Robinson Helicopter Company R44 II N440EM accident near Lyons, Nebraska

Probable cause

The engine's low cylinder compression, which resulted in low rotor rpm and a subsequent dynamic rollover following landing on soft terrain.

NTSB analysis

The airline transport pilot of the agricultural aerial application flight stated that as the helicopter entered the field to be sprayed, the low rotor rpm annunciation sounded. He attempted to recover rotor rpm, but the rpm continued to decay rapidly. He then used the remaining rotor rpm to cushion a landing onto the field. As the helicopter touched down, it slid about 150-200 ft before the skid dug into soft dirt and the helicopter rolled over. Post-accident examination of the engine by the operator revealed that the no. 6 cylinder had more leakage/lower compression than the previous 100-hour recorded compression checks. Reduced compression on one or more cylinders would have reduced excess power available to maintain a rotor RPM and altitude.

Text from the NTSB. Open the NTSB docket for CEN19LA250 for the full record.