Investigation Summary
The crash of Flight 427 was a watershed moment for aviation safety, particularly regarding the Boeing 737, the world's most widely used jetliner. For years, the NTSB had investigated similar incidents, including the 1991 crash of United Airlines Flight 585, without a definitive conclusion. The breakthrough came when investigators discovered that the dual servo valve in the rudder's Power Control Unit could jam in a way that reversed the rudder's response. If the valve was subjected to thermal shock (hot hydraulic fluid entering a cold valve) and the primary slide overtraveled, a pilot's attempt to correct a left roll with right rudder would actually force the rudder further left. This rudder hardover occurred at an altitude too low for the crew to diagnose or recover from the resulting aerodynamic stall.
Final Conclusions
The NTSB determined the probable cause was a loss of control resulting from the movement of the rudder surface to its blowdown limit. The rudder deflected in a direction opposite to that commanded by the pilots due to a jam of the main rudder power control unit servo valve secondary slide. This jam, combined with the overtravel of the primary slide, caused the reversal. The investigation also highlighted the 'crossover airspeed' phenomenon, where the aircraft's roll spoilers cannot overcome the rolling moment produced by a fully deflected rudder.
Photographic Evidence (1)
Video Analysis
Investigation of USAir Flight 427 Boeing 737 Rudder Hardover — This video examines the 1994 crash near Pittsburgh caused by a mechanical failure in the Boeing 737 rudder power control unit, leading to an uncommanded roll and the longest investigation in NTSB history.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Crew Experience
Captain
Germano logged approximately 12,000 flight hours, including 4,064 on the Boeing 737
First Officer
Emmett logged 9,119 flight hours, 3,644 on the 737.
Systems & Failure Modes
Rudder Power Control Unit (PCU)
The hydraulic actuator that moves the rudder. A jam in its dual servo valve caused the rudder to move opposite to pilot input.
Dual Servo Valve
A component within the PCU containing two slides; a secondary slide jam led to the rudder reversal.
Flight Data Recorder (FDR)
The FDR on N513AU only recorded 11 parameters, which initially hindered the investigation into the rudder's movement.
Interesting Facts
- 01The investigation lasted four years and seven months, one of the longest in NTSB history.
- 02The crash resulted in the first-ever mandatory redesign of a critical flight control system on an existing jetliner fleet.
- 03All 132 people on board were killed instantly; the impact was so severe that the aircraft disintegrated.
- 04The findings helped solve the previously 'unsolved' crash of United Airlines Flight 585 in Colorado Springs.
- 05The accident led to the passage of the Aviation Disaster Family Assistance Act of 1996.
Safety Actions & Advisories
Boeing 737 Rudder Redesign
The FAA mandated a complete redesign of the Boeing 737 rudder control system, including a new valve designed to prevent reversals and the addition of a redundant actuator.
FDR Parameter Expansion
The NTSB recommended significantly increasing the number of parameters recorded by Flight Data Recorders, specifically requiring the monitoring of pilot control inputs and control surface positions.
Upset Recovery Training
Airlines were required to implement advanced training for pilots to recognize and recover from unusual attitudes and uncommanded flight control inputs.
Location
ACCIDENT SITE · Coordinates taken from the wikidata coordinate.