Primary finding
Probable cause
The engine's low cylinder compression, which resulted in low rotor rpm and a subsequent dynamic rollover following landing on soft terrain.
Investigator assessment
Analysis narrative
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.
Source record
Factual narrative
On August 3, 2019, at 1615 central daylight time, a Robinson Helicopter Company R44 II helicopter, N440EM, experienced a loss of engine power during an aerial application of a field near Lyons, Nebraska. The helicopter impacted the ground during an attempted recovery by the pilot. The helicopter sustained substantial damage. The airline transport pilot received minor injuries. The helicopter was registered to an individual and operated by Wallin Agricultural Aviation under Title 14 Code of Federal Regulations Part 137 as an agricultural flight that was not operating on a flight plan. Day visual meteorological conditions prevailed at the time of the accident. The flight originated from an unknown location and an unknown time. The pilot stated that he entered a field to be sprayed and about 100-120 ft agl, the low rotor rpm annunciation sounded. As he attempted to recover rotor rpm, 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. At the previous recorded compression check, the no. 6 cylinder had a 8.75% of leakage (73 psi held from a reference of 80 psi). The no. 6 cylinder had 35% leakage (52 psi held from a reference of 80 psi) during the examination. No further examination of the cylinder was performed.