Earlier this summer, I was helping to move gliders back into the hangar, at the end of a flying day. One of the other helpers noticed that the position of the Citabria propeller was going to limit how tightly we could pack the next glider. As he reached for the prop, I instinctively asked him if he had checked the magneto switches (it turned out that they had been left in the ON position). In reply to my question, I received a blank stare. Having grown up hand-propping airplanes that had no starter motors, I have first-hand knowledge of how dangerous it can be to move a prop by hand. So, I proceeded to give a short course in propeller safety. Since that incident, it has occurred to me that there is nothing in a glider pilot’s training about this issue, which is second-nature to a power-airplane pilot. So, the following is a summary of the issue, for the benfit of glider pilots.
The ignition systems of aircraft piston-engines are typically sparked by a pair of magnetos. The primary field of a magneto is generated by permanent magnets, rather than the 12-volt windings of an alternator. This arrangement provides a very reliable ignition source, which keeps the engine running in case the aircraft electrical system fails. For ground safety, each magneto is fitted with an ON/OFF switch. After shutting down an engine, the pilot is supposed to flip the switch to the OFF position, which grounds the secondary coil, preventing a spark if the prop is moved.
But doesn’t a magneto need to be spun fairly fast for a spark to be generated? The answer is yes. However, to make engine starting easier, most aircraft magnetos are connected to the engine via an ingenious mechanical device called an impulse-coupler. As the prop turns through a very small arc, a spring in the coupler is wound up until it suddenly releases, temporarily spinning the magneto to an RPM high enough to generate a strong spark. It only takes one spark to the cylinder near top-dead-center to impart lethal energy to the prop.
- Always check to make sure that the mag switches are both OFF before touching a propeller.
- Always assume that the engine will start, even if the mag switches are OFF. Never stand in the plane of the prop when moving it. Faulty magneto grounds are not unheard of, particularly on older airplanes.
- Pawnee and Citabria pilots: Kill the engine by using the mixture idle-cutoff before turning the mag switches OFF. This reduces the possiblity of enough fuel vapor in the cylinders to ignite, even if someone moves the prop with the mag switches ON.