This is an interesting discussion.
A number of different options have been discussed in training over the years from considering the deductions to climb gradient Glen has discussed depending on the AOB to setting V2+10 and allowing the use of full bank for following the SID or the EFATO procedure.
It’s alway more of a discussion point than I think it should be. I think it should be black and white unless you have a very good reason for it not to be followed.
Should one day we have to account for our actions the question would be asked why we chose to ignore a valid performance calculation. How did we determine in the heat of the moment, oh that gap over there looks alright, Ill turn here, there’s an airport over there etc etc and be completely, 100%, bet my life on it that I am right and I can still clear all the terrain and obstacles. Just because you can see where your going doesn’t mean you can clear all the terrain.
If we use APG and it provides a procedure, and you accept that procedure to determine your MTOM you would have to have an exceptional reason for not following it.
Once you have issued your Mayday you may do as you please in order to protect the safety of your aircraft, at MTOM that means following the APG procedure if it gave you one. You tell ATC what your doing and tell them to standby, then you get on with it. I would suggest there is a greater risk of hitting the ground if your wandering around and off your procudere than there is hitting another aircraft even if your routing does take you somewhere you would rather not go.
Beijing is a great example by Glen where Guru used to suggest a right turn after departure ( maybe it still does I am on APG now ) on the northerly runways right across the departure tracks. I agree it would be challenging to get ATC to understand your intention, however, following that procedure would guarantee you terrain separation. Non approved methods ( even if correct, excel spreadsheets etc ) might be difficult to justify, I would suggest. After all, APG has done the work for you.
Fortunatly, our APG now gives sensible procedures based on the RNAV departures so its follow the SID for a couple of waypoints and then take up the hold but it does sometimes throw up anomalies and as someone already said perhaps its not quite as good as it could be. Previous airline ops had the benefit of fewer and known airports but there would never be any discussion about not following the engine out procedure.
You might have excess performance, you still flew the procedure because you would never really know how much extra you had, and if you started wandering off all over the place it was simply guesswork as to your terrain clearance.
Just my thoughts but its a good question by the OP and one of my favorite subjects.
Regards,
SFFH