What happened
While descending to land at an altitude of approximately 9,800 feet MSL, the flight crew of a MD-11 encountered heavy precipitation. During this period, a false ground proximity warning system (GPWS) alert occurred. In response to the alert, the crew initiated an abrupt escape maneuver. The GPWS warning subsided during the execution of the maneuver, and the aircraft subsequently landed unevently. Both flight crewmembers were not injured.
The investigation
A post-flight inspection of the aircraft revealed that the outboard portion of the left elevator, including the balance weight, had partially separated. The investigation determined that the escape maneuver performed by the captain was aggressive but consistent with the procedures published in the Federal Express airplane flight manual for a "GPWS Pull Up Warning."
Technical analysis showed that the damage to the elevator resulted from the aircraft exceeding its design aerodynamic stall buffet boundary, which caused severe vibration of the elevator component. The investigation also found that the false readings from the number 1 and number 2 radio altimeters, which prompted the erroneous GPWS warning, were most likely caused by signal return from the heavy precipitation. No mechanical deficiencies were found in the flight controls, avionics, or the elevator structure itself.
Findings
- The MD-11 exceeded its design aerodynamic stall buffet boundary during the maneuver.
- The false GPWS warning was prompted by erroneous radio altimeter readings caused by heavy precipitation.