Robinson Helicopter Company R44 II N233JD accident near Lake Sutherlin, Montana

Probable cause

The pilot’s transition into an out-of-ground-effect hover in high-density altitude conditions that exceeded the helicopter’s performance capabilities, which resulted in loss of rotor rpm.

NTSB analysis

The airline transport pilot was conducting a cross-country, personal helicopter flight. He reported that, while looking for a place to camp, he initially landed in one area; however, he decided it was unsuitable and lifted off, at which time he noticed the rotor rpm was low. He then twisted the throttle to full, and the rotor rpm immediately increased. The pilot then proceeded across a small ravine toward a hill traveling about 10 to 20 knots and about 25 ft above ground level. The low rotor warning horn sounded, and the engine began to lose power. The pilot added that, because he had already set the throttle to full power, he decided his only option was to lower the collective and fly downhill to gain airspeed and regain rotor rpm. The pilot leveled the helicopter just above the water's surface and increased the collective to cushion the landing. As the helicopter settled into the water, it rolled over onto its left side and then became partially submerged.

Examination of the engine did not reveal evidence of any preaccident mechanical malfunctions or failures that would have precluded normal operation. Given the reported gross weight of the helicopter, accident site elevation, and temperature, the helicopter was operating at the upper limit of its performance capabilities. Performance charts indicated that the helicopter would not have been able to hover out of ground effect at that elevation; therefore, it is likely that, when the helicopter crossed the ravine and entered an out-of-ground-effect hover, there was insufficient available power to sustain the out-of-ground-effect hover due to the throttle already being positioned to full power, which resulted in the loss of rotor rpm.

Text from the NTSB. Open the NTSB docket for WPR17LA168 for the full record.