Date: September 08, 1994
Time: 1903
Location: Near Aliquippa, Pennsylvania
Operator: USAir
Flight #: 427
Route: Chicago - Pittsburgh
AC Type: Boeing B-737-3B7
Registration: N513AU
cn / ln: 23699/1452
Aboard: 132   (passengers:127  crew:5)
Fatalities: 132   (passengers:127  crew:5)
Ground: 0
Summary: While on approach, the aircraft went into a sudden nose dive from 6,000 ft. and crashed into a wooded ravine 6 miles northwest of Pittsburgh International Airport. The accident was caused by a loss of control of the aircraft resulting from the movement of the rudder surface to its blowdown limit or an uncommanded rudder reversal. The rudder surface deflected in a direction opposite to that commanded by the pilots as a result of a jam of the main rudder PCU servo valve secondary slide to the servo valve housing offset from its neutral position and overtravel of the primary slide. The most likely sequence of events that led up to the accident included the jamming of the PCU servo valve, the application of light left rudder followed by hard right rudder which caused the rudder to reverse in the opposite direction the pilot commanded it to go. The application of hard right rudder was possibly initiated because the plane flew into the wake vortex of a B-727 which rolled the plane to the left. Rudder hardover is normally corrected with the stick (ailerons) but because the plane was flying at the crossover speed of 190 knots with flaps 1, using the stick would not correct the situation. When the right rudder was applied the rudder went to its fullblown left position causing the plane to roll further left, stall and go into a dive. Some speculate if the pilot-in-command pushed forward on the yoke to gain some speed rather than pull back, the accident possibly could have been avoided. Blame was not placed on the crew because there was no mention of this type of recovery by the manufacturer nor was there any training for such an occurrence.

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