You are mixing two concerns. First, the takeoff bit and then the subsequent departure. Both need to be considered and the more critical determines the final departure weight.
I actually believe YOU are the one mixing things up. I don't know why you want to talk about preflight planning.
I am talking about the ACTUAL takeoff. In 99.999% of the cases you are NOT going to be experiencing an engine failure (for which your takeoff performance calculations have been made).
So your takeoff will not be a "takeoff bit"... it will ONLY be the departure, as long as your engine doesn't fail.
The "takeoff bit" will only be relevant if your engine fails. It's a purely hypothetic "what-if" calculation. The ACTUAL departure is very real - every time.
P.s.
...it's funny that you say I am
mixing things up, when I started out in the first post to say that
It's common that certification requirements are being mixed up with procedural requirements.
And I there after split the post in
bold sections.
Facts are:
- You can safely takeoff with gear extended (except in cases like Cologne NOR8F, with an extreme procedural gradient requirement).
- You do not need to apply MEL for flying with gear extended, if retraction is operable.
- It will have no impact on your takeoff performance calculations and doesn't need to be accounted for (as you can retract the gear if necessary)
- If an engine fails, you retract the gear as usual.
End of story.