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PG Graphic: Flight 427 update
Wednesday, March 24, 1999 By Steve Thomas and Ted Crow, Post-Gazette Staff Artists
A rudder failure has long been suspected of causing the crash of USAir Flight 427, and have also been suspected of the crash of a United Airlines 737 in 1991 and the temporary loss of control of an Eastwind Airlines 737 in 1996. The rudder is the hinged part of the vertical tail section that is used to help keep the plane's nose pointed straight on takeoffs and landings. It is not used in normal flight, except for minute adjustments made by automatically by a device that prevents the plane from fishtailing.
A rudder reversal could have caused Flight 427 to crash. Under that scenario, when the plane started to roll to the left, a pilot would have pushed on the right rudder pedal to point the nose to the right, but instead of turning right, the rudder turned left in the opposite direction, making the problem even worse and leading to a loss of control.
Rudder movement: Signals from the pilot's rudder pedals are transmitted by cable to the input rod, which is connected to the external summing lever. The lever transfers the commands to a hydraulic servo valve, which powers a piston and rod assembly. The rod is attached to the rudder. When the rod extends, it pushes the rudder left, causing the plane to move left. When the rod retracts, it pulls the rudder right, causing the plane to move right.
Servo valve cutaway
At the heart of the rudder's power control unit is the servo valve. The cutaway drawing shows the valve and its secondary and primary slides. The movement of the slides inside the servo valve channels hydraulic fluid through the power control unit, which in turn controls rudder movement.
Potential disaster: One set of tests showed that in some cases, the secondary slide in the servo valve could move too far and go past the protruding "stops" that should have halted it. That caused fluid to flow incorrectly, making the rudder either stick in one position, or move opposite the direction the pilot intended.
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