Robinson R22 N923SH accident near Fulton, Missouri

Probable cause

A total loss of engine power due to carburetor icing, the pilot's delayed application of carburetor heat, and the pilot's delay in entering an autorotation following the loss of engine power, which resulted in a main rotor stall due to low rotor rpm and an uncontrolled descent into terrain.

NTSB analysis

The pilot was conducting a solo cross-country flight in the helicopter as part of his instruction to obtain his rotorcraft rating. The intent of the flight was to fly solo cross-country to perform touch and go landings at another airport. About 36 minutes after departing on the flight, the helicopter impacted terrain. One witness reported that he saw the helicopter flying and heard the engine stop. He stated that the helicopter began to lose altitude and began to spin in a downward spiral. Another witness reported that he observed the helicopter flying toward him with its nose pointed downward about 45°. He stated that the blades were turning slowly and that the helicopter was making a circular motion along its axis, but not a spiral, as it flew toward the ground.

The helicopter impacted terrain in a nose-low, left-skid-down attitude. Examination of the airframe and engine did not reveal any mechanical pre-impact anomalies. The main rotor blade damage was consistent with low rotor rpm at the time of impact, and the low rotor rpm caution light filament was stretched, consistent with the light being illuminated at the time of impact. Examination of the rotating engine components did not reveal any evidence consistent with rotation at the time of impact. The carburetor heat control in the cockpit was found unlocked and found in the "on" position. According to a carburetor icing probability chart, the atmospheric conditions about the time of the accident were conducive to moderate icing at cruise power settings and serious icing at descent power settings. Thus, it is likely that the helicopter experienced a loss of engine power due to carburetor ice accumulation. Although the carburetor heat was found in the on position, it is likely that the pilot applied the carburetor heat to address the loss of engine power but it was too late and at too low an altitude to restore engine power.

Following the loss of engine power, the main rotor rpm would have rapidly begun to deteriorate unless the pilot immediately reduced collective and entered an autorotation. Given the signatures of low rotor rpm present at the site, it is likely that the pilot failed to initiate the autorotation in a timely manner, which allowed the main rotor rpm to deteriorate to the extent that the main rotor stalled and the pilot would be unable to recover.

Text from the NTSB. Open the NTSB docket for CEN19FA009 for the full record.