue personel extinguished the fire, preserving the right wing which housed the No. 5 slat can area. Investigators were thereby able to determine how the downstop assembly had been forced through the slat can wall, initiating the leak.

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