What happened
On 10 April 2003, a Robinson R44, registration G-NIOL, was conducting a private flight from Lydd toward a landing site near Heathfield, West Sussex. During the cruise phase at 1,200 feet, the pilot encountered the edge of a rain shower and applied carburettor heat to mitigate the risk of icing. Shortly after, the pilot observed fluctuations in the pitot-static instruments, including variations in airspeed and vertical speed.
In an attempt to stabilize the instruments, the pilot reduced the airspeed to 70 kts. During this maneuver, the pilot noted the governor opening the throttle in response to a yaw, followed by a decay in both engine and rotor RPM. The pilot initiated an autorotation and attempted a powered recovery. However, during the attempt to regain power, the rotor RPM dropped to a critical level, leading the aircraft into a high rate of descent and uncontrollable yawing.
Seeking to maintain forward airspeed despite the low altitude, the pilot directed the aircraft toward a large garden. The helicopter struck trees at the end of the garden, resulting in the destruction of the helicopter. The pilot sustained minor injuries and was able to evacuate the wreckage without assistance.
The investigation
The investigation examined the engine and its components following the accident. The engine was placed on a test bed and performed successfully, exceeding its rated power figures with no defects identified. While fuel was found in the carburettor, the engine and its accessories showed no mechanical failures capable of causing the power loss.
Investigators also looked into the pitot-static instrument fluctuations. It was determined that these were likely caused by moisture in the system and were unrelated to the mechanical failure of the engine, though they may have increased the pilot's workload. Additionally, the carburettor air temperature gauge was found to be under-reading by 5°C when calibrated, meaning the actual temperature was higher than indicated.
Findings
- The exact cause of the initial engine power loss remains undetermined, though the ambient conditions were suitable for carburettor icing.
- The aircraft likely entered a vortex ring state after the pilot's attempt at a powered recovery caused the airspeed to decay.
- The pilot's focus on attempting a power recovery likely distracted from the critical monitoring of rotor RPM.
- The engine may have recovered to full power prior to the final touchdown, as evidenced by the pilot's ability to maintain tail rotor control during the ground slide.