Accident Sequence
According to witnesses, the left engine appeared to lose power just after rotation and liftoff. The airplane initially achieved a positive rate of climb but then immediately yawed left as it cleared 30-foot-high power lines perpendicular to the flight path. The airport is located at the east end of a lake in a mountain valley; the airplane departed westward over the lake. Approximately 2 miles from the runway, witnesses observed dark smoke emanating from the left engine, and the smoke significantly increased as the flight continued. The airplane then banked hard left until its wings were perpendicular to the ground, after which it nosed in vertically. The landing gear remained extended throughout the accident sequence.
Post-Accident Examination
On-site examination revealed that the top spark plugs for the left engine were black and sooty. A detailed examination determined that the left turbocharger turbine wheel shaft had fractured and separated. Extreme oxidation of the fracture surfaces prevented identification of the failure mode; however, the oxidation indicated exposure to high temperatures, meaning the fracture occurred while the turbocharger was at elevated temperature during operation. The multiple planes exhibited by the fracture were not consistent with a ductile torsional failure, as would be expected from a sudden stoppage of either rotor. No evidence of mechanical malfunction was found in the right engine.
Manual Information
The Cessna Owners Manual for the airplane notes that the most critical time for an engine failure is a 2–3 second period late in the takeoff while the airplane is accelerating from the minimum single-engine control speed of 87 KIAS to a safe single-engine speed of 106 KIAS. Although the airplane is controllable at the minimum control speed, its performance is so far below optimum that continued flight near the ground is improbable. Once 106 KIAS is achieved, altitude can more easily be maintained while the pilot retracts the landing gear and feathers the propeller. The best single-engine rate-of-climb is 108 KIAS with flaps up below 18,000 feet msl. Section VI of the manual provides operational data for single-engine climb capability, valid only under specific conditions: gear and flaps retracted, inoperative propeller feathered, wing banked 5 degrees toward the operating engine, 39.5 inches of manifold pressure if below 18,000 feet, and mixture at recommended fuel flow.
Probable Cause
The official findings state that the probable cause was a failure of the turbine wheel shaft in the left turbocharger during the takeoff initial climb for undetermined reasons, and the pilot's failure to attain and maintain safe single-engine airspeed that led to a loss of control.