Amateur-built SONEX SA N123SX accident near Oshkosh, Wisconsin

Probable cause

The pilot's failure to maintain sufficient airspeed following a loss of engine power during initial takeoff climb, which resulted in the airplane's wing exceeding its critical angle-of-attack and a subsequent aerodynamic stall. The reason for the loss of engine power could not be determined because examination of the wreckage revealed no mechanical deficiencies. Contributing to the accident was the pilot's decision to conduct an intersection takeoff.

NTSB analysis

The pilot and passenger departed on a local flight in the amateur-built experimental airplane using an intersection departure from the runway. Several frames of a surveillance video that captured the airplane during the initial climb from the runway showed a thin white trail of vapor/smoke behind the airplane. GPS data recovered from the airplane indicated that the airplane climbed to about 100 ft above ground level, leveled off, turned left at the end of the runway, and entered a descent. Shortly thereafter, the airplane collided with parked vehicles about 0.25 miles from the departure end of runway. Examination of the accident site indicated that the airplane impacted in a near-vertical attitude, consistent with an aerodynamic stall.

During examination of the wreckage, no evidence of airframe malfunction or failure was detected. The propeller displayed no evidence of rotational damage, consistent with a loss of engine power before impact. The engine's turbocharger would not rotate, and the turbocharger compressor housing displayed impact marks made by the compressor wheel, indicating that there was little relative motion between the blades and the housing at the time of the impact. The turbocharger's wastegate system was tested on an exemplar engine, and no anomalies were noted. It could not be determined whether the turbocharger would not rotate due to impact damage or whether it seized in flight resulting in a partial loss of engine power. Engine test runs with a turbocharger in a seized condition could not be conducted.

The mixture control lever was found near the idle-cutoff position with a corresponding witness/impact mark around the circumference of the control stem, indicating that the lever was in the cutoff position at the time of impact. It could not be determined at what point during the flight the lever was moved to the cutoff position. Attempts to produce white smoke from the engine by leaning the mixture to the cutoff position and by inducing a loss of engine power due to water contamination were unsuccessful. The exemplar engine's failure to produce white smoke when the mixture was leaned suggests that the loss of engine power was not the result of the pilot moving the mixture control lever to the cutoff position during takeoff. It is possible that the pilot moved the lever to the cutoff position after the airplane departed from controlled flight. The reason for the loss of engine power could not be determined.

The circumstances of the accident are consistent with the pilot failing to maintain sufficient airspeed following a loss of engine power during takeoff, resulting in the airplane's wing exceeding its critical angle-of-attack and a subsequent aerodynamic stall. Instead of using the full runway length of 6,179 ft, the pilot elected an intersection takeoff with about 2,570 ft of available runway. Calculations showed that, had the pilot used the entire runway for takeoff, sufficient runway for a landing following the loss of engine power would likely have been available.

Text from the NTSB. Open the NTSB docket for CEN15FA249 for the full record.