FAA Powerplant Written Practice Test

What happens to the primary circuit when the switch is off in a magneto ignition system?

1. It becomes energized

2. It is grounded

3. It short circuits

4. It activates the secondary circuit

The correct answer is: It is grounded

In a magneto ignition system, when the switch is off, the primary circuit is grounded. This grounding effectively prevents the magneto from generating a spark. The grounding of the primary circuit interrupts the flow of electricity, which is necessary for the magneto to function and create a spark in the secondary circuit. When the switch is turned off, the magnetic field collapses, and without a current flowing through the primary winding, there is no induction of voltage in the secondary winding, which is responsible for igniting the fuel-air mixture in the engine's cylinders. This is critical for both safety and operational control, as it ensures that the engine cannot inadvertently fire when the ignition system is not intended to be active. Understanding this function is crucial for safe aircraft operation and maintenance, as it highlights how essential the ignition switch is in controlling the ignition events in an aircraft engine.