Porch Monkey…………
the plane will burn about the same per hour at that speed/level as it does at .78/FL380
Your Fuel Flow may be the same
BUT your TAS will be quite a lot slower and thus the
kg/gnm will be a lot worse.
Obviously the aim is to achieve the best kg/gnm, which FL that is at will depend on wind gradient.
It's all down to fuel flow/GS calculations.
Sorry, im not trying to teach you how to suck eggs!! I suspect we are all on the same page.
