Accident Details
The accident occurred during a post-modification functional evaluation flight (FEF). The pilot flying (PF) applied inappropriate aft control column pressure during the clean stall maneuver recovery attempt, which did not conform to the stall recovery procedure in ABX's operations manual. The pilot not flying (PNF), occupying the right seat and serving as pilot-in-command and instructor, failed to recognize, address, or correct the PF's erroneous inputs. The aircraft subsequently impacted mountainous terrain.
Stall Warning System
The stall warning system, including the stick shaker, was inoperative. This prevented the flightcrew from receiving indications that the airplane was in a full stall during the recovery attempt, which could have reinforced the need for proper recovery action.
Simulator Fidelity
The flightcrew had been trained on the ABX DC-8 flight training simulator, which reproduced the aircraft's stall characteristics with low fidelity. This influenced the PF to maintain aft (stall-inducing) control column inputs when the airplane began to pitch down and roll, as the simulator did not accurately replicate the real stall behavior.
Programmatic Shortcomings
The investigation noted that the accident could have been prevented if ABX had formally instituted and the flightcrew had used the revised stall recovery procedure for FEF flights that was agreed upon in 1991. The informality of ABX's FEF training program allowed two pilots, neither of whom had ever handled the flight controls during an actual stall in the DC-8, to be paired for the flight.
Probable Cause
The official probable cause findings state: The inappropriate control inputs applied by the flying pilot during a stall recovery attempt, the failure of the non-flying pilot-in-command to recognize, address, and correct these inappropriate control inputs, and the failure of ABX to establish a formal functional evaluation flight program that included adequate program guidelines, requirements and pilot training for performance of these flights. Contributing to the causes of the accident were the inoperative stick shaker stall warning system and the ABX DC-8 flight training simulator's inadequate fidelity in reproducing the airplane's stall characteristics.