FAA Powerplant Written Practice Test

When the switch is off in a magneto ignition system, what happens to the primary circuit?

1. It is open

2. It is powered

3. It is grounded

4. It is disconnected

The correct answer is: It is grounded

In a magneto ignition system, when the switch is turned off, it effectively grounds the primary circuit. This grounding action allows the magnetic field in the magneto to collapse, preventing current from flowing to the ignition coil. As a result, the ignition system is disabled, which is essential for safety to ensure the engine does not inadvertently start when the pilot does not want it to. Grounding the primary circuit serves multiple purposes. Primarily, it interrupts the path for the electrical current that would otherwise allow the ignition system to generate a spark. By grounding the circuit, the system is made safe by ensuring no electrical energy can build up to fire the spark plugs. In contrast, when the switch is in the "on" position, the primary circuit is powered, allowing the ignition system to function correctly and generate sparks at the spark plugs for combustion. If the circuit were simply open or disconnected, these terms would not describe the grounding aspect, which specifically refers to making the circuit inoperative in a controlled manner.