USAir Flight 427

Investigation

The NTSB investigated the crash. All 127 passengers and 5 crew members on board were killed. For the first time in NTSB history, investigators were required to wear full-body biohazard suits while inspecting the accident site. As a result of the severity of the crash impact, the bodies of the passengers and crew were severely fragmented, leading investigators to declare the site a biohazard, requiring 2,000 body bags for the 6,000 recovered human remains. USAir had difficulty determining Flight 427's passenger list, facing confusion regarding five or six passengers. Several employees of the United States Department of Energy had tickets to take later flights, but used them to fly on Flight 427. One young child was not ticketed. Among the victims of the crash was noted neuroethologist Walter Heiligenberg.

Both the cockpit voice recorder (CVR) and flight data recorder (FDR) were recovered and used for the investigation. 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.) during the accident sequence. However, two parameters recorded were crucial — the aircraft's heading and the pitch-control yoke position. During the approach, Flight 427 encountered wake turbulence from Delta 1083, but the FAA determined "the wake vortex encounter alone would not have caused the continued heading change that occurred after 19:03:00." The abrupt heading change shortly before the dive pointed investigators immediately to the rudder. Without data relating to the rudder pedal positions, investigators attempted to determine whether the rudder moved hard over by a malfunction or by pilot command. The CVR was heavily scrutinized as investigators examined the pilots' words and their breathing to determine whether they were fighting for control over a rudder malfunction or had inadvertently stomped on the wrong rudder pedal in reaction to the wake turbulence. Boeing felt the latter more likely, while USAir and the pilots' union felt that the former was more likely. 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 under significant sideslip.

Reading the control-yoke data from the FDR revealed that the pilots made a crucial error by pulling back on the yoke throughout the dive, with the stick shaker audible on the CVR from the onset of the dive. 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. Because the aircraft had entered a slip, pulling back on the yoke only further aggravated the bank angle. 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. 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 NTSB remarked that no airline had ever trained a pilot to properly recover from the situation experienced by the Flight 427 pilots and that the pilots had just 10 seconds from the onset of the roll to troubleshoot before recovery of the aircraft was impossible.

Investigators later discovered that the recovered accident rudder power control unit was much more sensitive to bench tests than new control units. 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. The jam left no trace of evidence after it occurred, and a Boeing engineer later found that a jam under this controlled condition could also lead to the slide moving in the opposite direction than that commanded. Boeing felt that the test results were unrealistic and inapplicable, given the extremes under which the valve was tested. It stated that the cause of the rudder reversal was more likely psychological and likened the event to a circumstance in which an automobile driver panics during an accident and accidentally presses on the gas pedal rather than the brake pedal. 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.

After the longest accident investigation in NTSB history — lasting more than four and a half years — the NTSB released its final report on March 24, 1999. The NTSB concluded that the accident was the result of mechanical failure:

The NTSB concluded that similar rudder problems had caused the previously mysterious March 3, 1991, crash of United Airlines Flight 585 and the June 9, 1996, incident involving Eastwind Airlines Flight 517, both Boeing 737s. The final report also included detailed responses to Boeing's arguments about the causes of the three accidents.