Originally Posted by Sycamore
The act of unfeathering should start the engine as well..
That may be true with single-shaft engines like the Dart or similar, where a spinning propeller will turn the compressor as well. On the PW120 series, the propeller is not connected to the turbomachinery proper except via the power turbine. Here, unfeathering the engine will only result in drag and not in a restarted engine.
Again, only speaking for the DH8, but every engine restart in flight required the use of the electric starter both on the -300 and -400 series. A windmill relight is no option. The propeller is only unfeathered after the engine restart has been successful and the turbomachinery is running normal again. The associated procedure is rather long (2 full pages in the QRH) and takes much longer to complete than available when already in a circling approach at rather low altitude. IŽd guess that facing such a predicament, using the available energy to glide to some sort of landable field would be the better option.
Which is of course rather academic and leaves aside the startle factor and other things on the crews mind at such a moment.