On September 8, 1994, USAir Flight 427, a Boeing 737-300 on a scheduled flight from Chicago to Pittsburgh, crashed while maneuvering to land at Pittsburgh International Airport. The airplane was destroyed by impact forces and all 132 persons on board were fatally injured. Three years of investigation has failed to yield conclusive proof of why the aircraft crashed. What is known is that the aircraft encountered wake turbulence from a preceding aircraft while at 6,000 feet and 190 knots. The effects of the wake should have been easily recoverable. However, a few seconds after encountering the wake vortex, the 737's rudder deflected full-left and remained in that position for 23 seconds until the aircraft impacted the ground in a near-vertical position.