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The Pittsburgh 737 Crash

PUBLISHED: 12:42 23 June 2011 | UPDATED: 14:06 10 October 2012

Excerpts from the U.S. National Transportation Safety Board's report on the 1994 loss of USAir Flight 427, a Boeing 737 300, highlight detective work behind its conclusion that the accident was caused by uncommanded rudder movement.

Excerpts from the U.S. National Transportation Safety Board's report on the 1994 loss of USAir Flight 427, a Boeing 737 300, highlight detective work behind its conclusion that the accident was caused by uncommanded rudder movement.

The crew's exclamations of surprise when they encountered wake turbulence at 6,000 feet on the approach to Pittsburgh International Airport less than thirty seconds before the crash were evident in the cockpit voice recorder transcript.

The first officer applied right roll input to level the wings, but the aircraft then started to yaw, off heading, to the left. The captain said 'whoa' and in the NTSB's surmise the first officer reacted by pressing on the right rudder pedal; the flight data recorder showed that the aircraft then responded as if full left rudder had been applied.

The NTSB has since shown how this might happen by supplying the accident aircraft's main rudder power control servo valve with hotter than normal hydraulic fluid, causing one component to jam and resulting in temporary control reversal.

Sounds of grunting on Flight 247's voice recorder were evidence of the first officer's physical effort in holding pedal pressure against rudder feedback. This was relaxed for a moment when the crew disconnected the autopilot in an effort to restore control an action recorded as distinct clicks and a sound similar to the autopilot disconnect warning horn.

As the nose dropped the control column was pulled back. Consequently, speed fell below the critical, 187 knot 'crossover airspeed' where maximum aileron input can no longer counter full rudder and the left bank rapidly exceeded ninety degrees. There was insufficient height to recover before the aircraft struck the ground at 250 knots, killing all on board.

From the U.S. Flight Safety Foundation's journal Accident Prevention.

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