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Pilots gave high praise to Captain Takahama for keeping his stricken 747 in the air for at least 32 minutes after the tail damage was sustained over Sagami Bay. "In spite of such terrible conditions, the plane was kept aloft by engine thrust only," said Mitsuo Nakano, JAL's deputy chief of 747 pilots. "That is an incredible performance." A U.S. expert, Captain Homer Mouden of the Flight Safety Foundation in Arlington, Va., agreed. "The crew exhibited great courage and skill in trying to keep it sea flying," he said. But the odds loose," a United Air Lines pilot said. But why did so much of the tail break away in the air? That mystery was being probed by investigators from the Japanese Ministry of Transport as well as an advisory team from the U.S. National Transportation Safety Board and a group of experts from the Boeing Co. in Seattle. Initial speculation that the rear cabin door, mentioned by the crew over the radio, may have broken loose and struck the tail above it was abandoned when the door was found amid the wreckage on Mount Osutaka, still firmly attached to a part of the fuselage.
As the week went on, the experts' suspicions were also directed at the aircraft's rear bulkhead, an aluminum-alloy partition that separates the pressurized cabin from the non pressurized tail assembly. Hiroshi Fujiwara, deputy investigator for the Ministry of Transport, said that the bulkhead was found at the crash site and that it had been "peeled like a tangerine." It was possible, he said, that if the partition had cracked in flight, the air rushing from the cabin could have had enough force to dislodge the hollow tail fin. American experts theorized that the large number of takeoffs and landings, each involving a pressurization or depressurization of the cabin, required in the short-range use of the 747SR could have accelerated metal fatigue in the bulkhead. The crashed aircraft had made some 18,000 "cycles" (a takeoff and a landing).
One part of the history of JA8119 (the plane's serial number) particularly attracted the probers' attention. On June 2, 1978, the aircraft approached a landing at Osaka with its nose too high. The tail and the rear part of the fuselage slammed into the tarmac at 320 m.p.h.; the impact ripped aluminum skin panels from the belly of the plane. JA8119 was grounded for a month while Boeing engineers supervised repairs that included replacement of the lower part of the rear fuselage.
The tail assembly and the repairs were examined visually after the work was done, according to Hiroaki Kohno, general manager of JAL's maintenance planning department. There was no need for X-ray examination, he said, because "we had full access to the damaged area from the underside." Could some damage have been overlooked at that time? "That cannot be completely ruled out," Kohno conceded, "although that probability would be very low."
A British aviation expert remained suspicious of the botched Osaka landing as a possible cause of Flight 123's crash. William Tench, recently retired chief inspector of accidents at the Royal Aircraft Establishment in Farnborough, said he knew of cases in which it took three years before a crack became visible after an aircraft was heavily jolted. Japan's Ministry of Transport promptly ordered that the tail areas of all 747s registered in that country be re-examined, with special attention to the link holding the fin to the fuselage.