Rudder problems may also have been responsible for an earlier accident involving a United Airlines 737 in 1991.
The National Safety Transportation Board unanimously endorsed findings that the pilots aboard USAir Flight 427, en route from Chicago to Pittsburgh on 8 September 1994, lost control of the jet when the rudder started doing the opposite to their foot-pedal commands.
The board identified a hydraulic valve in the rudder system that may have caused the malfunction. According to a computer reconstruction of the flight, the rudder abruptly swung to the left, flipping the plane hard to one side. When the pilot attempt to move it back to the right by applying pressure on his pedal, he was, in fact, pushing it farther left.
The aircraft spiralled 6,000 feet into the ground, killing all 132 people on board. The disaster was immediately linked to the 1991 United accident, in which a Boeing 737 crashed outside Colorado Springs, killing all 25 on board. That accident was never solved, although strong winds were considered a possible factor.
In another incident in 1996 near Richmond, Virginia, the pilots of an Eastwind Airlines 737 struggled to maintain controlbecause of rudder problems, but managed to land safely.
The board's findings could have expensive repercussions for the makers, Boeing, and could trigger up to 10 new safety recommendations from the federal government. Among these could be an order that Boeing redesigns the 737 to ensure that the rudder system can be "reliably redundant" - enabling other mechanical systems to override the rudder.
Yesterday's ruling rejected suggestions by Boeing that the USAir accident may have been caused by pilot error.
"A rudder reversal scenario will match all three events," said Dennis Crider, chairman of the board's aircraft performance group.
The investigation has been closely watched because of the worldwide popularity of the twin-engined Boeing 737. There are 3,100 of them in service, and up to 800 may be in the air at any one time. In attempting to emphasise the jet's wider safety record, Boeing has pointed out that a 737 is taking off somewhere in the world every few seconds.