Operating in environments where ISA+15 or ISA+20 and high elevation airports are the norm, I recently asked one of my instructors "how much does engine-out performance is reduced when the prop is not feathered?" He said in a very straight forward way: "Long story short, you will realise your life is sitting on the auto-feather".
While practising engine failure in flight he proceeded to prove his statement. He reduced one engine to idle, we proceeded with the immediate action items, and as soon as we feathered the propeller it felt like the airplane immediately cut loose from a rope that was holding it back. If it felt like that already in cruise configuration, at altitude, levelled and at cruise speed one better feather that thing
Our standard briefing states the following:
"Failures at or above V1:
- Failures other than engine failure: No corrective actions below 400ft
- Engine failure: (...) Confirm auto-feather operation. If auto-feather failed, execute manual feathering as follows:
PF "protects" live engine; PM feathers the propeller on the failed engine at PF's command"