I've looked through our abnormal checklist for this and it (contrary to what is siad in the Q400 QRH) that the powerlever should not be moved until made certain that the prop is controllable. We are to try and carefully modulate the conditionlevers towards min to see if the Np is controllable and if so, we'll operate it normally with an increased monitoring on tha engine.
I guess that moving the powerlever could alter the pitch on the prop towards a more fine setting which would worsen the problem (constant speed prop: power reduced, prop goes to fine to correct Np). If it's a oneway controllability issue, this would indeed cause the problem to worsen.
What is intersting in this I conclude is how the prop is brought to feather. The autocoarsen is not feather, but a coarsened pitch. This position will be the least drag position but not fully feathered as I have understood things. It'd be interesting to know if the autocoarsen uses the same mechanical components as in normal prop governing (which I presume it does) or not. And if this ability is lost, will it be possible to feather the prop anyhow?
I realise these are all questions I should be able to read up on myself, but as I said I don't have access to the manuals at present.
/LnS