Introduction
A twin-engine turboprop cargo airplane operating an international flight experienced a loss of control and broke up during descent at night. The accident occurred about 40 minutes into the flight under visual meteorological conditions. The airplane was not equipped with a flight data recorder or cockpit voice recorder, and there were no avionics with downloadable memory, limiting available data.
Flight Sequence
Air traffic control cleared the airplane to descend to 7,000 feet, then further to 3,000 feet, and advised the flight crew to expect an instrument landing system approach. During the descent, at approximately 7,300 feet and 290 knots, the airplane entered a shallow left turn, followed by a 45-degree right turn and a rapid, uncontrolled descent. The airplane broke up about 1,500 feet over uneven terrain.
Investigation Findings
Postaccident examination revealed no evidence of in-flight mechanical failures that would have caused the loss of control. The airplane was loaded within limits, and continuity of flight controls was confirmed to the extent possible. Both engines were operating at the time of impact. One propeller blade from the right propeller separated from a fractured hub, but no preexisting anomalies were found. The electric pitch trim actuator showed no evidence of runaway; measurements indicated a slightly nose-low trim, consistent with the phase of flight. The fuel cross feed valve was closed, suggesting fuel imbalance was not a concern for the flight crew. The positions of the aileron and rudder trim wheels could not be determined due to separation of the wings and tail.
Analysis of Control Loss
Investigators noted that similar accidents involved unequal fuel burns or autopilot issues. Although this airplane lacked an autopilot, historical examples showed that sudden yaw or roll could lead to loss of control. It is likely that during the descent, the flight crew regained some control, but darkness and rapid descent at low altitude led to aggressive yoke inputs. The flight crew also lowered the landing gear, possibly in an attempt to slow or regain control. The excessive elevator input during recovery caused the wings and horizontal stabilizers to fail upward as the airplane overstressed.
Probable Cause
The flight crew's excessive elevator input during a rapid descent under night lighting conditions, which resulted in the overstress and breakup of the airplane. Contributing to the accident was an initial loss of airplane control for reasons that could not be determined because postaccident examination revealed no mechanical anomalies that would have precluded normal operation.