Because only 25 people, including crew, were killed on the short hop across the Rocky Mountains from Denver, the crash never received wide public attention. But when the embarrassing truth is finally acknowledged, that the nation's top airline crash investigators do not know what caused the disaster despite the most painstaking investigation in their history, United's Flight 585 will be known as the most troubling in modern aviation history.
The National Transportation Safety Board (NTSB), Boeing, the US pilots association and others have devoted thousands of hours and millions of dollars to the investigation, but the cause of the crash of the world's largest- selling airliner remains a mystery. The airline industry is bedevilled with the question of why the nine- year-old twin-engine jet should have flipped over at an altitude of less than 6,000 feet, hitting the ground with such force that the fuselage folded like an accordion, opening a 39ft wide and 10ft deep crater in the process.
Theories range from a sudden mechanical failure in the 'yaw damper', an automatic device on the rudder that prevents jets from fishtailing, to the effects of a sudden wind rotor or 'horizontal tornado' coming off the Rocky Mountains, or even suggestions that the pilots were arguing while steering through turbulence to the runway. The NTSB remains tight-lipped about its findings.
Unfortunately, the aircraft was equipped with an older flight data recorder that could only record five flight parameters. A state-of- the-art black box can log some 250 parameters, including everything from the lights to whether a microwave was on. Thanks to the lobbying of the US Federal Aviation Authority by aircraft manufacturers and operators, the aircraft would not require a sophisticated black box until 1994.
The accident report was originally due to be published in July, but unable to say with any confidence what caused the crash for the first time in its history of investigating disasters, the NTSB will not now issue its report until the end of September. Airline officials are concerned that the persistent mystery could affect travellers' confidence in one of the world's safest airliners.
Unforthcoming on most details of the investigation, the NTSB is adamant that the conduct of the pilots, Captain Harold Green, 52, and Patricia Eidson, 42, was nothing less than professional. 'It certainly wasn't an argument between the two crew members' that caused the crash, one investigator said. 'I know because I have listened to the cockpit tape.' The NTSB' preliminary investigation, which runs to more than 100 pages, bears this out. The pilots appear to have been model professionals.
United is one of the world's largest airlines, with an average of 2,000 daily departures and with 7,251 pilots on its payroll. Its safety, training and maintenance records are considered excellent.
Six days before Flight 585 crashed, another crew reported problems with the aircraft's yaw damper. While climbing through 10,000 feet the pilot felt the plane 'yaw' to the right for three or four seconds, while the co-pilot felt several rapid 'jerks'. A flight attendant was nearly flung into the bulkhead. Two days later the same problem was reported and repaired. Some independent investigators think the most likely cause of the crash was a failure of the yaw mechanism at a crucial moment, possibly even while the aircraft was being hit by a rare horizontal tornado effect, causing it to flip over on its descent.
Dr John McCarthy of the National Center for Atmospheric Research believes the conditions may have been ripe for just such a freak accident. While the sky on Sunday 3 March 1991 was a crisp blue, high winds gusting to 60mph were coming in from the mountains. Such high winds cause 'mountain waves' of wind, which at times create rotors or horizontal vortices of wind. Dr McCarthy speculates that the aircraft could have been flipped by just such a horizontal tornado with a 120-knot wind differential. 'A plane flying into such a rotor would be rolled over, with each wing being hit by a 60mph wind, one going up, the other down,' he said.
No commercial airliner is known to have come to grief from a rotor before. But in 1966 a BOAC 707 encountered the savage effects of a mountain wave just after take-off from Tokyo en route to Hong Kong. At 15,000 feet a mountain wave coming off the sacred Japanese volcano of Mount Fuji ripped the aircraft apart, with the loss of all 124 on board.