Incident Summary
On a flight operating an MD-11 aircraft, the flight crew initiated an abrupt maneuver while descending to land. The maneuver was triggered by a false ground proximity warning system (GPWS) alert that activated as the airplane encountered heavy precipitation at an altitude of approximately 9,800 feet above mean sea level. The GPWS warning subsided during the maneuver. The airplane subsequently landed without further incident. Both flight crewmembers, who were the sole occupants, were not injured.
Investigation Findings
The investigation determined that the escape maneuver was aggressive and consistent with procedures published in Federal Express' airplane flight manual for "GPWS Pull Up Warning." Post-flight inspection revealed that the outboard portion of the left elevator, including its balance weight, was partially separated. The flight crew reported no handling difficulties after the escape maneuver.
Further examination indicated that the elevator damage resulted from an exceedance of the airplane's design aerodynamic stall buffet boundary, which caused a severe vibration of the elevator. The stall buffet boundary was exceeded due to the captain's initiation of the escape maneuver. The false GPWS warning was prompted by false readings from the no. 1 and no. 2 radio altimeters, most likely caused by signal return off heavy precipitation. No mechanical deficiencies were found in the flight controls, avionics, or elevator structure after the event.