Bendix drives.

I have a customer with a pair of factory reman IO 540s with less than 100 hours
on them. After landing he will sometimes find the bendix drives on the
starters partially and occasionally fully engaged. It does not happen every
flight but sometimes every other flight. On at least two occasions the bendix
drives were not engaged after shut down but were almost engaged when the pilot
returned from lunch an hour later! We have changed the starters twice now
since Kelly Aerospace admits too certain problems on some starters even though
these do not fall into the serial number range of those on a recent bulletin.

The bendix is supposed to be held back until the starter winds up and thrown
back in position when the engine speed exceeds the starter speed. So what is
going on here. One source doubts the starter motor is actually engaged as the
speed of the starter driven by the ring gear would through the windings off the
armature and ruin the starter. Is is possible some kind of transient voltage
spike at shut down is attempting to drive the starter? Why would an unengaged
bendix become engaged while the aircraft is shut down and the crew is at lunch?