Are there situations on a 500 where you might do an auto with the engine set back at idle?
One possibility is a Pc line failure or leakage, which can cause the engine to decel to idle, with no provision for manual override. Happened to an acquaintance of mine in a Soloy B47 4 or 5 years ago.
here's an AB that explains it:
https://www.casa.gov.au/file/79386/d...token=zml52uhP