SPRINGFIELD, Va. (AP) -- Government investigators said today there is evidence that an uncommanded reversal of the rudder panel on the Boeing 737 may have caused two airplane crashes and nearly a third.
The finding was unveiled during a National Transportation Safety Board hearing into the cause of the September 1994 crash of USAir Flight 427 outside Pittsburgh. It may explain not only that crash but also the 1991 crash of United Airlines Flight 585 outside Colorado Springs, Colo., and a 1996 incident in which the pilots of an Eastwind Airlines 737 briefly lost control of their aircraft as they approached Richmond, Va.The rudder panel runs up the tail of an airplane and sweeps the nose left and right. In an uncommanded reversal, a mechanical snafu causes the rudder to go in the direction opposite what the pilot intends when he presses on the rudder pedals in the cockpit.
Under those circumstances, a pilot trying to recover from a minor control problem with his plane could actually make the situation worse.
"A rudder reversal scenario will match all three events," Dennis Crider, chairman of the NTSB's Aircraft Performance Group, told the board members.
The board staff members drove home their point in graphic terms, playing three computer simulations showing the USAir and United planes flipping over and flying into the ground, and the Eastwind plane wobbling back and forth as the pilots fought to regain control.
The Boeing Co., which makes the 737, the world's most common jetliner, hotly contests such a finding, saying there is no physical evidence that supports rudder reversal scenarios in the three incidents.
The company points to pilot error in the Pittsburgh crash, a rogue wind in the United Airlines crash and a misrigged yaw damper -- another element of the rudder system -- for the Eastwind incident.
Since none of the planes had modern, highly informative flight data recorders, the board staff was forced to make assumptions in developing its hypotheses. That approach raised questions with one board member.
"When I see all these assumptions, I get a little twitchy," said Robert T. Francis II, the board's vice chairman.
USAir Flight 427 plummeted 6,000 feet to the ground in Aliquippa, Pa., on Sept. 8, 1994, as it neared the end of a flight from Chicago. All 132 aboard were killed.
All the parties to the investigation agree that the plane's rudder went to its full leftward position shortly after Flight 427 flew through the wake of the plane ahead of it.
Beyond that, there is disagreement.
Boeing suggests the pilots may have mishandled the plane in reaction to the turbulence, with the first officer inadvertently holding the left rudder pedal to the cockpit floor as he and the captain pulled back on the control stick to break their plunge.
Some investigators suggest a part in the rudder linkage, a hydraulic valve known as the power control unit, may have jammed, causing a rudder reversal. If the first officer pressed on the right pedal to counteract a leftward movement of the plane, he would have held a flip-flopped rudder in the wrong position.
The FAA argues that no one will ever know the cause with any certainty, so it has focused on making the plane safer.
Working with Boeing, the FAA has redesigned the power control unit to prevent rudder reversals and also created a limiting device to reduce the amount of rudder that can be applied in high-speed situations. Both have also developed new training to help pilots recover from surprise changes in course.
Sources close to the board, who spoke on the condition of anonymity, said the four members have been divided about whether to accept the fixes or order more that could cost the airlines millions and possibly create service disruptions. The fifth NTSB member, John Goglia, is not participating in the hearing because he led an investigation into the accident while working as a mechanic for USAir.
A way out of that dilemma, the sources said, would be to point out the need for more changes but let the engineers and government regulators determine which ones to make.
Some 100 family members were attending today's hearing.
"I'm here to try to see what they are going to do to prevent this from happening again," said Jennifer Duffy-Burns, whose 28-year-old brother Joseph died in the crash.
Also watching the proceedings was the Seattle-based Boeing, which has a huge financial stake in the outcome.
The 737 is the most popular commercial airliner, with over 3,100 in the worldwide fleet. The plane has amassed over 91 million flight hours, and because of its widespread use, one takes off an average of every six seconds around the clock.