Robinson Helicopter R44 II N6960 accident near Herriman, Utah

Probable cause

A hard landing following a total loss of engine power due to fuel starvation as a result of the improper idle setting of the helicopter’s engine. Contributing to the loss of engine power was the instructor’s reduction of engine power past that recommended by the manufacturer for practice autorotations.

NTSB analysis

The helicopter flight instructor was demonstrating a practice autorotation with a power recovery for the student. The instructor stated that he reduced the throttle to idle and entered the autorotation. After noticing several caution lights illuminate, the instructor attempted to increase engine power and realized that the engine had lost total power. The cockpit video recorder showed that the engine tachometer needle decreased from 100% to below 46% rpm, below idle. The instructor continued the autorotation to a field and the helicopter landed hard. During the landing, the main rotor blades struck the tailboom, resulting in substantial damage.

A postaccident examination and engine run revealed that the engine would not start and idle unless the electric boost pump was set to ON. Subsequent bench testing of the mechanical fuel pump and fuel injection servo revealed no anomalies.

The flight instructor reported that he noticed the helicopter’s engine would idle about 50-52% rpm, but stated that the engine did not have any issues when he previously demonstrated practice autorotations. He added that he did not notify maintenance of the low idle setting, as he was not aware that it was adjustable. According to the operator’s maintenance manual, the idle setting for the accident engine mode should be 58-62%.

According to the pilot’s operating handbook (POH), “Idle mixture and speed may require adjustment as conditions vary from sea level standard.”

The accident helicopter’s maintenance logs did not indicate that the flight idle was adjusted as recommended by the manufacturer after the helicopter was received from the factory.

It is likely that, given the engine’s low idle setting, the instructor did not immediately recognize that the rpm continued to decrease past idle and the subsequent loss of engine power due to fuel starvation. It is also likely that, had the instructor followed the manufacturer's recommendation to lower tachometer needle separation to the desired level, usually below 80% but above idle, the engine would not have lost power.

Text from the NTSB. Open the NTSB docket for WPR24LA241 for the full record.