According to the ICAO Standard Phraseology Guide, MAYDAY indicates the need for immediate assistance, while PAN PAN means an urgent communication not requiring immediate assistance. Following a simple engine failure, the need for immediate assistance would, at the very least, be predicated on whether you are going to use your emergency authority to deviate from a clearance. If you can continue at the assigned altitude and routing until re-cleared, perhaps a MAYDAY call is not needed. Otherwise, it seems to me that it is.
Back in my more daring years, I flew all of the test flights on our fleet of Metro II's following the engine upgrade from the -3 to the -10UA. This involved an inflight shutdown and airstart, and certainly did not require an emergency declaration as I was bobbing around within an assigned block altitude in an otherwise empty airplane.
The difference in performance between the two engines on the Metro II was remarkable, however the airplane was still a Part 23 airplane with no genuine certification requirement for single engine performance. The Metro III was a "commuter" category airplane certificated to a nearly identical to Part 25, and did meet a much better performance standard with one engine inoperative. In fact, the III arrived with AFM data for single engine climb gradient, and it was apparent that an IFR departure from the mountainous Roanoke Valley in Virginia would be rather problematic without serious weight restrictions, which left one with a very chilly feeling toward the Metro II, which provided no such data.
Previous to the Metros I flew Convair 240's, and I recall the control forces with an engine inoperative on the Metro were similar to the Convair...heavy. However, I don't remember any of our numerous 110 pound female pilots having any difficulty, while some of our more brutish fellows broke a sweat now and then.
