Accident Boeing 737-3B7 N513AU,

ASN Wikibase Occurrence # 324908

Date:	Thursday 8 September 1994
Time:	19:03
Type:	Boeing 737-3B7
Owner/operator:	USAir
Registration:	N513AU
MSN:	23699/1452
Year of manufacture:	1987
Total airframe hrs:	23846 hours
Engine model:	CFMI CFM56-3B2
Fatalities:	Fatalities: 132 / Occupants: 132
Aircraft damage:	Destroyed, written off
Category:	Accident
Location:	near Aliquippa, PA - United States of America
Phase:	Approach
Nature:	Passenger - Scheduled
Departure airport:	Chicago-O'Hare International Airport, IL (ORD/KORD)
Destination airport:	Pittsburgh International Airport, PA (PIT/KPIT)
Investigating agency:	NTSB
Confidence Rating:	Accident investigation report completed and information captured

Narrative:
USAir Flight 427, a Boeing 737-300, crashed following a loss of control during the approach to Pittsburgh International Airport, Pennsylvania, USA. All 132 on board were killed.
The flight departed Chicago-O'Hare International Airport, Illinois at 18:10 hours on a flight to Pittsburgh. The first officer was Pilot Flying (PF) and the captain was the pilot-not-flying (PNF) on this leg. The flight until the descent part of the flight was uneventful.
About 19:02:22 the Pittsburgh TRACON controller issued instructions to turn to a heading of 100 degrees and told the crew there was other traffic in the area.
About 19:02:53, flight 427 was rolling out of the left bank as it approached the ATC-assigned heading of 100° and was maintaining the ATC-assigned airspeed (190 knots) and altitude (6,000 feet msl). About four seconds later the aircraft suddenly entered the wake vortex of Delta Airlines flight 1083, a Boeing 727, that preceded it by 69 seconds (4,2 miles).
Over the next 3 seconds the aircraft rolled left to approx. 18 deg 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 150deg/second. The airplane started rolling back to the right at an acceleration that peaked 36deg/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 30deg. The aircraft pitched down, continuing to roll through 55 degree left bank. At 19:03:07 the pitch attitude approached -20 degrees, the left bank increased to 70 degrees 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 degree 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 degrees nose low and 15 degrees 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 degree nose down, 60 degree 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."

Accident investigation: