Originally Posted by
gulliBell
Does the after-start magneto check require the pilot to observe an ERPM drop, and if no such drop is observed the aircraft is unserviceable and requires maintenance prior to the next flight?
Yes, that is exactly correct.
The procedure is to set 75% engine RPM, then from the BOTH position, select L, then BOTH, then R, then BOTH (or you can do R first, etc.). The checklist says "Mag Drop at 75% RPM...7% max. in 2 seconds", meaning no more than a 7% loss of RPM from 75%.
Interestingly, there is no minimum mag drop specified. However, it is important to allow more than just two seconds to elapse when performing these checks so that a definite mag drop can be seen, and the integrity of the magneto switching system verified.