Investigation Summary
During a routine descent through 5,000 feet, the captain felt a distinct kick in the right rudder pedal, followed by a sharp uncommanded roll to the right as the aircraft passed 4,000 feet. The captain attempted to counter the roll with full left rudder, but the pedals were stiff and unresponsive. To maintain control, the crew applied opposite aileron and increased thrust on the right engine to create asymmetric lift. As part of their emergency checklist, the crew also turned off the yaw damper. After several seconds of struggle, the rudder suddenly snapped back to a neutral position, and the aircraft returned to normal handling. The NTSB later determined that the higher airspeed at which this incident occurred (250 knots), compared to the fatal crashes of United 585 and USAir 427, provided enough aerodynamic authority for the ailerons to counteract the rudder hardover until the malfunction cleared.
Final Conclusions
The NTSB concluded that the incident was caused by a malfunction of the rudder Power Control Unit (PCU) dual servo valve, which could jam and cause the rudder to move to its limit (hardover) or move in the opposite direction of pilot input. The investigation into Flight 517, combined with the previous fatal accidents, led to a complete redesign of the Boeing 737 rudder control system.
Photographic Evidence (1)
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Systems & Failure Modes
Rudder Power Control Unit (PCU)
A hydraulic actuator that moves the rudder. The PCU contains a dual servo valve that was found to be susceptible to jamming or reversal when exposed to thermal shock or contaminants.
Ailerons
Primary flight control surfaces on the wings used to control roll. In this incident, the ailerons provided enough counter-force to prevent the aircraft from rolling into an inverted dive.
Interesting Facts
- 01The aircraft was the first Boeing 737 to survive a rudder hardover event, allowing for a successful investigation.
- 02Prior to this incident, the same aircraft had a history of reported rudder-related 'bumps' and uncommanded movements.
- 03The NTSB determined that higher airspeed during the event provided the pilots with enough control surface authority to prevent a crash.
- 04The incident aircraft, N221US, was the same plane that later witnessed and reported the explosion of TWA Flight 800 in July 1996 while operating as Eastwind Airlines Flight 507.
Safety Actions & Advisories
Boeing 737 Rudder Redesign
The NTSB recommended, and the FAA mandated, a complete redesign of the Boeing 737 rudder system, including a new PCU with redundant valves to prevent jams and reversals.
Location
APPROXIMATE POSITION · AIRPORT LEVEL · Coordinates are the estimate published in the entered by an editor. The ring shows the accuracy the record supports, not the point of impact.