| Date of Accident: | 08 September 1994 |  |
| Airline: | USAir |  |
| Aircraft: | Boeing 737-3B7 | |
| Location: | Aliquippa, Pennsylvania | |
| Registration: | N513AU | |
| Previous Registrations: | --- | |
| Flight Number: | 427 | |
| Fatalities: | 132:132 | |
| MSN: | 23699 | |
| Line Number: | 1452 | |
| Engine Manufacturer: | CFM International | |
| Engine Model: | CFM56-3B2 | |
| Year of Delivery: | 1987 | |
| Accident Description: | The aircraft, on a flight from Chicago's O'Hare International Airport, was approaching runway 28R at Pittsburgh when Air Traffic Control reported traffic in the area, which was confirmed in sight by the first officer. At that moment, the aircraft was levelling off at 6000ft and rolling out of a 15° left turn with flaps at 1°, the gear still retracted, an indicated airspeed of 190 knots, and autopilot and autothrottle systems engaged. The aircraft then suddenly entered the wake vortex of a Delta Airlines Boeing 727 which had preceded it by approximately 69 seconds. Over the next 3 seconds, the aircraft rolled left to approximately 18° of bank. The autopilot attempted to initiate a roll back to the right as the aircraft went in and out of a wake vortex core, resulting in two loud "thumps." The first officer then manually overrode the autopilot without disengaging it by putting in a large right-wheel command at a rate of 150°/sec. The airplane started rolling back to the right at an acceleration that peaked 36°/sec, but the aircraft never reached a wings level attitude. At 19.03:01 the aircraft's heading slewed suddenly and dramatically to the left (full left rudder deflection). Within a second of the yaw onset the roll attitude suddenly began to increase to the left, reaching 30°. The aircraft pitched down, continuing to roll through 55° left bank. At 19.03:07 the pitch attitude approached -20°, the left bank increased to 70° and the descent rate reached 3600 ft/min. At this point, the aircraft stalled. Left roll and yaw continued, and the aircraft rolled through inverted flight as the nose reached 90° down, approx. 3600 feet above the ground. The 737 continued to roll, but the nose began to rise. At 2000 feet above the ground the aircraft's attitude passed 40° nose low and 15° left bank. The left roll hesitated briefly, but continued and the nose again dropped. The plane descended fast and impacted the ground nose first at 261 knots in an 80° nose down, 60° left bank attitude and with significant sideslip.
PROBABLE CAUSE: "The National Transportation Safety Board determines that the probable cause of the USAir flight 427 accident was a loss of control of the airplane resulting from the movement of the rudder surface to its blow down limit. The rudder surface most likely deflected in a direction opposite to that commanded by the pilots as a result of a jam of the main rudder power control unit servo valve secondary slide to the servo valve housing offset from its neutral position and over travel of the primary slide." |
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| Related Links/Information: | Accident Photos - AirDisaster.Com Photo Database
Investigation Report
Cockpit Voice Recorder Transcript |
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