Accident Overview
On August 14, 2021, about 1030 mountain daylight time, a Robinson R-22B helicopter, registration N226WM, sustained substantial damage in an accident near West Jordan, Utah. The flight instructor was seriously injured, and the pilot receiving instruction (PRI) sustained minor injuries. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight.
Flight History
The PRI, who held a private pilot certificate with an airplane single-engine land rating, was on his second instructional flight toward a rotorcraft rating. They departed around 0800 and performed basic rotorcraft maneuvers. During this practice, the low rotor rpm indication occurred several times; during a hover, they had to manually rotate the throttle to add power. After about an hour, they landed and refueled, then departed to a practice area for pinnacle approaches.
The Accident
About 20 minutes after departure, they arrived at the practice area. The PRI orbited a ridge initially at about 500 ft above ground level (AGL), then descended to 300 ft AGL. While descending further, he selected a landing site but overshot it. As the helicopter reached about 30 ft AGL, both occupants heard the low rotor rpm horn and saw the light. The instructor told the PRI to lower the collective and advance the throttle, but the throttle was already fully open. The PRI then pushed the cyclic forward to gain airspeed, but the instructor immediately took control and lowered the collective to increase rotor rpm. The helicopter descended into the ground; the left landing skid struck a large rock, causing the helicopter to roll over onto its left side.
Postaccident Examination
The helicopter's tailboom sustained substantial damage. The engine was examined and found to have mechanical continuity, compression, and proper magneto timing. An engine and governor functional test showed normal operation, with the governor engaging without delay. The forward vee-belt was mostly missing, with only a 5-inch section recovered; the aft vee-belt was dislodged. Rub marks and score marks were noted on sheaves and structural tubes. The instructor stated he performed a preflight inspection and a quick walkaround after refueling, checking vee-belt tension without specific criteria. The operator's records indicated three vee-belt-related squawks in the prior year, including slow blade engagement and belt tightness adjustments.
Governor and Belt Issues
The instructor noted a slight delay in the governor advancing the throttle to 104% rpm during pattern work before the accident flight but proceeded without reporting it. However, postaccident testing found no delay. The manufacturer offered a scenario where belt misalignment could lead to belt failure and subsequent slip, potentially reducing rotor drive.
Low Rotor RPM and Pilot Actions
The FAA Helicopter Flying Handbook describes low rotor rpm as a condition where rotor speed falls below safe operating range, potentially leading to rotor stall. The pilot's operating handbook (POH) states that a low rpm light/horn indicates rotor speed below 97% and recommends lowering collective, twisting throttle on, and applying aft cyclic in forward flight. The POH also includes Robinson Safety Notice SN-29, warning airplane pilots against abruptly pushing the cyclic forward in a helicopter.