It was an unusually clear and cloudless day in Tokyo on the afternoon of March 5, 1966. British Overseas Airways flight 911 was scheduled to leave Tokyo at 1:30pm for Hong Kong with Captain Bernard Dobson on the flight deck. In addition to the cabin crew, there were 124 people aboard the Boeing 707 that day.

911 Falls in a Flat Spin

Due to the clear weather, Dobson asked for an amendment to the scheduled flight plan, climbing visually via Mt. Fuji to allow his passengers a rare up-close view of the Japanese landmark. 911 left Tokyo at 1:58pm and began climbing southwest towards Mt. Fuji, reaching 17,000ft. Passing over Gotemba, the aircraft turned northwest and began a slow descent towards the 12,400ft mountain. Shortly afterwards, witnesses reported seeing the aircraft trailing white vapour. The aircraft was then seen to be shedding pieces and then a large puff of vapour came from the aircraft's tail. It pitched up and entered a flat spin, the tail assembly and engines seen to be missing and the starboard outer wing had failed. The forward fuselage then broke off and the aircraft continued in a flat spin until impacting the base of Mt. Fuji. All aboard 911 were killed.

911 Falls Near Mt. Fuji

Clearly the aircraft suffered some sort of in-flight structural failure. The weather was clear at the time and no explosion was seen other than the puff of white vapour. The FDR was recovered, but was destroyed in the resulting ground fire. Reconstruction of the wreckage trajectory showed that the vertical fin and port tailplane failed first, followed by the failure of the starboard outer wing, resulting in the flat spin and forward fuselage failure. Metallurgical study showed that the wing had failed upward and the engines had all failed to the left, as had the tail surfaces. Paint markings on the port tailplane indicated that the vertical fin had smashed into the tailplane, causing it's failure. No structural defects or control system failures could be found. Boeing officials said that the fin must have failed due to a sudden extreme load imposed on the surface, but in testing, the fin only failed when a load greater than 110% it's design limit gust load was imposed. Investigators were able to locate an 8mm camera in the wreckage. The camera showed, from out the window, the aircraft's progress over Gotemba and continuing northwest towards Fuji when it suddenly skipped frames and then showed blurred images of the passenger cabin before ending. Obviously, the camera supported the idea that a sudden load had been imposed on the aircraft, but it gave no clue as to what the cause was. Study of the meteorological conditions at the time helped to unravel the mystery.

A strong pressure gradient lay over Japan at that time, causing strong west to northwest winds and clear visibility. At the time of the accident, a weather station at the base of Mt. Fuji was registering winds of 60-70kts. Winds of such velocity are very conducive to the formation of mountain waves. These waves are formed by smooth air flowing at a high velocity encountering and obstruction such as a mountain and "boiling over" or breaking up, resulting in turbulence and gusts. In moist conditions, these waves can be visualized by the formation of standing rotor or lenticular clouds, but in dry conditions, there may be no cloud formation. Study of satellite photos just prior to the accident showed the formation of both rotor and lenticular clouds in the mountains south of Mt. Fuji.

911 Impacted Nearly Vertical

Reports from other aircraft which had flown in the vicinity or Mt. Fuji showed that nearly all of them had encountered moderate to severe turbulence. A U.S. Navy aircraft dispatched to search for the wreckage of 911 encountered extreme turbulence near the same area the crash occurred. After landing, the aircraft's G-meter was found to have registered plus 9g to negative 4g during the flight. It was concluded that, while approaching Mt. Fuji's leeward side, flight 911 was violently impacted by a severe mountain wave which led to vertical fin failure and subsequent in-flight break-up, the white vapour being jet fuel flowing out of the aircraft after separation of the engines.