Autopilots Blamed In Sudden 747 Rolls

A mysterious flaw discovered in the autopilot computer system used on more than 700 Boeing 747 jumbo jets could cause the plane to veer - uncommanded - into a severe roll and high-speed dive.

The National Transportation Safety Board has asked the Federal Aviation Administration to order Boeing to redesign the autopilot system used on all 747-100s, 747-200s and 747-300s. Boeing delivered 724 of those models between 1969 and the late 1980s. (A different autopilot system is used on 747-400s, which Boeing has been making exclusively since 1989.)

In a letter to FAA Administrator Barry Harris, safety-board Chairwoman Susan Coughlin said "the potential exists for loss of an airplane as a result of undetected autopilot-system failures." Coughlin recommended that all 747 operators be forewarned of this.

However, Boeing downplayed Coughlin's recommendations. "We believe the autopilot system is safe," said Boeing spokesman Chris Villiers.

Villiers said the company will not send any warnings about autopilot flaws to 747 operators until formal findings are completed in an investigation, headed by Canadian authorities, into the sudden high-speed dive of an Evergreen International Airlines cargo flight last December.

The plane, a 747-100, was on a flight from New York to Anchorage and cruising on autopilot at 31,000 feet above Nakina, Ontario, when it inexplicably rolled 90 degrees to its right and dove two miles at near-super-sonic speed.

Three large wing panels ripped away, leaving a 3-by-15-foot hole in the right wing as the pilots scrambled to shut off the autopilot and regain control. They righted the craft at 22,500 feet and made a safe emergency landing at Duluth, Minn.

Coughlin said tests by Canadian authorities revealed that stray "inputs" told the autopilot on the Evergreen plane to put the plane in a roll. It is unknown what caused the erroneous signals.

Coughlin's letter cites two other incidents involving severe uncommanded rolls - blamed on failure of the autopilot system: a Sept. 1, 1990, Air Canada flight and an April 30, 1990, British Airways flight from Miami to London. In both cases, no one was injured.

The safety board regularly makes recommendations for improving aviation safety to the FAA, but has no enforcement powers.

The FAA is not treating Coughlin's findings as an emergency, and could take up to 90 days to respond, said FAA spokesman Dave Duff. The FAA could take several months beyond that to make a decision.