Robinson R22 Beta N522SA accident near Spokane, Washington

Probable cause

A loss of engine power for reasons that could not be determined because postaccident examination of the airframe and engine did not reveal any anomalies that would have precluded normal operation and the student pilot's failure to maintain main rotor rpm. Contributing to the accident was the student pilot's low total flight experience.

NTSB analysis

After flying one time around the traffic pattern with the student pilot, who had made three previous solo flights, the flight instructor exited the helicopter. The student then completed about five touch-and-go landings. On the next approach, the helicopter appeared to be at a normal altitude and attitude for the base leg. The engine sputtered, and several witnesses heard one or two pops, and the rotor sounds went silent. Pieces flew off the helicopter, and then it began to spin clockwise, went nose low, and fell straight to the ground. One witness stated that the tail was v-shaped. Examination of the wreckage revealed strike marks consistent with main rotor blade contact with the tail boom, which separated from the airframe. The examination also revealed witness marks consistent with the engine not rotating at ground impact. The instructor observed the student pilot use carburetor heat while he was on board, but it could not be determined if it was applied on the accident flight. The conditions encountered in this accident were conducive for serious icing at cruise power. Due to numerous approaches in short succession, the engine may have never completely warmed to the point where it would shed all ice on each approach. The accumulation could have finally built up to the point that it caused a loss of engine power and subsequent low main rotor blade rpm condition if the student did not immediately lower the collective. However, it could not be determined if carburetor icing occurred. A safety notice issued by the helicopter’s manufacturer noted that main rotor blade stall due to low main rotor blade rpm caused a very high percentage of helicopter accidents. It pointed out that the stall would cause the main rotor blade rpm to rapidly decrease and lead to an immediate uncontrolled descent.

Text from the NTSB. Open the NTSB docket for WPR10FA277 for the full record.