What happened
On 26 August 2012, a Rotorway 162F helicopter, registration G-FLIT, was conducting a private flight near Haslem and was en route from Dunsfold to a landing site near Petersfield. While cruising at approximately 1,000 ft, the aircraft experienced a sudden and violent yaw. This was accompanied by a rapid increase in engine RPM, which moved into the red sector of the tachometer.
Recognizing the emergency, the pilot closed the throttle within two seconds to initiate an autorotative descent. Due to the low altitude, the pilot had limited options for a landing site. The aircraft was at a height of only 700-800 ft when the pilot attempted to climb towards the south. However, the terrain in the chosen field was uneven. As the pilot flared for landing, the helicopter struck an upslope, causing the tail rotor blade tips to contact the ground. The pilot escaped the incident without injury, and the aircraft sustained only minor damage.
The investigation
The investigation focused on the sudden loss of drive to the main rotor system. Examination of the aircraft's drive-train revealed that the secondary driveshaft had failed, which disconnected the engine from the main rotors. While the tail rotor continued to be driven until the throttle was closed, the main rotor lost all power from the engine.
Components were sent to a manufacturer in the USA for metallurgical analysis. The examination of the 35 mm diameter shaft showed that the fracture occurred near the upper bearing. Investigators identified evidence of mechanical wear and longitudinal scores on the shaft surface. The metallurgical report confirmed that the fracture originated from rotational bending fatigue induced by a fretting process near the upper bearing.
Findings
- The primary cause of the accident was the fatigue failure of the secondary driveshaft.
- The failure originated from a region of fretting on the shaft surface, which likely initiated a fatigue crack.
- The presence of fretting and bending fatigue suggests that the shaft may have been subject to misalignment during installation.
- The aircraft was equipped with the updated 35 mm shaft design, which was intended to replace a previous 30 mm version prone to similar failures.