Page 748 - PPL-engelsk 2025

Aircraft general knowledge

When the contact in the primary circuit is opened by the cam on the shaft, the

field lines collapse and supported by the capacitor, this will cause a high voltage

to be induced in the secondary winding.

The voltage is approximately 20,000 volts, and the power distributor ensures

that the voltage reaches the correct spark plug.

Note the magnet selector switch at the bottom. When you select the magnet

OFF, the switch closes, and there is ground on both sides of the primary winding.

If there is a fault in the system in the form of a broken wire, or a contact that will

not close, this causes the magnet to be ON all the time. This is good, because
then the engine will not stall.

It is not always that such a fault is discovered.

Never turn the aircraft propeller unless you do it carefully and know the risk that

there may be a fault so that the engine starts just by turning the propeller!

Unfortunately, there are many examples of people getting off the wrong way.

The link below has good advice on handling propellers. Some aircraft actually

have to be started by hand, so see how.

https://www.youtube.com/watch?v=EQm0hfEES6o

8.9.7.3 Impulse coupling

An impulse coupling is used to help with starting. At least one of the 2 magnets

is equipped with an impulse coupling. Without this, the engine is almost
impossible to start.

Flyveteori PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 748