The crash of BOAC flight 911: Analysis
On the 5th of March 1966, the passengers of a BOAC flight from Tokyo to Hong Kong watched with a mixture of curiosity and horror as their jet taxied past the wreckage of a Canadian Pacific DC-8 that had crashed at Haneda Airport the previous day. Some of them may have drawn a little bit of comfort from the unusual sight — after all, what were the odds of another crash so soon after the last one?
But just minutes later, as the crew of BOAC flight 911 treated their 113 passengers to a close-up view of Mount Fuji, disaster struck violently and without warning. Extreme turbulence in the wake of the volcano ripped the Boeing 707 apart in midair, sending it spiraling downward toward the world’s most iconic mountain in full view of hundreds of witnesses. None of the 124 people on board survived the catastrophic plunge from 16,000 feet. But what could have ripped a large jet airliner out of the sky so suddenly on a perfectly clear, cloudless day? Delving into the circumstances of the crash, investigators discovered the true danger of the poorly understood mountain wave phenomenon, and the accident served as a wake-up call to the industry about a threat few had appreciated.
In aviation, the 1960s were very much a different world from today. Commercial flying had not quite lost all of its romance, and because romance and tragedy so often seem to go hand in hand, it was also considerably less safe.
One flight that was emblematic of its time was British Overseas Airways Corporation flight 911, a multiday marathon beginning in London and ending in Hong Kong with stopovers in Montreal, San Francisco, Honolulu, and Tokyo. In 1966, direct flights between distant cities were usually not viable; instead, airliners on long international trips made numerous stops along the way to refuel and shuffle passengers, a model somewhat more similar to a sea or rail route than the average modern flight.
BOAC, Britain’s main international airline outside of Europe, operated flight 911 using a four-engine, narrow body Boeing 707, part of the first generation of jet airliners. The flight progressed without incident until the night of the 4th of March, while the plane was en route from Honolulu to Tokyo. Weather conditions in Tokyo that night were poor, with heavy fog obscuring the airport. At 6:00 p.m. local time, the 707 diverted to a US Air Force base in nearby Fukuoka, where it was forced to stay until conditions improved.
Some other flights nevertheless attempted to land at Tokyo’s Haneda Airport, despite the bad weather. Canadian Pacific Airlines flight 402, a Douglas DC-8 arriving from Hong Kong on the first leg of a flight to Vancouver, had been holding over the airport for some time. Just as the captain considered diverting to Taiwan, air traffic control informed the crew that conditions had improved above the legal minimums. While approaching the airport around 8 p.m. in darkness and fog, the DC-8 descended below the glide slope and struck the runway approach lighting system. The plane continued forward, plowing into a seawall before crashing onto the runway in flames. In the impact and subsequent fire, 64 of the 72 people on board were killed. The passengers of the diverted BOAC flight from Honolulu could not have received a more chilling reminder of why diversions, while annoying, are sometimes necessary.
The following morning, the low pressure system that had brought the bad weather to Tokyo moved off over the Pacific Ocean. At the same time, an area of high pressure in mainland Asia created a steep gradient over Japan, which sat right between the high and low pressure zones. Air naturally flows from areas of high pressure to areas of low pressure, creating what we know as wind — and the winds that blew over Japan on the 5th of March 1966 were quite strong, particularly at high altitudes. The blast of cool, dry air resulted in beautifully clear weather with near perfect visibility, a welcome contrast to the previous night.
At 1:50 p.m., fully fueled and loaded up with 113 passengers and 11 crew, BOAC flight 911 pushed back from the gate at Haneda for the last leg of its journey to Hong Kong. Among the passengers was a group of 75 Americans consisting of employees from the Minnesota-based Thermo King Corporation, a company that manufactured refrigerators. The employees and their families had been offered a 17-day trip to Asia, paid for by the company, as a reward for excellence in the workplace. Currently seven days into the vacation, they had completed their sight-seeing tour of Japan and were headed onward to China.
The Boeing 707 taxied out to runway 33L, passing by the still-smoking wreckage of Canadian Pacific flight 402. A spectator shot a video of it passing by the debris (shown above). The disconcerting sight must have been a major topic of conversation for the 707’s passengers, but it was soon put behind them as flight 911 accelerated down the runway and took to the sky at 1:58.
