With the speed window closed, I think your FMC target speed in descent mode (speed window closed) is normally going to be whatever is on VNAV 3/3, or your max flap limit speed, whichever is lower.
If you select a flap setting whose max speed limit is less than 240 knots (more than flaps 10), then the FMC target speed should try to prevent you from exceeding that flap limit speed. In your scenario, the speed window is closed and you are selecting flaps. If you were already at 240 knots (that's what you would probably have in VNAV 3/3), when you select flaps 20, the FMC target speed should go to 230 knots or perhaps five knots less than that.
Notwithstanding that, I would prefer to have the speed window open well before I got to flaps 20 on a VNAV approach and not rely on the FMC to prevent a flap overspeed.
[EDIT] It looks like AtoBsafely got it before me. You can also manually enter a speed at the course fix or final fix on the LEGS page and I think the aircraft will slow to that speed when within a certain distance of it. As mentioned above, some programs even have lower speeds coded in on the LEGS page which is handy to prevent the power surge when you select VNAV and the speed window closes just prior to the FF during a VNAV approach.
Last edited by cf6-80c2b5f; 23rd Sep 2015 at 07:53.
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