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?

Vessbot

The physical interaction between flowing air and the fuselage/wings/speedbrake/etc of the airplane, are not effected by the autopilot mode or FMA annunciation. If X airspeed and Y vertical speed are maintained at idle thrust and Z speedbrake setting in one mode, it'll be the same in all modes.

Because whoever told you this was making an offhand exaggerated remark, that taken literally went beyond his real point. Or, his real point could be plainly in error, too. Even if they're an instructor.

Check Airman

VS mode priorities VS over IAS

1. If you select a high VS, the speed brakes won’t change your descent rate. They’ll just keep the speed in check if it’s still accelerating at idle.

2. Speed mode does just that, but is limited by what the engines can give you. If your VS is too high to maintain at idle thrust, it’ll simply accelerate. Take a look at the mode reversions.

hans brinker

In addition to the info above:

First of all, you will not descend faster without the speed increasing if you use V/S instead of open descend. Either the instructor was talking about V/S giving a faster response when starting a descend, or there was some other misunderstanding/miscommunication.

1)
If you select a high V/S and the speed goes over the selected/managed speed, the speedbrakes will work. The only reason I can think of that you were told the speedbrakes are useless in V/S is in the situation that you are descending in V/S, speed is at the selected speed. