What happened
On 17 August 2001, an Aerostar SA Yak-52, registration G-BVMU, was involved in an incident at Little Granscent during a private flight operation. Following two training sorties, the aircraft had been landed and refuelled. During the preparation for a solo flight, the pilot was in the front cockpit while an instructor assisted with the engine start sequence.
As part of the standard procedure to prevent hydraulic lock, the propeller was being manually pulled through by the instructor. After approximately six blades had been turned, the engine unexpectedly ignited and continued to run. During this event, the propeller struck the instructor's right hand, resulting in one serious injury to a person on the ground.
The investigation
The investigation examined the sequence of events leading to the engine ignition. It was noted that the instructor had reminded the pilot to ensure the front cockpit magneto switches were in the OFF position. While the pilot initially declined assistance, he later accepted the instructor's help with the start-up process, explicitly informing the instructor that the rear cockpit magneto switches remained ON.
Investigators looked into the possibility of the engine starting due to compression while the cylinder head temperature was still high, though the instructor estimated the temperature was likely below 70°C. The investigation also considered the pilot's configuration of the cockpit switches. The pilot noted that his familiarity with American-style switches, which are turned OFF anticlockwise, might have led to an error, as the Yak-52 switches require a clockwise motion to turn OFF.
Furthermore, the instructor reported hearing the starter solenoid open during the propeller rotation, suggesting the pilot may have initiated the starting sequence by pressing the start button, despite the pilot's assertion that he had not done so.
Findings
- The engine ignited while the propeller was being manually rotated.
- For the engine to have continued running after the initial ignition, the magneto switches in both the front and rear cockpits must have been in the ON position.
- Potential contributing factors included the possibility that the front cockpit magnetos were left ON after the previous shutdown, or were incorrectly switched to the ON position by the pilot during his checks.
- The possibility of the engine firing due to compression while still warm was considered, though temperature levels were likely low.