What happened
On 9 April 2017, a Piper PA-28-161 Cherokee Warrior II, registration G-BNSZ, was prepared for a private local flight at Halfpenny Green Airfield, Staffordshire. Following standard procedures, the pilot primed the fuel system and attempted to start the engine. After three unsuccessful attempts to turn the engine over via the starter motor, the pilot shut off the fuel and electrics and removed the ignition key.
To check for a potential engine seizure, the pilot exited the cockpit and attempted to rotate the propeller manually. During this process, the propeller suddenly spun for approximately five revolutions. The pilot attempted to move away from the arc of the propeller, but the blades struck his right elbow. The impact resulted in a minor injury, specifically a small puncture wound accompanied by swelling and bruising, though no long-term damage was sustained.
The investigation
The investigation focused on why the engine rotated despite the ignition key being removed. The pilot noted that the engine had been primed with fuel during the previous failed start attempts. It was suggested that residual fuel in the cylinders may have ignited, causing the engine to run briefly.
While the pilot had placed the ignition switch in the position required to remove the key, investigators considered the possibility that the magnetos were not properly earthed. This would mean the ignition system remained capable of providing a spark even with the key removed.