By the fall of 1996, a few months before Jim Hall made the case against the Boeing 737 in his long letter to the FAA, Bernie Loeb and Tom Hauteur and their colleagues at the NTSB had a fairly good idea of what had happened to cause the crash of Flight 427:
Flight 427 encountered the wake vortices of Delta Flight 1083 and rolled.
The pilot (or the yaw damper) applied right rudder to correct the roll.
The main rudder power control unit (PCU) servo valve jammed.
That jam led to a rudder reversal.
That reversal was a hardover.
It happened at 190 knots, close to the crossover speed for Flaps 1, where the ailerons cannot overcome the effect of the rudder.
The aircraft yawed, rolled, stalled, went into an uncontrollable spin, and crashed.
KeywordsNeutral Position Flight Control Wake Vortex Servo Valve Power Control Unit
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