Riddle of the auto pilot

Press, 13 March 1985, Page 17

Riddle of the auto pilot

The terrifying, 10,000-metre plunge of a China Air Lines 747 jet flying from Taipei to Los Angeles last month may have been caused by the plane being left on automatic pilot for too long after one of its engines failed. As the full horror of the incident is documented, it is clear that the 325-ton jet was saved only by chance.

As it spiralled from 12,500 metres, the airliner, carrying 272 people, rolled over so that it was virtually on its back, at one stage falling almost vertically. It was subjected to forces of 5g — five times that of gravity and twice the level civil airliners are built to withstand. The g-forces were so great that the massive bolts holding up the

main undercarriage were sheered and the landing gear was pulled down. It was probably that, rather than the skill of the crew, which slowed the aircraft and allowed the pilot to regain control. He was able to make an emergenncy landing an hour later, and won a round of applause from his shaken passengers. The United States National Transportation Safety Board,

which is investigating the incident, has established that, contrary to the crew’s account, there is no evidence that power was lost from more than one engine. The crew had said that the number four engine “flamed out” during severe turbulence, and power was then lost from the three remaining power-plants. Investigators have discovered that even during the precipitate descent, electric generators powered from the jet engines con-

tinued to function. Even with a total loss of thrust the 747 should have gone into a glide. Investigators are therefore trying to explain how the aircraft came to stall — or fall to a speed where aerodynamic lift from the wings-is lost. Stall speeds at high altitudes such as 12,500 metres are dangerously close to the limit for

subsonic jets. Because the air is more rarefied, the lift generated by the wings at any given speed is much reduced. As a result, despite true speeds as high as 510 knots, the stall speed can creep up to as little as 30 knots less. Because of increased fuel savings when planes fly above 12,000 metres these altitudes are used increasingly.

Some safety investigators are coming to a view that the crew may not have disengaged the auto pilot soon enough after the loss of thrust from the number four engine. The auto pilot has strictly limited authority over the aircraft’s navigation and, in turbulence with reduced thrust, it would be unable to maintain both height and speed.

With a specified altitude locked into the on-board computer, the auto pilot would sacrifice speed to maintain height until the plane slowed to stalling point. Footnote: Despite the terrifying experience and buffeting, only five people were detained in hospital at San Francisco where the plane made an emergency landing. Up to 50 passengers suffered injuries, mostly minor. The Boeing had been flying from Taipei, Taiwan, to Los Angeles.

By

VICTOR SMART,

London

“Observer” air correspondent