Bell OH-58A N861CC accident near Guernsey, California

Probable cause

The pilot’s overpitching of the main rotor blades while maneuvering at low altitude in a high-temperature environment at high gross weight, which resulted in a reduction of engine and rotor speed, excessive main rotor blade flapping, mast bumping, and impact with terrain.

NTSB analysis

The pilot was conducting an aerial application flight and initiated a climbing left turn to return to the field. The helicopter’s low rotor rpm warning light illuminated, and the pilot heard the low rotor audio tone in his headset. The pilot lowered the collective to attempt to regain engine and rotor speed while maneuvering to avoid a drainage culvert. While the pilot was maneuvering, tThe tailboom separated, the right skid contacted the culvert, and the helicopter rolled over onto its right side. T; the helicopter sustained substantial damage to the fuselage and the tail rotor had separated from the tailboom.

Examination of the helicopter, including the transmission and hydraulic systems, revealed no preaccident mechanical malfunctions or failures that would have precluded normal operation. The tailboom was completely separated just aft of the horizontal stabilizers, severing the tail rotor and gear box. Witness marks from the main rotor blades were observed on the tail rotor driveshaft cover, aft of the horizontal stabilizers. A test run of the engine revealed no anomalies.

The helicopter was loaded about 21 pounds below its maximum gross weight and operated in a high-temperature environment. Performance calculations based on the pilot’s estimates of environmental conditions and the helicopter’s weight revealed 5minute maximum power availability of about 92% torque. It is likely that while trying to maintain the climbing left turn, which required increased lift due to the increased load factor, the pilot demanded more power than the engine could produce and overpitched the main rotor blades. The resulting low rotor speed led to a reduction of lift and stiffness and lag in the flight control response, which the pilot interpreted as a hydraulics failure.

It is likely that while the pilot maneuvered to avoid the culvert, the low rotor speed allowed the main rotor blades to flap down and sever the tailboom. At the same time, the main rotor hub teetered downward and fractured the mast, consistent with mast bumping, which separated the main rotor system from the transmission.

Text from the NTSB. Open the NTSB docket for WPR23LA313 for the full record.