What happened
On 22 September 2008, a Robinson R22 Beta, registration G-BZYE, was conducting a private flight from Blackbushe Airport. During the transition from a hover into forward flight, the pilot experienced intermittent and rapid variations in engine power. The aircraft began to yaw violently to the right, accompanied by an increase in manifold pressure and an unplanned climb.
In an attempt to regain control, the pilot lowered the collective and applied left pedal. After stabilizing the aircraft, the pilot attempted a slow running landing. However, during the final touchdown, the aircraft experienced a second period of difficult control, leading to a heavy landing. The pilot and one passenger were on board; the passenger sustained one minor injury, while the crew member was unhurt. The landing caused damage to the engine frame, rear undercarriage legs, and engine side panels.
The investigation
The investigation established that the previous day, a student pilot had performed an engine start that resulted in a high engine RPM. The student failed to ensure the throttle was fully closed, causing the engine to exceed its rated speed. Although the instructor suspected an overspeed, they decided to fly G-BZYE to check its condition because no alternative aircraft was available.
Upon inspection at a maintenance facility, engineers discovered that the plastic gear for the left engine magneto had broken, which explained the rough running and fluctuating manifold pressure. Furthermore, the inspection revealed evidence of a confirmed engine overspeed, including excessively worn valve guides and stepped valve springs across all cylinders. The engine cooling fan had also moved on its shaft, a condition consistent with an overspeed event.
Findings
- The primary cause of the flight instability was an engine overspeed that had occurred during a student's solo flight the previous day.
- The pilot continued the flight despite the student's report of high RPM because the aircraft initially appeared to behave normally.
- The maintenance organization had not been notified of the suspected overspeed, and no entry was made in the technical log.
- The broken magneto gear and movement of the cooling fan were both consistent with the stresses of an overspeed event.