Robinson Helicopter R22 BETA N184SH accident near Caldwell, Idaho

Probable cause

Abnormal ground contact during a 180 degree autorotation, resulting in a severed tail rotor driveshaft by a main rotor blade. A factor contributing to the accident was the rotor blowback condition due to the aft tilting of the main rotor disk.

NTSB analysis

The flight instructor reported that the student pilot entered a practice 180 degree autorotation with a power recovery maneuver. He reported that as the student pilot started to flare and raise the collective, the helicopter "fell dramatically and began to shake," at which time he took the flight controls. The flight instructor recovered the helicopter and landed. The helicopter landed with forward momentum and departed the runway to the right. He reported that "it did not seem as though the landing was very hard." A postflight inspection revealed that a main rotor blade had struck the tail boom, severing the tail rotor driveshaft.

The flight instructor reported there were no pre-impact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.

Text from the NTSB. Open the NTSB docket for GAA15CA181 for the full record.