What happened
On 2 November 2004, a Robinson R44 Raven 2, registration G-IVEN, was conducting a private VFR flight from Cork Airport toward Weston. While flying approximately 1,100 feet above mean sea level, the pilot experienced intense vibrations through the cyclic control. Suspecting a mechanical issue with the main rotor head, the pilot initiated an immediate descent to perform a forced landing.
During the descent, the pilot notified Cork Tower of a hydraulic emergency. The flight controls became heavy and difficult to maneuver due to the loss of hydraulic assistance. To avoid obstacles such as wires and trees, the pilot executed a turn and successfully landed the aircraft in a field roughly two miles east of Cork Airport. The pilot exited the aircraft safely, and no injuries or fire were reported.
The investigation
An investigation was able to identify the source of the hydraulic fluid loss. An engineer arrived at the scene and used a hydraulic ground rig to confirm that the system had lost fluid, causing the controls to operate without servo assistance. The engineer discovered an oil spray on the firewall beneath the auxiliary fuel tank.
Upon inspecting the hydraulic pump, the investigation found that the leak originated from a tee-piece union on the pump's output side. The investigation revealed that the O-ring retainer had been improperly seated. Specifically, the union nut had been over-tightened, causing the retainer to distort. This defect was traced back to the manufacturing process of the Robinson R44 Raven 2.