Accident Overview
The Australian Transport Safety Bureau (ATSB) investigated an accident involving a Robinson R44 II helicopter. The helicopter was operating over water in calm wind conditions, with a glassy water surface. During the flight, the helicopter experienced a decay in rotor RPM, leading to a collision with the water.
Aircraft Performance
The helicopter was below the published maximum take-off weight and within the weight limits for hovering in and out of ground effect. At the speed of the accident, the power required was less than that required for hovering, as the minimum power speed for the R44 II is approximately 55 knots. In the reported calm conditions, the helicopter should have had sufficient power available to maintain rotor RPM. However, the ATSB was unable to determine the cause of the RPM decay.
Operational Circumstances
The manufacturer recommends a take-off profile where the helicopter reaches a height of 25 feet at an airspeed of 50 knots. In this instance, the helicopter was at 5 to 10 feet while traveling at 50 to 60 knots, providing the pilot with very little reaction time after the low rotor RPM warning sounded. The pilot reported no outstanding maintenance issues, and the helicopter had operated normally on a previous flight only minutes before the accident. The helicopter had not been recovered from the water at the time of the investigation, preventing an engine inspection.
The helicopter had recently undergone significant engine maintenance, mostly on the cylinders, and was consuming more oil than normal but within expectations for a running-in period. The pilot had topped up the oil prior to the first flight of the day. The pilot stated that after the low rotor RPM horn activated, no warnings were observed before the collision, but there was very little time to react.
Investigation Findings
The pilot commented that the glassy water surface made it difficult to accurately assess height above the water, and operating at an estimated 5 feet did not allow sufficient time to react to an engine failure or temporary reduction in performance. The ATSB could not determine why the rotor RPM decayed.