Ignition Systems

The ignition switch controls the magnetos and the starter.

The ignition system provides a spark to ignite the air/fuel mixture in the cylinders of a piston engine. Most modern aircraft engines use magnetos to generate the spark. Although not as sophisticated as the electronic ignition systems used in the latest cars, magnetos are useful in aircraft because:

1. They produce a hotter spark at high engine speeds than the battery system used in automobiles.
2. They do not depend on an external source of energy such as a battery, a generator, or an alternator.

Getting Started

Magnetos generate electricity when they rotate. So, to start the engine, the pilot must engage a battery-powered starter that rotates the crankshaft. After the magnetos begin rotating, they supply the spark to each cylinder to ignite the air/fuel mixture and the starter system is disengaged. The battery no longer has any part in the operation of the engine. If the battery (or master) switch is turned off, the engine continues to run.

Dual Ignition

Most aircraft engines are equipped with a dual ignition system—two magnetos that supply electrical current to two spark plugs for each cylinder. One magneto system supplies the current to one set of plugs; the second system supplies the current to the other set of plugs. This is why the ignition switch has five positions: OFF, L (LEFT), R (RIGHT), BOTH, and START. With the switch in the L or R position, only one magneto supplies current and only one set of spark plugs fires. With the switch in the BOTH position, both magnetos supply current and both sets of spark plugs fire.

Advantages of Dual Ignition

Aircraft have dual ignition systems for safety and efficiency.

Operating the Ignition System

You should turn the ignition switch to BOTH after starting the engine and leave it on BOTH during flight. Turn it OFF after shutting down the engine. If you leave the ignition switch on BOTH or LEFT or RIGHT, the engine could fire if the propeller is moved from outside the airplane—even if the electrical master switch is off.

Before Takeoff Check

To make sure both ignition systems are operating properly, check each system during the engine runup before takeoff. The normal procedure is to set the power at about 1700 RPM. Turn the ignition switch from BOTH to RIGHT, then back to BOTH, then to LEFT, and then back to BOTH. You should see a slight drop in RPM each time you switch from BOTH to either RIGHT or LEFT. If both magnetos are functioning normally, the drop should be no more than about 75 RPM.

Shutting Down the Engine

You should not shut down a piston engine by turning the ignition switch to OFF. Instead, move the mixture control to the idle cutoff position to turn off the fuel supply to the cylinders. After the engine stops, turn the ignition switch to OFF. This procedure ensures that no fuel remains in the cylinders and that the engine won't start accidentally if someone turns the prop or if carbon deposits inside the cylinders create hot spots that ignite residual fuel.