BFO93IA175

During cruise flight, a rudder ratio warning light appeared, followed by uncommanded right rudder movement and left aileron input. The airplane remained under control and the crew counteracted the rudder movement manually. The captain elected to abort the flight and landed safely at the departure airport. The rudder ratio warning was displayed on two subsequent flights, despite repeated attempts to resolve the discrepancy. After the fourth warning was displayed in about two months, an examination of the left yaw damper module revealed a malfunction in the left hydraulic pressure switch. The unit was removed and replaced. No further incidents involving a rudder ratio warning were reported.

On September 13, 1993, about 0855 hours eastern daylight time, N757NA, a Boeing 757-200ER, operating as North American Airlines Flight 5, had a flight control problem during cruise flight. The flight had departed from John F. Kennedy International Airport (JFK), Jamaica, New York, en route to San Francisco, California, but returned to JFK and landed uneventfully. None of the eight crewmembers or 77 passengers were injured. Visual meteorological conditions prevailed and an instrument flight rules flight plan was filed. The flight was conducted under 14 CFR 121.

According to a FAA aviation safety inspector, a "rudder ratio" warning message was displayed in the cockpit as the airplane climbed through Flight Level 270 after departure. The crew performed the published company troubleshooting procedures, including resetting the rudder ratio circuit breaker light, and the warning was eliminated. About one hour later during cruise flight at Flight Level 350, the warning was displayed once again followed by gradual uncommanded right rudder movement. The autopliot was engaged at the time and two units of left aileron were inputted by the autopilot. The airplane remained under control and the crew counteracted the rudder movement manually. The captain elected to abort the flight and return to JFK.

An examination of the airplane's electrical system revealed that the fault ball on right rudder control module had popped out. The module was reset and successfully passed a functional check. The airplane was placed back into service.

The rudder ratio warning message was again displayed on two flights subsequent to this event. The flights terminated uneventfully. According to company maintenance records, maintenance activities to correct the situation during these events involved readjustment of the rigging, functional testing of the fault module, and replacement of the rudder ratio actuator. The airplane was placed back into service after each of these events.

On November 19, 1993, the rudder ratio light was displayed for the fourth time in about two months. An examination of the left yaw damper module revealed a malfunction in the left hydraulic pressure switch. The unit was removed and replaced. No further incidents involving a rudder ratio problem were reported.