On Thursday, September 8, 1994, the Boeing 737 flying this route crashed in Hopewell Township, Pennsylvania while approaching Runway 28R at Pittsburgh, which was USAir's largest hub at the time.
The National Transportation Safety Board (NTSB) determined that the probable cause was that the aircraft's rudder malfunctioned and went hard over in a direction opposite to that commanded by the pilots, causing the plane to enter an aerodynamic stall from which Captain Peter Germano and First Officer Charles B. Emmet III were unable to recover.
[1]: 35 The flight crew consisted of Captain Peter Germano,[3] aged 45, who was hired by USAir on February 4, 1981,[1]: 8 and First Officer Charles B.
"[1]: 6 As air traffic control noticed Flight 427 descending without permission, Germano keyed the microphone and stated, "Four-twenty-seven, emergency!
[1]: ix For the first time in NTSB history, investigators were required to wear full-body biohazard suits while inspecting the accident site.
Because of the limited parameters recorded by the FDR, investigators did not have access to the position of the flight-control surfaces (rudder, ailerons, elevator, etc.)
[1][11] The FDR revealed that after the aircraft stalled, the plane and its occupants were subjected to a load as high as 4 g throughout the dive until impact with the ground in an 80-degree nose-down attitude at approximately 300 mph (480 km/h) under significant sideslip.
This raised the aircraft's angle of attack, removed all aileron authority, prevented recovery from the roll induced by the rudder and caused an aerodynamic stall.
[11] Boeing's test pilots reenacted the dive in a simulator and in a test 737-300 by flying with the same parameters recorded by the accident FDR, and found that recovery from a fully deflected rudder at level flight, while at 190-knot crossover speed, was accomplished by turning the wheel to the opposite direction of the roll, and not pulling back on the yoke to regain aileron authority.
[11]: 153 The FAA later remarked that the CVR proved that the pilots failed to utilize proper crew resource management during the upset while continuing to apply full up elevator after receiving a stall warning.
The exact mechanism of the failure involved the servo valve, which remains dormant and cold for much of the flight at high altitude, seizing after being injected with hot hydraulic fluid that has been in continuous action throughout the plane.
This specific condition occurred in fewer than 1% of the laboratory tests, but explained the rudder malfunction that caused Flight 427 to crash.
[14][11] The official position of the Federal Aviation Administration (FAA) was that sufficient probable cause did not exist to substantiate the possibility of rudder system failure.
"[19] However, the FAA changed its attitude after a special task force, the Engineering Test and Evaluation Board,[14] reported in July 2000 that it had detected 46 potential failures and jams in the 737 rudder system that could have catastrophic effects.
[14] USAir submitted to the NTSB that pilots should receive training with regard to a plane's crossover speed and recovery from full rudder deflection.
[6] As a result, pilots were warned of and trained how to deal with insufficient aileron authority at an airspeed at or less than 190 knots (352 km/h), formerly the usual approach speed for a Boeing 737.
[1]: 96–100 [20] Boeing agreed to redesign the rudder control system with a redundant backup and paid to retrofit the entire worldwide 737 fleet.
[16] In 2016, former investigator John Cox stated that time has proven the NTSB correct in its findings, because no additional rudder-reversal incidents have occurred since Boeing's redesign.