Are you new on Boeing? Former Airbus? It sounds like you have little knowledge of Boeing VNAV logic.
The FMC controls speed in cruise. The FMC controls speed in descent based on idle (with no above or below constraints) or based on a geometric descent (if there are constraints). Since it's FMC commanded the speed window blanks so that you know you that its following the plan that you made. If you open the speed window (outside of approach logic conditions) speed will be the priority and it will diverge from the path. The VNAV path is set by you, and a few rules that you really need to know in order to visualise the path. The speeds commanded by the FMC can vary from Descent speed (above FL100) to 240 (below FL100) to a speed restriction below xxxxx to any speed selected at any waypoint - culminating at the (default) 170 for final approach. The more speed changes you give it, the more flat spots you'll see in your path. The VNAV path is usually based on idle and is affected by wind, which means it's hardly ever correct. This means that in descent when the thrust levers go to idle and the airplane pitches to follow the path - it will usually be to fast or too slow. If it's too slow you can add thrust, but if it's too fast you can either 1. use speedbrake, 2. let the speed creep up or 3. speed intervene (which will decouple you from the path and probably add to your problems). Most people just let the speed creep up.
SOPs permitting, Non-ILS approaches CAN be flown in VNAV from top of descent until minimums, with absolutely no requirement for a level segment - The entire logic is designed to fly a CDA. You just have to understand what's happening.
Last edited by rudestuff; 18th Apr 2024 at 17:43.