Managed descent, altitude constraints, radar headings

[pineteam]

Gimbal Error you might want to check the FCTM. You can select speed below characteristics speed provided it’s above VLS + 5kt. It’s only at high altitude that you should not intentionally fly below Green dot speed. This is a common misunderstanding.
FCTM -PRO-NORMAL PRO- SOP-APPROACH
Like B2N2 mentioned beautifully: There is no substitute for experience.
If you were flying into China where ATC ask you to fly at 180kt nm 60 miles from touchdown and your S speed is 185kt you will get use to actually fly below S speed. It’s usually more than 25kt from VLS. Totally fine. Of course you can select flaps 2 but you will burn more fuel.
the 3 times I will definitely select speed below characteristic speed that I can recall are when:
I’m flying at 250kt and ATC asked me to reduce speed to 180kt ( again very common in China) I will immediately activate approach phase if not done already and select 180 kt and ask for flaps 1 at Green Dot and Flaps 2 if required. Yes you can do the Fcom way and managed speed when speed reaches Green Dot ask for flaps 1 and so on but doing so the engines will spool up and down. Not so great.
Second case it’s when I fly a very heavy A321 and green dot speed is less than 5kt from VFE. If VFE is 230 and Gd is 227 and you don’t select a lower speed the engines won’t be on idle and flying into turbulence or cloud you would easily overspeed. The safest way there is to select a speed below Green Dot, allowing the engines to go on idle and once the speed goes below Green dot with a speed trend going down then ask for flaps 1.

3rd case. Anytime during configuration if it will prevent thrust variation.