Robinson Helicopter Company R44 II N323TT accident near Bluestem, Washington

Probable cause

The pilot’s failure to manage main rotor rpm during an autorotation, which resulted in a loss of helicopter control. Contributing to the accident was a loss of power for reasons that could not be determined.

NTSB analysis

The pilot and pilot-rated passenger were returning the helicopter to its home airport after it had undergone routine maintenance, which included the installation of a fuel flow transducer. While in day visual meteorological conditions (VMC), in cruise flight, and about 745 ft above ground level (agl), the helicopter entered a descent. During the descent, the main rotor blades impacted the tailcone, severing the empennage, while about the same time the main rotor’s pitch horns and control rod ends fractured and separated. The lack of tail rotor and main rotor components resulted in a total loss of helicopter control. The helicopter impacted terrain at a high descent rate in a nose-high attitude as evidenced by the flattening and upward bending of the tailcone and the fracturing of the engine case. A postaccident fire destroyed most of the wreckage.

Examination of the wreckage revealed evidence of a total loss of engine power as indicated by the lack of rotational scoring on the upper sheave and the undamaged section of the engine cooling fan. Main rotor blade impact with the tailcone and damage to the main rotor system were indicative of main rotor blade blow back and stall due to low rotor rpm. Examination of the freewheeling unit (sprag clutch) revealed normal operation. Multiple B-nuts on the fuel lines associated with the transducer and fuel servo were found loose; however, due to the thermal exposure, it could not be determined if the loss of torque to the B-nuts was due to heat damage or other factors.

The pilot was a highly experienced airplane pilot but had only accumulated about 350 hours of flight experience in helicopters. Based on the helicopter’s altitude and the wreckage signatures, after the loss of total power the pilot likely did not reflexively and simultaneously lower the collective, apply aft cyclic, and trim the helicopter to enter an autorotation while in cruise flight, and/or improperly managed the rotor rpm during the remainder of the autorotation. The lack of timely action allowed the main rotor rpm to reduce to an unrecoverable speed and likely resulted in the main rotor blades stalling, as detailed in the manufacturer’s Safety Notices and FAA guidance documents. The reason for the pilot’s failure to manage rotor rpm during an autorotation could not be determined. According to a Safety Notice published by the manufacturer, pilots with many hours of flight experience in airplanes and fewer hours experience in helicopters may instinctively make flight control inputs that are inappropriate for helicopters, which can result in catastrophic consequences.

Text from the NTSB. Open the NTSB docket for WPR24FA200 for the full record.