Accident Details
On July 21, 2019, about 0845 central daylight time, a Robinson R-44 (Raven I), registration N459TA, sustained substantial damage in an accident near Ida Grove, Iowa. The helicopter was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. The commercial pilot sustained minor injuries.
Pilot Report
The pilot reported that the purpose of the flight was to apply fungicide to a field. He was spraying the field using north-south and south-north swath runs. Due to obstacles such as trees, power line poles, and power lines near the field, he conducted teardrop turns to safely facilitate the swath runs.
At the end of his second north-to-south swath run, the helicopter exited the field and executed a gentle tear drop turn to the left at about 25 knots. The pilot estimated his altitude at approximately one and a half to two telephone pole lengths high. Upon completing the turn and approaching the target field, the helicopter yawed hard to the left. The pilot attempted to correct with right tail rotor pedal input, and the low rotor rpm light and horn activated. He observed engine rpm below half and dropping, along with main rotor rpm, which had a "good split." The pilot reduced the collective and attempted a high hovering autorotation. Due to lack of airspeed and altitude, the helicopter impacted a bean field "very hard." The engine was not operating when the helicopter came to rest on its left side. The pilot egressed without further incident.
Operational Information
At the time of the accident, the reported temperature was about 64°F, with wind from about 030° at 6 knots. The pilot reported paying close attention to carburetor heat temperature, with the carburetor heat lever pulled out "a little over half" and the needle kept out of the yellow arc on the carburetor temperature gauge. For engine power settings during swath runs, he reported manifold pressure below 15 inches when exiting the field and never exceeding 21 inches when level.
About 15 minutes prior to the accident, a loader from the operator reported loading 30 gallons of water, 105 ounces (0.82 gallons) of AFrame fungicide, and 5 gallons of 100 low lead fuel.
Postaccident Examination
The helicopter sustained substantial damage to the main rotor system, fuselage, tail boom, and tail rotor system. A postaccident examination by an FAA aviation safety inspector found no preimpact mechanical malfunctions or failures that would have precluded normal operation. Photographs from law enforcement showed the carburetor heat was on at the time of impact.
The reported temperature and dewpoint at the time indicated a probability of serious icing conditions at glide power settings per the FAA Special Airworthiness Information Bulletin CE-09-35 Carburetor Icing Prevention. The helicopter was equipped with a carburetor heat assist device. The Robinson R-44 Pilot's Operating Handbook states that the carburetor heat assist correlates application of carburetor heat with collective setting changes to reduce pilot workload, with a friction clutch allowing pilot override.