The transport ministry's Aircraft and Railway Accidents Investigation Commission, the U.S. National Transport Safety Board and Boeing Aircraft Co. have made progress in their investigation of the fire and explosions on a China Airlines Boeing 737-800 at Naha airport on Aug. 20.
At first, a theory was floated that a fuel pipe inside the starboard pylon attachment broke, causing a large amount of fuel to leak and catch fire from residual engine heat. Now it is thought that a bolt and a nut in an arm connected to a starboard slat came off and pierced a fuel tank. It is hoped that the investigation will determine why the bolt and nut came off.
The bolt and nut is supposed to remain in a hole in the arm, which is used to extend or retract the starboard slat — a control surface along the leading edge of the wing to increase lift during take-off and landing. Since the diameters of the bolt and the nut are smaller than the diameter of the hole, a washer and two other parts with larger diameters must be placed between the head of the bolt and the nut. But it was discovered that the washer and two other parts were not attached to the bolt and nut that pierced the fuel tank. Instead, they were found near the tank. It is thought that the bolt and nut on the China Airlines passenger jet broke loose when the slat was retracted after landing.