Ok so I’ve taken off according to SID chart, climbed out to cruising altitude using correct vertical separation altitude, calculated TOD…ready to start my descent.
To make it realistic IRL is the correct procedure:
reduce speed to below 250 knots before starting descent?
descend to just above 10,000 feet and then reduce speed to below 250 knots?
Well… This is actually up to the captain to decide, some go fast and steep, some go slow and steady. According to some SOP's, including the one I follow, state that you reduce your mach speed until a little under climb, and the mach speed will then decrease while the IAS will increase. Once my airspeed is around 280kts, I switch from mach to IAS, and keep 280 until 12-13k. Then I decrease it to 230-240kts, and further decrease it if a STAR speed restriction is in place. If not, I do it at my own discretion to extend flaps as soon as possible. :)
If you left VNAV in on a real world aircraft 2 is what would happen - so you would descend at a speed higher than 250, and when you get within a few hundred feet of 10,000 the aircraft would slow the decent to reduce to 250 before continuing to descend further.
Certainly no need to do 1 and reduce speed to less than 250 above 10,000. Ideally your descent is at a rate which maintains speed but where the power setting is at idle to maximise fuel efficiency.