Impulse Coupler

[TractorBoy]

Apologies if this is the wrong forum for this, but it looks like there are technical questions here...

Recently had a flight cancelled as after startup we noticed a catastrophic mag drop on one mag, with some rather alarming misfire. The engineer who fixed the problem has diagnosed an impulse coupler failure on the mag in question.

What would cause this to occur? We'd just landed from a flight over the Irish Sea for some fuel and it happened when we were due to depart the airfield. Were we on the lucky side that it didn't occur during the crossing, or would starting the engine have caused the failure?

I've read up a little on the impulse coupler. How could it fail mid flight, as it doesn't seem to engage above 250rpm?

[cwatters]

I'm no expert but perhaps read the explanation here..

http://www.aeroelectric.com/articles/MagnetoSwitchOptions.pdf

"The energy storage spring used to “snap" the shaft during cranking now keeps the magneto in proper step with the engine for running"

So damage to that spring caused during the start could cause miss-firing during running.

[TractorBoy]

"The energy storage spring used to “snap" the shaft during cranking now keeps the magneto in proper step with the engine for running"

That's the bit I can't get my head round. The retardation is caused on startup by the pawls catching on the stop nut, winding up the spring which suddenly releases and gives a burst of energy. However once the engine is up and running the pawls no longer catch on the stop nuts due to centrifugal force and the spring is no longer wound.

I'm guessing that if the spring snaps then the pawls aren't held in place correctly and can catch on the stop nuts again. But I'd like an expert to confirm this !