Aircraft engines have dual ignition systems for increased detonation margins, NOT for reliability from redundancy. Aircraft combustion chambers are so freaking big that by the time a flame front has traversed the chamber the remaining gas has been compressed and heated to the point of detonation.  Starting the fire in two places shortens the combustion time significantly and reduces the opportunity for detonation. This is also the mechanism for the RPM drop during a mag check.  When you turn off one mag the combustion event takes longer which has a similar effect as starting the process later, AKA "retarding the timing". 

Hi Brent,

 

From what little I know about the combustion process, the above paragraph seems to contradict it's self. Retarding the spark timing helps to prevent detonation. If turning off one mag has the same effect as retarding the timing (as you stated above), wouldn't running on one mag help to prevent detonation rather than contribute to it? After all, peak cylinder pressure will be lower while running on one mag and thetaPP will move further away from TDC. Lower pressure means lower CHT's which should also improve the detonation margin. I think redundancy and improved fuel efficiency may be the reason two mags are used on aircraft engines.

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Ron Laughlin
Legacy N44L (res)