USAir Flight 427



N513AU, the aircraft involved in the accident, wearing the 1979 livery

Occurrence
Date: September 8, 1994
Summary: Loss of control due to rudder hardover
Site: Hopewell Township, Beaver County, Pennsylvania
Coordinates: 40.60393°N 80.31026°W

Fatalities: 132
Survivors: 0

USAir Flight 427 was a scheduled flight from Chicago 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 while approaching runway 28R of Pittsburgh International Airport, located in Findlay Township, Allegheny County, Pennsylvania, which at the time was the largest hub for the airline.

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 the aircraft were killed.

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Involved

Aircraft

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]

Crew

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 they were both very experienced: Captain Germano logged approximately 12,000 flight hours (including 4,064 on the Boeing 737), while First Officer Emmett logged 9,000 flight hours, with 3,644 of them on the Boeing 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.

Crash

During its arrival into 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. Flight 427 was on approach at 6,000 feet (1,800 m) altitude, at flaps 1 configuration, and at approximately 190 knots.

At 7:02:57 p.m., the aircraft entered the wake turbulence of the Delta 727 it was sequenced behind, and there were three sudden thumps, clicking sounds, and a louder thump, after which the 737 began to bank and roll to the left. As the aircraft stalled, Germano exclaimed "hold on" numerous times, while Emmett, under physical exertion, said, "oh shit." Germano exclaimed, "what the hell is this?" As air traffic control noticed Flight 427 descending without permission, Germano keyed the mic and stated, "four-twenty-seven, emergency!" The mic was keyed for the rest of the event, resulting in the following exclamations in the cockpit being heard in the tower at Pittsburgh. The aircraft continued to roll while pitched nose-down at the ground, at which point against sharply rising G-forces, Germano yelled "pull" three consecutive times before screaming, during which Emmett stated "God, no" seconds before impact. In an 80-degree nose-down position, banked 60 degrees left and traveling at approximately 300 mph (480 km/h), the 737 slammed into the ground and exploded at 7:03:25 p.m. in Hopewell Township, Beaver County, near Aliquippa approximately 28 seconds after entering the wake turbulence.

Investigation

Main article: Boeing 737 rudder issues

The National Transportation Safety Board investigated the crash. All 127 passengers and five crew members were killed. For the first time in NTSB history, investigators were required to wear full-body bio-hazard 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 bio-hazard, 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 CVR and FDR were recovered and used for the investigation. Due to the limited parameters recorded by the FDR, investigators did not have access to the position of the flight-control surfaces (rudder, ailerons, elevator, etc.) for the accident. However, two parameters recorded by the FDR were crucial, one being the aircraft's heading, the other being the pitch control yoke position. During the approach, Flight 427 encountered wake turbulence from Delta 1083; the FAA, however, determined "the wake vortex encounter alone would not have caused the continued heading change that occurred after 19:03:00."[1]:245 The abrupt heading change shortly before the dive pointed investigators immediately to the rudder. Due to the absence of rudder pedal positions from the data, investigators had to determine whether the rudder moved hard-over by a malfunction or by pilot command. This in turn led to the CVR being more heavily scrutinized than most other recordings as statements and breathing from the pilots could potentially tell investigators if they were fighting for control over a rudder malfunction or inadvertently stomped on the wrong rudder pedal in excitement from the wake-turbulence. Boeing felt the latter more likely, while USAir and the Pilot's Union felt the former was more likely.[1][11] The FDR revealed that after the aircraft stalled, it along with the crew and passengers were subjected up to a 4 g load throughout the dive until impact with the ground in an 80 degree nose-down attitude traveling 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 control 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 aggravated the bank angle further. Boeing's test pilots reenacted the fatal dive in both a simulator, as well as a test 737-300 by flying the same parameters on 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 on the proper recovery technique for the situation experienced by the pilots and that the pilots had just 10 seconds from the onset of the roll to troubleshoot before recovery of the aircraft was impossible.



Impact crater of USAir Flight 427



Recovered wreckage under examination

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. In light of this, Boeing felt that the test results were not real-world and not applicable due to the extremes under which the valve was tested. Boeing stated that the rudder reversal was more likely psychological, likening it to examples when a human panics and intends to step on the brake during an automotive accident, but accidentally presses on the gas pedal instead while under duress. The FAA's official position was that there was not enough evidence for probable cause 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 due to 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.

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.

Aftermath

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 director of aircraft certification, issued a statement on the same day it was issued which 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, of which there were then more than 3,400 in the USA alone.

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[1] Aircraft Accident Report – Uncontrolled Descent and Collision With Terrain, USAir Flight 427, Boeing 737-300, N513AU, Near Aliquippa, Pennsylvania, September 8, 1994 (PDF). National Transportation Safety Board. March 24, 1999. NTSB/AAR-99/01. Archived (PDF) from the original on March 4, 2016. Retrieved January 17, 2016.]

[11] Adair, Bill (2002). The Mystery of Flight 427: Inside a Crash Investigation. ISBN 1-58834-005-8.