Flight 911’s filed flight plan called for a southerly takeoff followed by a 40-degree right turn to head southwest toward Hong Kong. However, many pilots flying out of Tokyo enjoyed giving their passengers a close-up view of Mount Fuji. The commander of flight 911, one Captain Dobson, couldn’t resist the great views on offer on such a clear day. Before takeoff, he requested permission from air traffic control to make a close pass just to the east of the majestic volcano before returning to the designated airway, and his request was quickly granted.
On board the 707, one of the American passengers sat at a window seat with a movie camera rolling. The silent film included shots of the airport terminal before boarding, then the cameraperson started shooting again as the plane climbed out of Tokyo, taking in sweeping vistas of distant mountains. In the cockpit, the pilots leveled out at about 17,000 feet, aimed toward Mount Fuji, and began a shallow descent.
Mount Fuji rises from sea level to a height of 12,387 feet (3.776m) in just a few kilometers, making it one of the world’s most prominent peaks. As the tallest mountain in Japan, its height and its isolation from other mountains meant that it stuck directly up into the stream of wind blowing over the islands from west to east. At a weather station on the summit of the volcano, meteorologists clocked sustained wind speeds that day in excess of 110kph. Like a boulder interrupting the flow of a river, Fuji bisected this fast-moving flow of air, disrupting its passage and creating waves and eddies in its wake.
A flowing air mass passing over a ridge or an isolated peak can loop back on itself some distance behind and above the topographical source of the disturbance. This creates a rotor, a horizontal standing wave in the wake of the mountain that sits in place and spins around and around like a washing machine. Successive rotors can be found extending downwind of the mountain, although the closest one is always the strongest. In clear, dry weather, this chain of rotors — known as a mountain wave — is utterly invisible and can be encountered without warning.
All the planes that flew near Mt. Fuji on the 5th of March reported heavy turbulence, but that was nothing an airliner couldn’t be expected to handle. Descending through 16,000 feet downwind of the volcano, the pilots were likely prepared to encounter invisible clear air turbulence. What they found instead was something far more vicious than any jet had flown into before.
As a mountain wave curls in on itself, rotors can accelerate to incredible speeds over short distances, generating vertical wind gusts in excess of 100 kilometers per hour. An airplane can easily handle a 100-kph horizontal wind because it does not differ much from the normal aerodynamic forces experienced in flight. But a vertical wind of equivalent strength is extremely rare, and a plane designed to withstand horizontal wind loads may not be able to withstand those same loads from a different direction. A 100-kph vertical gust could easily tear an airplane apart.
18.5 kilometers southeast of Mt. Fuji, BOAC flight 911 suddenly flew into a monstrous standing rotor caused by Fuji’s “mountain wave.” An enormous gust far in excess of the 707’s design limits slammed into the plane with catastrophic effect. The plane was subjected to a momentary gravitational load in excess of +7.5G, outright killing some of the passengers, particularly any who had their seat belts unfastened. The video camera, which was recording at the moment of the upset, malfunctioned and skipped two frames under the massive G-load, then briefly captured blurred images of the cabin interior before it abruptly stopped filming.
The violent wind gust also fatally damaged the airplane, ripping off the 707’s tailfin and smashing it over against the left horizontal stabilizer. The stabilizer also broke away, causing the plane to pitch steeply upward in a fraction of a second. The sudden pitch-up overstressed all four engine pylons to the breaking point, and the engines separated from the wings, followed almost instantaneously by the empennage as far forward as the rear exit doors.
Far below in the vicinity of Mount Fuji, witnesses on the ground caught sight of the plane losing altitude, trailing white vapor and shedding debris in its wake. As they watched, the crippled Boeing 707 lost the outboard section of its right wing. Black smoke mixed with the white cloud of fuel escaping from the fuel tanks. The airliner began to plunge from the sky in a flat spin, falling like a leaf as it spun around over and over. People snapped photographs of the jet corkscrewing downward from 16,000 feet, capturing its death spiral before of the imposing face of the snow-capped volcano.
As the plane fell, the breakup continued. More pieces ripped off both wings and the cockpit separated, taking with it the first few rows of the passenger cabin. Witnesses saw people falling out of the plane, accompanied by a rush of clothes and other items liberated from the passengers’ luggage. And then, just a couple of minutes after encountering the mountain wave, it was over. The scattered wreckage of BOAC flight 911 crashed to earth on the slopes of Mount Fuji, destroying what remained of the plane and killing all 124 people on board.
