Page 749 - PPL-engelsk 2025
P. 749
Aircraft general knowledge
The impulse coupling delays the ignition timing so that the piston has been in
the top position before the mixture ignites. This is to prevent the engine from
backfiring.
The impulse coupling delays the ignition by about 25 degrees to avoid backfiring.
This provides a higher voltage at the start moment and thus a better spark, so
that the spark that is formed is stronger and thus has an easier time igniting the
mixture.
8.9.7.4 Fouled spark plugs and other faults
The engine usually starts on the magneto that is equipped with an impulse
clutch. If there is too much loss in RPM on a magneto, it may be due to fouled
spark plugs.
Fouled spark plugs can be caused by taxiing on the ground for too long with the
engine idling. This can lead to too much loss of RPM when the engine is only
running on one magneto during testing of the ignition system before take-off.
This can be remedied by running the engine up to approx. 2000 rpm for approx.
half a minute while leaning the mixture. Then, test again. If this has not helped,
check the spark plugs etc.
Faults in the form of a broken wire can cause the magnetos to be on!
Propellers must be treated as if the ignition is on constantly!
8.9.7.5 Testing the magnetos
The next page describes how to test the magneto ignition system before take-
off.
Flyveteori PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 749
