Many lives and limbs have been lost as a result of spinning propellers, either by someone walking into their revolving paths or by someone inadvertently starting an engine by moving a propeller. I recently spoke with a gentleman who was fortunate to walk away from an incident in which his airplane started up briefly as he moved the prop.
The prop started spinning because one of the magnetos in the engine was not grounded. Magnetos in piston powered airplane engines are the magical little devices that are responsible for the ignition of the fuel that powers the moving parts that make the propeller or propellers spin. They are very simple magnetic units that send electrical messages to the spark plugs to fire. But how do you make sure that the magnetos don’t start the engine when you turn your propeller?
The thing that prevents a spark from being created inadvertently is a wire called a P-lead, which effectively grounds the magneto. But if the P-lead is not connected properly, the magnetos can create its electrical signal. So it is important to verify that the magnetos are properly grounded.
You need neither to be a mechanic nor dig into the engine to make sure that the P-lead is properly connected. There is an easy check you can perform in the cockpit to make sure your mags are grounded. Before you shut down the engine, simply turn the key briefly to the OFF position. If the engine quits when you turn the key to OFF you can rest assured that the mags are grounded.
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