Aircraft Accident/Incident Report

Richmond, Virginia 23231
Sunday, June 9, 1996 22:00 EDT

The Boeing 737-200 experienced a yaw/roll upset while operating at an airspeed of about 250 knots and an altitude of about 4,000 feet msl in visual flight rules (VFR) conditions. The pilots were able to regain control of the airplane and land at the destination airport without further incident. As a result of previous Boeing 737 rudder anomalies, including the fatal accident involving USAir flight 427, a Boeing 737-300, which entered an uncontrolled descent and impacted terrain near Aliquippa, Pennsylvania, on September 8, 1994, Safety Board investigators initiated an investigation into this incident. 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. As a result, the Safety Board concluded that the rudder reversed, moving to its right blowdown limit when the captain commanded left rudder, 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. The complete report of this incident investigation, including all analyses, is contained in the final report of the USAir flight 427 investigation which can be found on the NTSB web site at the following link: http://www.ntsb.gov/publictn/1999/AAR9901.pdf

The yaw/roll upset 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.