GAA17CA401 · Robinson Helicopter Company R44 II

***This report was modified on June 13,2018. Please see the docket for this accident to view the original report.***

The helicopter pilot reported that he was performing an agricultural application flight and was in a hurry to spray the field because of a previous 3-hour delay caused by a broken fuel truck. The pilot reported that the weight of the helicopter was 40 lbs below the helicopter's maximum gross weight and that he "approached the field with the understanding that I was heavier, and it was hotter and more windy than I originally planned for."

The pilot reported that the helicopter was headed south and that the wind was from the southwest about 15 knots. He made a low-altitude, low-airspeed, left turn to the north, and the helicopter entered a nose-low attitude and sank more rapidly than he anticipated. The pilot aborted the turn and chose to maintain a southeast heading to use the crosswind to avoid power lines. He attempted to arrest the sink rate by increasing the collective, but the low rotor rpm horn sounded.

The helicopter impacted the ground, bounced, and then rolled onto its left side. The helicopter sustained substantial damage to the tailboom and the main rotor system.

The nearest METAR was about 32 miles northwest of the accident site, and it reported, at the time of the accident, wind from 230° at 9 knots, gusting to 16 knots. The temperature was 88°F and the dew point was 20°F. The field elevation was 696 ft, and the density altitude was 2,786 ft.

According to the Federal Aviation Administration (FAA) Helicopter Flying Handbook, FAA-8083-21A (pg. 2-4, para. 5), "turns in a helicopter increase the load factor exponentially, ultimately increasing the power requirement that is necessary to maintain the helicopter's altitude. Left pedal turns increase the quantity of anti-torque produced by the tail rotor, by demanding additional power from the 245-brake horsepower engine. Pilot flight control inputs demanding more power than the engine is capable of producing, with respect to the atmospheric conditions, adversely affects the helicopter's ability to sustain its altitude. Available engine power is directly correlated to main and tail rotor RPM. When the engine fails to produce the required power to sustain airspeed and altitude at high DA [descent altitude], the main rotor blades will exceed their critical angle of attack, consequently decreasing main rotor blade RPM."

The pilot reported that there were no preaccident mechanical malfunctions or failures with the helicopter that would have precluded normal operation.

Verbatim NTSB analysis and probable cause from the NTSB dataset