Wrapping up a two-day hearing into the cause of the USAir accident, the board concluded unanimously that the current design of the rudder system on the Boeing 737, the type of plane involved in all three cases, is not "reliably redundant."
The 737 is the only commercial aircraft that has two rudder control systems that merge in a single hydraulic valve.
The board agreed that a jam of that valve caused the crash of Flight 427, 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 struggled to maintain control of their aircraft as they prepared to land in Richmond, Va.
The USAir crash killed all 132 aboard; all 25 on the United plane died.
The Boeing Co., which makes the 737, the world's most common jetliner, hotly contests the board's finding, saying there is no physical evidence that supports its scenario.
The board also approved 10 safety recommendations, including one asking Boeing and the Federal Aviation Administration to convene a board to ensure the rudder system is made redundant.
The investigation into why USAir Flight 427 plunged into a Pennsylvania hillside has been one of the longest and most expensive air crash investigations in U.S. history, reports CBS News Correspondent Bob Orr.
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.
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 - where a mechanical malfunction causes the rudder to move in the opposite direction from which it was intended. It 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.
On a calm summer evening, Flight 427 was flying just above 6,000 feet on its final approach to Pittsburgh.
It hit a bump of rough air caused by the wake of another plane. It rocked to the left, nearly leveled out and then rolled over on its back and spiraled to the ground, killing all 132 people on board.