Incident Details
A lineman was assisting a pilot during the starting sequence of an aircraft. The pilot reported that the propeller was turning slowly and requested the lineman's assistance. In response, the lineman asked that all switches be turned off. Once the switches were confirmed off, the lineman proceeded to turn the propeller manually. The moment the lineman rotated the propeller, the engine immediately fired and continued to run. As the propeller began to rotate under its own power, it struck the lineman's arm.
Mechanical Finding
Following the incident, a thorough examination of the engine's ignition system was conducted. The investigation revealed that a P-lead on the magneto had become pulled out of the lug at the noise filter. This disconnection rendered the magneto electrically hot at all times, meaning the ignition system remained active regardless of the position of the ignition switches. The continuous energization of the magneto caused it to be capable of firing the spark plugs even when the switches were turned off, as had been done upon the lineman's request. The unexpected engine start occurred because the magneto was always hot due to the disconnected P-lead.