Aviation Accident Summaries

Aviation Accident Summary CEN21LA369

Wheeling, IL, USA

Aircraft #1




The pilot reported a decrease in engine and rotor rpm during cruise flight. The pilot reported that he heard sounds from the engine “like detonations,” and he placed the helicopter into autorotation and landed on a road. The helicopter rotor blades struck a pole and the helicopter rolled onto its right side. Downloaded data from the engine monitoring unit confirmed the decrease in engine and rotor rpm. Functional testing of the engine and electronic governor control revealed no anomalies. Bench testing of the engine rpm sensor and the electronic governor control unit also revealed no anomalies. During functional testing, it was confirmed that the manual throttle control could be used to override the governor control. Based on the available information, the reason for the decrease in engine rpm reported by the pilot could not be determined.

Factual Information

On August 11, 2021, about 0640 central daylight time, a Robinson R44 helicopter, N4043B, was substantially damaged when it was involved in an accident near the Chicago Executive Airport (PWK), Wheeling, Illinois. The pilot received minor injuries. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. According to the pilot, the day before the accident flight, he had flown the helicopter from his private helipad in Morton Grove, Illinois, to several airports in central Wisconsin, returning that same day. Upon returning, the winds were strong and gusting, and the pilot did not feel comfortable landing at his private helipad, so instead landed at PWK. He left the helicopter there overnight, instructing ramp personnel to fill the fuel tanks prior to his return the following day. On the day of the accident, the pilot arrived at PWK and planned to fly the helicopter from PWK to his private helipad. He reported that after takeoff, when the helicopter was about 200 ft above ground level, at 40 knots airspeed, he noticed the rpm decreasing and then heard the low rpm warning. He then saw the rpm increase “above the governor” followed by a decrease and once again the low rpm warning activated. He said that he heard sounds from the engine “like detonations.” He entered an autorotation and selected a road to land on. During the landing the helicopter’s main rotor blades struck a pole and the helicopter flipped onto its right side. The helicopter was equipped with an engine monitoring unit (EMU) that recorded engine data that were downloaded during a postaccident examination. The EMU showed that the engine speed and rotor rpm increased to initiate the flight, stabilizing at about 101% and 101% respectively where they remained for about 1.5 minutes. Both the engine and rotor speed then decreased briefly, followed by a brief increase before decreasing to 69% and 87%, respectively, where they remained for the remainder of the flight. The helicopter’s right landing skid was broken during the landing and as a result the helicopter was leaning toward its right side during a postaccident examination. A test run of the engine was performed and the engine started normally and idled, but due to the positioning of the helicopter a complete run up of the engine was not possible. After the initial test run, another test was completed on a separate date with the helicopter blocked up to a level attitude. Engine operation and functioning of the electronic governor system was performed. No anomalies regarding engine operation or governor operation were detected. The ability to override the governor using the manual twist throttle was also tested. Although there was resistance to throttle movement with the governor engaged, the manual throttle control could be used to override the governor inputs. The governor control unit and the engine rpm sensor were removed for further bench testing. Testing at the manufacturer’s facility showed no discrepancies and both the governor controller and the engine rpm sensor passed all test procedures.

Probable Cause and Findings

A partial loss of engine power for reasons that could not be determined based on available information.


Source: NTSB Aviation Accident Database

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