Incident Overview
A Boeing 737-200 aircraft encountered a yaw/roll upset while operating at an airspeed of approximately 250 knots and an altitude of about 4,000 feet above mean sea level in visual flight rules (VFR) conditions. The flight crew successfully regained control of the airplane and completed a landing at the intended destination airport without any further incident.
Investigation Initiation
The Safety Board launched an investigation into this incident due to prior rudder anomalies on Boeing 737 aircraft, including the fatal accident of USAir flight 427, a Boeing 737-300, which entered an uncontrolled descent and struck terrain near Aliquippa, Pennsylvania, on September 8, 1994.
Investigation Scope and Methods
Investigators conducted detailed examinations of pertinent rudder control components, reviewed flight test data, performed a computer simulation, and completed an analysis of human performance data to determine the cause of the upset.
Findings
The Safety Board concluded that the rudder reversed, moving to its right blowdown limit when the captain commanded left rudder. This behavior was consistent with 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.
Report Availability
The complete report of this incident investigation, including all analyses, is contained in the final report of the USAir flight 427 investigation, available on the NTSB website at http://www.ntsb.gov/publictn/1999/AAR9901.pdf.