What happened
On 26 September 1999, a Replica Campbell Cricket, registration G-BXCJ, was conducting a private flight near Swansea Airport. After departing normally and climbing to 500 feet, the student pilot initiated a left turn and continued to an altitude of 700 feet. Upon leveling the aircraft, the pilot observed that the autogyro failed to accelerate, even though the Rotax 532 engine appeared to be running at a constant and normal speed.
As the pilot turned downwind, the aircraft was unable to maintain its altitude. Following an emergency radio transmission, the pilot decided to perform an emergency landing in a nearby grass field. While the engine continued to run at its usual speed, the actual power available to the aircraft steadily declined. The pilot successfully avoided cattle in the field, but the aircraft struck the ground heavily, resulting in damage to the wheel axle and rotor blades. No injuries were sustained by the pilot.
The investigation
The investigation focused on the mechanical connection between the engine and the propeller. The Rotax 532 engine utilizes an integral reduction gearbox where an output gear wheel on the crankshaft meshes with a larger gear on the propeller shaft. This system relies on a 1:10 taper to transmit torque via friction, held in place by a central bolt.
Upon inspecting the engine, investigators discovered that the central bolt was not tight. This allowed the output gear to rotate independently of the crankshaft, which effectively reduced the rotational speed of the propeller. Although the bolt threads showed evidence of locking film, the connection had failed. The engine had only been in use for approximately 32 hours, though it was unclear if the gearbox had been previously dismantled.