The rudder system was also designed with an explosive charge to detach the rudders in the event they became jammed.

They eventually cured the problem by adding a movable rudder system, now found on nearly all aircraft.

"There is nothing that is unique about the rudder system," he said.

One theory centers on the yaw damper, an automatic device that sends signals to the rudder system to make small turns to prevent fishtailing.

During ascent, one of four flight control channels for the rudder system, the speedbrake, was behaving differently from the others, a malfunction known as a "force fight."

But she added that it "fails to direct attention to the problem of the rudder trim system that created the environment in which the accident occurred."

Hatecke had recognised that it was possible to obtain control even in diving flight by using the lift force itself instead of any mechanical rudder system.

Horizontal rudder system elevates the angle of rudders proportional to the angle of displacement the depth meter detect.

Q: The Seattle Times won a Pulitzer Prize this year for a series investigating what it described as technical problems with the rudder system on the 737.

Bolts in the rudder system that have shown signs of stress and corrosion will be replaced, although they are not linked to the two crashes.