Magneto Failure Modes and Effects - or why you can't afford to neglect magneto maintenance!
The manufacturer's maintenance manuals spell out the maintenance tasks required to keep the magneto airworthy but they don't always make it clear (apart from phrases like "possible engine failure") about what may happen if those tasks are neglected.
(1) Impulse Coupling failures:
(a) IC spring failure.
The "clockwork" spring in the impulse coupling can and does break. This will be very obvious when the magneto is off the engine because when you hold the magneto body in one hand and turn the IC/shaft with the other hand, there will either be no spring action at all or a very weak action. The broken spring will result in the magneto firing at the wrong time. This is particularly dangerous if the magneto is mounted on an engine without a starter motor, where the propeller has to be hand swung to start the engine - as with many vintage aircraft. Incorrect timing due to the broken spring will cause the engine to kick back with a risk of serious injury to the person swinging the prop.
Running a mag beyond the recommended 500hr inspection, may mean that fatigue cracks which might be seen during the inspection will grow to failure.
(b) Pawl rivet wear.
The 500hr inspection involves checking the pawl rivets for wear. Excessive wear allows the heel end of the pawl to start contacting the stop pin which will eventually cause the latching end to be thrown out. Centrifugal force at normal engine speeds keeps the latching end clear of the stop pin but when this fault occurs the sudden engagement with the stop pin will destroy the coupling which may then shed broken metal into the engine accessory case. This will likely result in both magnetos being put out of action plus further damage to the engine. Such an occurrence in a single engine aircraft will leave you looking for a suitable field in a hurry.
See this youtube video: https://www.youtube.com/watch?v=Pw0FOB54tV0
Inspection at the recommended interval detects excessive wear and allows replacement of the coupling before such a catastrophic failure can occur.
(2) Distributor Gear/Rotor gear failure
(a) The distributor and rotor gears are both nylon and teeth can break off these gears. This will result in mis-timing of sparks which will manifest itself as abnormal engine noise and vibration. This failure mode is not as serious as the impulse coupling failure modes because the affected magneto can be grounded and the engine will still run on the remaining magneto.
(b)
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