The only time we really look at or think about the ignition system is when the aircraft engine fails to respond at the turn of the key. Today, let’s examine this little device that could make or break your day in the sky.

The aircraft ignition system is very simple and normally very reliable. It consists of a starter switch or "mag switch", wiring harness with leads, couple of magnetos, and the spark plugs. All these are interconnected.

To understand and care for your aircraft ignition system, we need to understand what exactly happens when the pilot turns the switch.

When the ignition is turned on:

To being with, in the “ON” position, the battery gets connected to the starter and preps the magnetos. When the key is further turned to the “Start” position, the starter is engaged which in turn, causes the propellers to turn.

The propellers are connected to the crankshaft which in turn rotates a rotor inside the magneto. As you know, the magnetos produce electricity which runs through the wiring harness and into the spark plug in each cylinder. When this spark plug receive the electrical jolt, they produce a strong spark that ignites the fuel-air mixture in the cylinder and the aircraft engine roars to life leading to the start of another glorious day of flying.

As the propeller rotates, the alternator kicks in and begins generating electricity which charges the battery that in turn runs the lights etc. The alternator and the magneto are two different things – the former is solely used to charge the battery while the latter is used to generate high-voltage current for the spark plugs.

When the ignition is turned off:

When the pilot turns the key to "OFF", the electrical Fuel Scan 450 Twin to the magnetos is interrupted. Any residual electrical energy is “sent to ground” via a wire called the "P-lead". With no current flowing from the magneto to the spark plug, there is no spark and the combustion stops.

Testing the P-lead:

Turning the key momentarily to "OFF" then back to "ON" should result in the Aircraft Engine Monitoring Systems faltering and then catching on again. If this did not happen, it could mean the P-lead is faulty and did not complete stop current flow to the magnetos. Should this be the case, just turning the propellers manually will restart your engine and this can spell disaster because this is something you would not normally expect to happen.

Having explained how the aircraft ignition works and the processes that happen with the key is turned on or off, should there be a problem related to the ignition, you now know where to look.