What happened
On 25 February 2015, a Robinson R44 Raven, registration G-CEDG, was conducting a private flight from a strip near Salford, Bedfordshire, toward East Sussex. The aircraft was carrying the pilot and two passengers. Shortly after takeoff, while the helicopter was in a cruise climb at approximately 800 feet, the engine RPM began to hunt and increased beyond the normal green arc on the instrument gauge.
In response to the instability, the pilot deactivated the governor to attempt manual RPM control and began a descent for a precautionary landing. During this maneuver, a low rotor RPM warning was triggered. The pilot lowered the collective lever fully and, after an unsuccessful attempt to raise it, believed the engine had failed. The pilot then executed an autorotative landing in a field roughly 2 km south of the departure point. Upon touchdown, the helicopter slid forward approximately 3 meters before the skids caught the ground, causing the aircraft to roll onto its left side. The landing resulted in one minor injury to a passenger, while the pilot and the second passenger escaped without injury.
The investigation
The AAIB examined the flight history and the mechanical state of the engine and governor. It was noted that the previous day, the pilot had experienced a similar instance of high engine RPM during an engine start, which led to the aircraft being shut down for advice. Although the subsequent flight to Salford had been normal, the events of the 25th suggested a recurring issue. The investigation also reviewed the meteorological conditions present at the time of the accident.
Findings
- The pilot experienced engine RPM fluctuations and hunting during the climb.
- The pilot transitioned to manual control by switching off the governor.
- The aircraft experienced a low rotor RPM warning, leading the pilot to believe an engine failure had occurred.
- Weather conditions were consistent with a high risk of serious carburettor icing at any power setting.
- The pilot likely did not apply carburettor heat during the rapid descent due to the speed of the developing emergency.