Originally Posted by
Yeah nah
Yes, definitely no advantage going from open descent to V/S. As stated above, V/S can also be more difficult to manage because thrust is not at a fixed value in that mode.
Taking this a step further and assuming you are still in NAV mode and above the calculated vertical profile, changing from DES to OP DES will not change your descent rate either as both modes will be commanding Thrust Idle in that situation.
If you are way too high on the calculated profile and have limited use of the speed brake because of VLS margins, such as on the 321 when ATC have requested min clean speed, in my aircraft there is no shame in selecting flap early (below 20k and icing conditions permitting) which will reduce your VLS allowing for more speed brake extension. I always activate approach phase before selecting flap to prevent an embarrassing overspeed later in the arrival should managed speed be subsequently selected. Likewise there is no shame in retracting that flap later if unanticipated track miles have been added by ATC negating their continued use.
Alternatively, you could state your minimum clean speed is higher than GDot because you anticipate using speed brake to stay in touch of the calculated vertical profile, thereby handing the resulting separation management change back to ATC.
Either way, you have the opportunity to achieve a greater ROD than staying clean at GDot.
It can be useful. If you have free speed, want to target an exact RoD to put you at X mile final at X altitude then managing the speed with activated approach phase and allowing it to simply be managed with the speedbrake is very effective.
Descending from 10,000’ and ATC have given you a 10 mile final. Can set V/S of, for example, 2000’ a minute and then you know you will be at exactly 3,000’ at the ten mile final. Then all you have to do is manage the speed, the predictions for the level off arrow can vary wildly in Op Desc depending on aircraft state, what it’s expecting you to do with speed modes and speed brake etc.
Some would get upset about not flying the bugged speed but meh, it’s active use of modes to achieve a desired effect (idle power, exact level off point)