What happened
On 25 July 2009, a Robinson R44 Raven II, registration EI-UNI, was prepared for a solo local flight in Enniskerry, Co. Wicklow. The aircraft had been parked overnight in a field with protective covers installed to shield the cabin and the rotor head from expected bad weather. During the morning pre-flight routine, the pilot removed the cabin cover but was interrupted by a mobile phone call. After concluding the call, the pilot entered the aircraft, started the engine, and engaged the main rotor without having removed the smaller cover from the rotor mast.
As the rotor began to spin, the pilot detected a low-frequency vibration. Realizing the error, the pilot immediately shut down the engine. However, the rotation of the rotor had already caused the cover to become entangled in the mast, leading to mechanical damage and a subsequent strike of the rotor blade against the tail-boom.
The investigation
The investigation focused on the mechanical failure and the sequence of events leading to the damage. It was established that as the rotor gained speed, the ensnared cover tightened around the mast, deforming the pitch change rods. This deformation forced the blade into a negative pitch angle. The resulting aerodynamic forces caused the blade to dip lower than its normal flight path, causing it to strike the tail-boom. This impact knocked off the anti-collision light and punctured the tail-boom structure. The investigation also examined the pilot's actions during the pre-flight phase, specifically the impact of the mobile phone call on the completion of necessary safety checks.
Findings
- The failure to remove the main rotor cover was the primary cause of the accident.
- The pilot was distracted from essential pre-flight duties by a mobile phone call.
- The pilot did not restart the pre-flight inspection sequence from the beginning after the interruption, which would have ensured all covers were removed.
Safety action
Following the investigation, a safety recommendation was made to the Irish Aviation Authority (IAA) to issue an advisory to the aviation community. This advisory aims to increase awareness regarding the risks of using mobile phones during the flight preparation stage of operations.