What happened
On April 21, 2012, a Robinson R22 Alpha, registration N121MR, operated by Pollux Aviation Limited, was flying in a practice area near Palmer, Alaska. The pilot attempted to land the helicopter on a surface believed to be firm snow. During the landing, the aircraft broke through the surface, causing the tailrotor to strike the snow.
Following the strike, the pilot repositioned the helicopter. At that time, the tailrotor gearbox chip light illuminated.
The investigation
An inspection of the aircraft revealed that the tailrotor driveshaft had sheared.