Hello G,
A quick reply from my side since I need my rest asap.
but what you are not saying, is that if necessary it will fly it at idle thrust same as open descent.
Yes, when necessary it will fly engine idle thrust in DES mode but only IF you have a significant tailwind which was not accounted for or when high on profile and also at lower levels. (below 10000ī, ballpark figure).
In a nil wind managed DES on profile the proposed idle segment is not really idle. Itīs more around 40 to 45% N1 while the FMA
will read THR IDLE, but there is no idle flashing on your ewd. If you pull for OP DES at this stage you will notice the N1 decreasing while the FMA continues to read THR IDLE. The idle segment of a managed descent profile is more shallow than a true idle OP DES. This is as I said to account for reduced trackmiles/early reduction/anti ice requirement.
I personally always descend in managed descent as this is SOP and itīs their trainset, also being conservative (not being true idle) it leaves your options open.
Also on the last paragraph, you are contradicting yourself, if you select cost index of 0 your speed for the descent will be slower(not green dot, but closer to 250KTS on the A320) which means you will spend less time at altitude, that is really where you want to be for fuel savings.
My concept was that the longer the engines are operated at idle and not at an 80% N1 cruise setting the better. Green dot will maximise your time in descent thus reduces the time the engines are running at high N1. However the engines will be less efficient at lower levels. So I guess there is a bit of a trade off there. I stand corrected.
S.
PS javelin is not really doing testpilot stuff. he is just forcing geometric descents between constraints to become level with idle segments. this is more efficient. the same way a boeing behaves in VNAV. even if you object to this, you can do it by accident by re entering the QNH on the perf app page for example.