Originally Posted by
k.swiss
Hello All Question:
Descending with OP DES and green-dot-speed gives a very shallow ROD, (less than 700FPM with a standard weight) try and use speed-brake and the VLS shoots up, even worse on 321.
Instructor once told me better to use V/S if you want to get down faster when at green-dot-speed but in some cases if selecting for example -2,000FPM at green-dot the speed trend starts to creep up.
Question 1:
When in this situation of high V/S selected and speed trend is shooting up why am I told speed-brake is useless in V/S mode? Surely if the speed is shooting up because of the high ROD speed-brake will have some effect.
Question 2:
What is the true meaning of "speed" mode on FMA when in V/S, it keeps the ROD but how much is the speed trend allowed to fluctuate?
First thing first we need to understand why You are at green dot speed to start with. Is an ATC speed restriction? Is it because You are on Final Approach? In both cases the control of Your IAS is of primary importance, therefore you will need to keep the speed AND increase RoD if you need to because you are getting high for instance. If You are on Final Approach (ie. Close to the airfield) just start to configure as much as required to increase drag. Airbus considers the use of VS as a tactical mode to reduce the rate of descent or for small altitude changes. To increase the rate of descent the required mode is DES (if applicable above the profile) or OP DES. The only time you must use VS to increase your rate of descent is on a GS From Above technique. So keep your descent “disciplined”: if you need to maintain a certain speed for whatever reason then act on your drag.
Now if you are let’s say 30NM out and flying at Gdot because of sequencing, it will not be practical to configure so far out. At Gdot you descend approximately on a 3 degree glide path without speedbrakes but being far out you still have plenty of options in your pocket, first one asking for more track miles if it looks really tight (thumbs up to atc) and then configuring as you get closer.
To summarize: speedbrakes are brakes for speed
they create drag and you can of course use them with VS to avoid the speed increase but it is not how you should aim to increase your ROD.