Rescuers arriving on the scene found that the crash site was in fact more than 16 kilometers long, stretching from the town of Gotemba all the way to the resting place of the main fuselage, just short of the tree line. The cockpit was found nearby, having been consumed by fire after impact. This proved to be a major setback to the investigation, because in early 707s, the flight data recorder was located in the cockpit, and the fire had rendered it unreadable. At that time jets were not required to carry a cockpit voice recorder, and none was installed.
Meanwhile, the people of both Japan and Britain wanted answers. Not only was this the second fatal crash near Tokyo in 24 hours, it was the fourth fatal crash near Tokyo in the past 30 days. On the 4th of February, an All Nippon Airways Boeing 727 crashed into Tokyo Bay, killing all 133 people on board in what was then the deadliest single-aircraft accident in history. During the search for the plane, a Japanese military helicopter also crashed into Tokyo Bay, killing four. Then came the Canadian Pacific Airlines crash on the 4th of March, and the BOAC crash on the 5th. So far, investigators had not determined the cause of any of these accidents, and at first it seemed like BOAC flight 911 might meet the same fate.
However, a couple of lucky breaks allowed investigators to work around the loss of the flight data recorder and develop a theory. On the day of the crash, a US Navy Skyhawk participating in search and rescue efforts flew into the exact same mountain wave that brought down the 707. Despite encountering wild load fluctuations ranging from -4 to +9G, the pilot managed to regain control and lived to tell the tale.
Investigators also made a breakthrough using a method rarely seen in crash investigations: the video footage shot by a passenger on board the doomed plane. Because the fuselage didn’t catch fire, the film survived the impact and was able to be developed. Tests showed that to get the camera to skip two frames, it had to be subjected to a load of at least 7.5G. This was easily enough to rip the tailfin off a big, slow jet airliner, and in fact that appeared to be exactly what happened — based on its position in the wreckage trail, the tailfin was the first part to come off. From there, following the distribution of the debris, some understanding of the sequence of the breakup could be developed. It became clear that from the moment the plane encountered the mountain wave, disaster was inevitable. The pilots never stood a chance.
Japanese investigators set up a scale model of the terrain around Mount Fuji and ran wind tunnel tests to determine what kind of turbulence might have existed on the lee side of the volcano. They found that strong winds blowing over the cone created an area of unstable air extending up to 20km in the wake of the mountain, as well as upward from the summit to an altitude of 16,000 feet. Localized wind shear within this unstable area could occasionally be extreme enough to rip a plane right out of the sky. It seemed that a classic Japanese proverb held a significant kernel of truth: “When the sky is blue, Fuji is angry.”
At the time, the mountain wave phenomenon was not well understood. It was difficult to accurately model the complex airflow patterns and find the real strength of the turbulence created by different mountains and wind directions. And indeed, mountain waves would kill again. Countless general aviation aircraft have crashed in mountainous areas after encountering powerful rotors and breaking waves, taking the lives of many notable private aviators. But for airline pilots, BOAC flight 911 provided a simple lesson: when the wind is blowing, stay away from high mountains. Had the crew of the 707 stuck to their filed flight plan instead of trying to take their passengers sightseeing, they never would have strayed close enough to Mount Fuji to get into trouble. Flight 911 was only one of a large number of crashes that occurred due to sightseeing detours, and while it would not be the last, few occurred after it. By the start of the 1970s, both safety concerns and the rise of mass affordable air travel eliminated the expectation that pilots would go out of their way to give passengers a good time.
Although mountain waves did continue to bring down small planes, the phenomenon has not caused the crash of an airliner since flight 911, in part due to increased awareness of the danger as a result of the accident. However, perplexingly, the Japanese investigation made no safety recommendations relating to mountain waves or clear air turbulence, apparently writing off the encounter as an act of god. The only recommendations made in its final report concerned fatigue cracks found in the plane’s tail, a discovery which proved unrelated to the disaster. This was more than could be said for the other two major crashes in Tokyo that month. The cause of the All Nippon Airways crash was never determined, and investigators could only report that the Canadian Pacific crash occurred because the pilots “misjudged the approach” and descended too early. They were either unable to determine why they botched the descent, or they did not consider that question important. Altogether, the handling of the spate of crashes in Tokyo in the spring of 1966 serves as a stark contrast to the way investigations are conducted today.
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