How It Works: Making Sparks
BY MARC E. COOK (From AOPA Pilot, August 1992.)

For reasons of cost, durability, and redundancy, airplanes get along with ignition systems whose power player is the venerable magneto. Chief among the mag's virtues is the ability to provide its own power, but sophistication is not on its list of attributes. Being independent of the airplane's electrical system means that the motor will chug along even if you turn off the master switch or suffer a complete electronic shutdown in the cabin.

Broken down into its basic parts, the aircraft ignition system includes two magnetos, ignition leads, and the spark plugs. The mags make electricity and the leads carry it to the plugs, which in turn make the sparks needed for combustion. You have two mechanically and electrically independent magnetos for safety reasons; one could fail, and the engine will still run well enough for a safe landing. (Some Lycoming-powered airplanes have two magnetos in one housing, sharing the drive gear to the engine; these do not offer true redundancy. Look for a "D" at the end of the engine-model suffix, like the O-320-H2AD in late-model Skyhawks.)

Inside every magneto are bundles of wound wire (called coils, there is a primary and secondary) and permanent magnets. The stationary coils surround the spinning magnets to create the electricity that eventually makes its way to the spark plugs. Points, or contact breakers, determine when each pulse of electricity occurs, and a rotor assembly selects which ignition lead gets the juice.

In a perfect world, then, the magneto makes its energy, the points fire it off at exactly the right time, and the rotor, leads, and plugs turn it into a spark where (and when) it's needed most. But the world is not perfect, and that's why we test the ignition system prior to each flight. Though it seems simple at first, what you are really checking on the runup is the whole ignition system, not just the mags.

By turning the key from, say, Both to Left, you are in reality grounding the right magneto, causing its output to cease. (For reasons of description, magnetos have been labeled left and right, which is related to where the mag is on the engine, not which cylinders it serves. Typically, one magneto will serve all cylinders, with half the connections to the top plugs and the other half to bottom plugs. The exact arrangement varies by engine make and model.) The device that carries out this grounding is called the p- lead.

So when you turn the ignition switch from Both to Left, you should see a slight reduction of engine rpm. This is because a single spark plug is not as efficient at starting the combustion event as are two, and so less power is made. Then switch back to Both, let the engine speed stabilize, and try the other mag by itself. You should see the same results. Incidentally, the total mag drop and differential are set by the engine makers but are usually about 175 rpm and 50 rpm, respectively.

If there is no rpm drop on one magneto but a normal amount on the other, two things could be wrong. Either the p-lead has broken — because the system is designed to fail safe, a break in the wire will leave the engine running — or the suspect mag is timed incorrectly. No matter, the airplane should be inspected; a broken p-lead or bad ignition switch leaves the engine ready to run at the push of a prop, regardless of the position of the ignition switch. Be careful.

On the runup, you should also be looking for smoothness. A rough engine on one mag could indicate lead-fouled spark plugs — common on Cessna 152s used in the training environment but possible in any model — or another sort of magneto problem. Try running the engine at a slightly higher rpm and leaning until the rpm peaks, then re-try the mag check. If the engine seems to have smoothed out, it was probably lead; if not, have a mechanic take a look.

Ever wondered where that clicking sound comes from when you (carefully) turn the prop over? Because magnetos have a fixed relationship to the engine (in other words, their timing relative to the engine doesn't change) and most engines need slightly different timing to get good, consistent starts, there is a mechanism to aid starting. Called the impulse couplings, these devices use a clutch assembly to both retard the magnetos during low-speed operation (usually below 500 rpm) and provide a kind of slingshot effect that helps the mags overcome their natural tendency to be weak at low speeds. Still other airplanes use the so-called Shower of Sparks, which employs a battery-powered system to introduce more powerful sparks during starting.

Magneto ignition is not perfect, but it is what one might call (if somewhat tongue-in-cheek) "mature" technology and likely the only one we'll see in production airplanes for the foreseeable future.