Flight 611 Probe Finds Likely Rear Fuselage Fatigue Cracks
By Michael A. Dornheim/Aviation Week & Space Technology
06-Aug-2002 10:16 AM U.S. EDT
The China Airlines Boeing 747-200 that disintegrated in midair appears to have fatigue cracks up to 9 in. long in the rear fuselage section where the aircraft may have come apart, according to Taiwanese investigators.
China Airlines Flight 611 crashed in the Taiwan Strait near the top-of-climb on May 25, killing all 225 on board ( AW&ST July 1, p. 42).
Since the suspicious pieces of wreckage were mostly from Section 46, it is very likely that the aircraft first broke up there, said Kay Yong, managing director of Taiwan's Aviation Safety Council (ASC). Section 46 is Boeing's nomenclature for the rear part of the fuselage from aft of the wheel well to the rear pressure bulkhead. It includes the fourth and fifth passenger exits on the left and right sides, and the large aft cargo door on the lower right side and the smaller bulk cargo door behind it.
Investigators have categorized the wreckage as coming from four fields. The first field is before the point of last radar contact and contains relatively small and widely scattered pieces primarily from Section 46. If one were to reconstruct Section 46 with the pieces found so far, there would be large amounts missing from the rear part of the right side and the center part of the left side, Yong said. Another sonar scan of Field 1 was being conducted last week to try to find more pieces--less than 50% have been found so far.
The second field is closest to the last radar contact and contains the empennage and some cargo containers. About a mile farther to the southwest is the third field, which contains the greatest amount of wreckage--the cockpit, forward and center fuselage, and both wings. It appears this section hit the water intact and broke somewhat upon impact, Yong said. They are very large pieces and connected.
The fourth field is farther along and is the one most directly under the projected flight path. It contains heavier objects such as all four engines and the body landing gear. Inspection of the engines shows they were producing little or no power at impact and they probably came off the airframe from the gyrations of the inflight breakup, Yong said. The engines are less than 0.5-1 mi. apart.
Investigators are focusing on wreckage found in Field 1. Of particular interest is a 21.7 X 16.7-ft. segment that includes the intact bulk cargo door and surrounding structure down to the belly. A 7 X 2-ft. skin doubler near the belly on this segment is from a February 1980 China Airlines repair of damage caused by a tail strike. On the skin just adjacent to the doubler there is a 40-in. section with several flat fracture cracks typical of metal fatigue. The longest crack is about 9 in., the next longest is 6 in., and the sum of the crack lengths is 20 in., Yong said. The doubler runs from fuselage station 2080-2160, while the bulk cargo door is at about station 2100. He couldn't say if the periphery of the segment had a fatigue crack along it. The segment has been sent to a Taiwanese laboratory for scanning electron microscope analysis to determine the failure mode ( click here for ASC's archive of Flight 611 releases ).
Fatigue cracks around a repair reminded Yong of the Japan Air Lines 747 crash on Aug. 12, 1985, in which a faulty Boeing repair of the rear pressure bulkhead following a tail strike caused it to blow out and disable the flight controls (AW&ST Sept. 9, 1985, p. 97). We are going to review that accident very closely, he said. The Flight 611 pressure bulkhead was recovered by itself in Field 2 and was complete, except for a missing 5-deg. sector at about the one o'clock position looking aft.
The larger aft cargo door was found still locked to its surrounding structure in an 8 X 10-ft. segment, but only the lower part of the door was found--it was torn along a diagonal line from the rest of the door. Separately, skin from the aft cargo door was found about 6 km. (3.7 mi.) east of the last radar point.
There were flat fracture cracks likely caused by fatigue in about three places on stringers around the aftmost No. 5 left exit door. A repair doubler there was made of stainless steel, not aluminum. The current structural repair manual forbids stainless steel, Yong said.
Mechanics have to be careful about blending a repair in with surrounding structure. It is possible to have a repair that is strong in itself but alters stress paths and overloads surrounding structure.
The No. 4 left exit door had a puncture hole about the size of an orange at about fuselage station 1580. The hole has sharp teeth and appears to be caused by a high-energy impact, but there are conflicting indications of the direction of the impact, Yong said. The No. 4 right door has not been found.
The NTSB did a spectrum analysis of the cockpit voice recorder, and NTSB, ASC and Boeing officials concluded that the final chahhh sound was more likely to be a rapid decompression than an explosive decompression or an explosive charge, Yong said. They were not able to identify heartbeat-like sounds that occurred 7 min. before the breakup, but other mysterious noises may be the crew turning knobs.
The 747-200 has a known fatigue weakness in the nose Section 41, but records show the accident aircraft had that area reinforced at least 10 years ago.