Bell 47D1 N4251A accident near San Diego, California

Probable cause

The flight instructor's inadequate supervision of the student pilot during a simulated autorotation and the pilots’ failure to use carburetor heat while operating at reduced engine power while in carburetor icing conditions, which resulted in a loss of engine power.

NTSB analysis

During an instructional flight, a practice autorotation was initiated with the intention of performing a power recovery. The student maintained directional control while the certified flight instructor (CFI) adjusted the rotor rpm. The student initiated the power recovery, but the engine did not respond. The student then relinquished control of the helicopter to the CFI, who attempted to cushion the landing, but the helicopter landed hard.

The helicopter's engine was equipped with a float-type carburetor, and the weather conditions provided for the possibility of serious carburetor icing at glide power. The accumulation of carburetor ice likely resulted in the loss of engine power. Neither the CFI nor student applied carburetor heat during the autorotation, nor they did not monitor the carburetor temperature gauge during the maneuver. During the postaccident examination of the engine, no mechanical failures or malfunctions were revealed which would have precluded normal operation.

The CFI's flight experience in piston-powered helicopters was limited to the 6 hours of training he had provided in the accident helicopter, with his prior rotorcraft experience occurring exclusively in turbine-powered helicopters. This was also the first time he had performed an autorotation in the accident helicopter.

Text from the NTSB. Open the NTSB docket for WPR12LA029 for the full record.