Event Summary
Shortly after takeoff, the pilot contacted air traffic control and requested to return to the airport for landing. When asked if assistance was needed, the pilot replied that assistance was not required at that moment but that he needed to land as soon as possible. After being asked about the problem, the pilot requested to stand by. About 30 seconds later, while on a left downwind for runway 35R, the pilot declared an emergency and stated that an engine was being shut down, requesting emergency equipment.
Air traffic control and witnesses observed the airplane on base leg, then overshooting the final approach to runway 35R. The controller cleared the pilot to an alternate runway (runway 28), but received no response. The controller observed the airplane's landing lights turning toward the terrain and reported that the airplane was gone. A witness described the airplane making an immediate sharp bank to the left and descending nose-first at an angle slightly less than 45 degrees.
Investigation Findings
A performance study revealed that while on downwind, the airplane began banking left, maintaining a constant left bank of about 24 degrees as it turned to base leg. Twenty-three seconds later, the bank angle increased as the airplane turned to final approach, overshooting the runway. The angle of attack reached the stall angle of approximately 17 degrees. Subsequently, the flight path angle decreased by 22 to 25 degrees, calibrated airspeed decreased by 40 to 70 knots, and vertical speed indicated a descent rate of 3,000 feet per minute just before impact.
Examination of the airframe showed that the flaps were set to 20 degrees and landing gear was retracted. According to the airplane flight manual, flaps should remain at 5 degrees and landing gear should be extended during base leg; when landing is assured, flaps should be extended to 20 degrees while maintaining 125 knots calibrated airspeed (KCAS) during final and 110 KCAS over the runway. The minimum controllable airspeed (Vmc) for the airplane is 99 KCAS.
Propeller examination revealed that at impact, the left propeller was feathered and the right propeller was in the normal operating range. Examination of the left engine showed static witness marks on internal components, with no anomalies that would have prevented normal operation. The reason for the precautionary shutdown of the left engine was not determined. Examination of the right engine revealed rotational scoring and metal spray deposits on internal components. Four vanes of the oil pump transfer tube were separated and missing. The gearbox oil-scavenge pump was not free to rotate and was disassembled; one separated oil pump transfer tube vane was found inside. Pitting and wear damage were noted on all roller bearing elements and the outer bearing race of the propeller shaft roller bearing. No other anomalies were found.
Probable Cause
The official probable cause was the pilot's failure to maintain minimum controllable airspeed during the night visual approach, resulting in a loss of control and uncontrolled descent into terrain. A contributing factor was the precautionary shutdown of the left engine for undetermined reasons.