What happened
During a flight in calm conditions, a Robinson R44 II helicopter experienced a sudden drop in rotor RPM while operating at a very low altitude. At the time of the incident, the aircraft was flying between 5 and 10 feet above a glassy water surface. While the aircraft was within its published weight limits for hovering and was below its maximum take-off weight, the rotor RPM decayed below 97 percent. This loss of power occurred while the aircraft was traveling at 50–60 knots, a state where the power required was less than that needed for a hover. The pilot had very little time to respond to the low rotor RPM warning before the helicopter collided with the water.
The investigation
The ATSB examined the circumstances surrounding the flight, noting that the pilot had previously flown the aircraft minutes earlier without issue. The aircraft had recently undergone significant engine maintenance, specifically involving the cylinders, and was consuming a slightly higher volume of oil, which was considered normal for a running-in period. The pilot had topped up the oil levels prior to the day's first flight. Because the aircraft remained submerged in the water during the investigation, a physical inspection of the engine could not be performed. The investigation also noted that the glassy appearance of the water due to the lack of wind may have made it difficult for the pilot to accurately judge the aircraft's height above the surface.
Findings
- The Robinson R44 II was operating below its maximum take-off weight and possessed sufficient power to maintain a hover.
- The pilot was operating at an altitude of only 5–10 feet above the water, which provided minimal reaction time following the low rotor RPM warning.
- The ATSB was unable to determine the specific cause of the rotor RPM decay.
- The takeoff profile used did not align with the manufacturer's recommendation of reaching 25 feet at 50 knots, leaving the aircraft at a dangerously low altitude during the critical phase of flight.