Preflight check list: Magnetos check-RIGHT;BOTH;LEFT;BOTH-maximum drop 150 rpm(maximum differential 50)
This is an item that is accomplished thousands of times a day by pilots and is checking an ignition system that is so reliable that the pilot will not think of it until the next preflight check is performed.
The magneto is a self contained generator of electricity that sends the current to the correct spark plug at the proper time to make the controlled explosion of fuel and air in each cylinder.
One of my first jobs in general aviation maintenance was to overhaul magnetos. The units are glorious in their simplicity. A fond memory of those early days of my career was of a company employee who was not a technician that kept picking up components from my work station and not returning them to the same place. I finally found out who the person was and set up a trap. One of the components of a magneto is called a condensor (capacitor now-a-days)and its function is to store an electrical charge. I charged up several condensors from our tester and placed them in a convient spot on my bench for curious hands to pick up. The person I suspected wandered into the shop and just happened to pick up a charged condenser. The 500 volts just happened to discharge across his hands. It was worth having the component being destroyed as the involuntary muscle jerk caused the unit to be thrown into the wall across the room. I never had another component wander from its assigned place on the workbench.
A magneto uses a spinning magnet to generate the current and is completely independent from the aircraft electrical system. Some aircraft do not even have an electrical system. The engine is started by "hand propping" which is done by manually spinning the propeller. The magneto uses the force of the person turning the propeller to provide the spark to the spark plugs to start the combustion process which keeps turning the magneto.... and on and on.....until the engine runs out of fuel or the magneto is turned off. This same technology is used on most lawn mower engines.
Just in case of a problem, there is a second magneto as a backup including a second set of spark plugs. Either magneto or both together will allow the engine to operate properly.
The term "magneto timing" is common in the general aviation maintenance field. This term is simply the position where the magneto is installed so the spark is sent to the proper cylinder at the proper time for efficient combustion.
Magneto manufacturers require their units to be disasembled every 500 hours of operation, be inspected, components be tested, cleaned and relubricated. During the aircraft required Annual Inspections, the magnetos timing is checked and adjusted as necessary to compensate for slightly worn components.
I have seen a magneto that had its internal timing and timing to the engine be off by 15 degrees. The engine was running normally. This is a testimony for the inherent safety of the reciprocating aircraft engine.
Jet engines and turboprop engines do not use magnetos. They usually have high current ignition systems to start the engine and once combustion is begun, it is self sustaining. Some of the new Light Sport Aircraft engines have a completely different ignition system that we may address in General Aviation Maintenance at a later date.
Modern reciprocating engines are beginning to use a type of "electronic ignition" referred as FADEC (full authority digital engine control). Many modern General Aviation aircraft are beginning to use Diesel technology which does not use an ignition system at all. I am old-fashioned and think a magneto is "sexy". The magneto will be around for a long time.