USAir Flight 427
USAir Flight 427 was a scheduled flight from Chicago's O'Hare International Airport to Pittsburgh International Airport, with a final destination of West Palm Beach, Florida. On Thursday, September 8, 1994, the Boeing 737 flying this route crashed in Hopewell Township, Pennsylvania while approaching Runway 28R at Pittsburgh, which at the time was the airline's largest hub.
|Date||September 8, 1994|
|Summary||Loss of control due to rudder hardover|
|Site||Hopewell Township, Beaver County, Pennsylvania, US |
|Aircraft type||Boeing 737-3B7|
|Flight origin||O'Hare International Airport|
|Stopover||Pittsburgh International Airport|
|Destination||West Palm Beach Int'l Airport|
After the longest investigation in the history of the National Transportation Safety Board (NTSB), it was 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 the pilots were unable to recover. All 132 people on board were killed, making the crash the deadliest air disaster in Pennsylvania's history.
The aircraft involved was a Boeing 737-3B7, registration N513AU, and previously registered as N382AU. The aircraft was delivered in 1987 and was powered by two CFM56-3B2 engines. The aircraft had recorded approximately 18,800 hours of flight time before the crash.:1:6
The flight crew consisted of Captain Peter Germano, 45, who was hired by USAir in February 1981, and First Officer Charles B. "Chuck" Emmett III, 38, who was hired in February 1987 by Piedmont Airlines (which merged into USAir in 1989). Both were regarded as excellent pilots and were very experienced: Germano logged approximately 12,000 flight hours, including 4,064 on the Boeing 737, while Emmett logged 9,000 flight hours, 3,644 on the 737. Flight attendants Stanley Canty and April Slater were hired in 1989 by Piedmont Airlines. Flight attendant Sarah Slocum-Hamley was hired in October 1988 by USAir.:7–11
In its arrival phase approaching Pittsburgh, Flight 427 was sequenced behind Delta Air Lines Flight 1083, a Boeing 727-200. At no time was Flight 427 closer than 4.1 miles to Delta 1083, according to radar data.:2 Flight 427 was on approach at 6,000 feet (1,800 m) altitude, at flaps 1 configuration, and at approximately 190 knots.
At 19:02:57, the aircraft entered the wake turbulence of Delta 1083, and three sudden thumps, clicking sounds and a louder thump occurred, after which the 737 began to bank and roll to the left.:4 As the aircraft stalled, Germano exclaimed "Hold on!" numerous times,:138 while Emmett, under physical exertion, said, "Oh shit!":143 Germano exclaimed, "What the hell is this?":6 As air traffic control noticed Flight 427 descending without permission, Germano keyed the mic and stated, "Four-twenty-seven, emergency!":6 Because the mic remained keyed for the rest of the incident, the ensuing exclamations in the cockpit were heard in the tower at Pittsburgh. The aircraft continued to roll while pitched nose-down at the ground. Trying to counteract sharply rising G-forces, Germano yelled "Pull!" three consecutive times before screaming, during which Emmett stated "God, no" seconds before impact. Pitched 80o nose-down and banked 60o left while traveling at approximately 300 mph (480 km/h), the 737 slammed into the ground and exploded at 19:03:25 in Hopewell Township, Beaver County, near Aliquippa, approximately 28 seconds after entering the wake turbulence.
The National Transportation Safety Board investigated the crash. All 127 passengers and five crew members were killed.:ix 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 U.S. 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.":245 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.: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 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.:153
Investigators later discovered that the recovered accident rudder power control unit was much more sensitive to bench tests than other new such 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 lab 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 FAA's official position 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 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 blowdown 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 overtravel of the primary slide.:ix
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.:292–295 The final report also included detailed responses to Boeing's arguments about the causes of the three accidents.
At the time of the crash, Flight 427 was the second-deadliest accident involving a Boeing 737 (all series); as of 2020, it now ranks as the ninth deadliest. It was also the seventh-deadliest aviation disaster in the history of the United States, and the deadliest in the U.S. involving a 737; as of 2020, it ranks eleventh. The accident marked USAir's fifth crash in the period from 1989 to 1994. The Commonwealth of Pennsylvania spent approximately $500,000 in recovery and cleanup for the accident site.
The FAA disagreed with the NTSB's probable-cause verdict and Tom McSweeney, the FAA's director of aircraft certification, issued a statement on the same day on which the NTSB report was issued that read: "We believe, as much as we have studied this aircraft and this rudder system, that the actions we have taken assure a level of safety that is commensurate with any aircraft."
However, the FAA changed its attitude after a special task force, the Engineering Test and Evaluation Board, reported in July 2000 that it had detected 46 potential failures and jams in the 737 rudder system that could have catastrophic effects. In September 2000, the FAA announced that it wanted Boeing to redesign the rudder for all iterations of the 737, affecting more than 3,400 aircraft in the U.S. alone.
USAir submitted to the NTSB that pilots should receive training with regard to a plane's crossover speed and recovery from full rudder deflection. 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. Boeing maintained that the most likely cause of the crash was that the co-pilot inadvertently deflected the rudder hard over in the wrong direction while in a panic and for unknown reasons maintained this input until impact with the ground.:96–100 Boeing agreed to redesign the rudder control system with a redundant backup and paid to retrofit the entire worldwide 737 fleet. Following one of the NTSB's main recommendations, airlines were required to add four additional channels of information into flight data recorders in order to capture pilot rudder pedal commands, and the FAA set a deadline of August 2001 for airlines to comply. 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.
Following the airline's response to the Flight 427 accident, the United States Congress required airlines to deal more sensitively with the families of crash victims. USAir ceased using Flight 427 as a flight number. The crash was the second fatal USAir crash in just over two months, following the July 2 Flight 1016 accident at Charlotte-Douglas International Airport that killed 37. The crashes contributed to the financial crisis that USAir was experiencing at the time.
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- NTSB Accident Report
- NTSB Accident Investigation Docket (Archive)
- CVR Transcript (Archive)
- FDR Transcript (Archive)
- Boeing's report to the NTSB (Archive)
- ALPA's report to the NTSB (Addendum) (Archive) (Addendum archive)
- Accident description at the Aviation Safety Network
- Photos of the accident aircraft from Airliners.net
- 25 years ago, USAir Flight 427 crashed near Hopewell despite perfect conditions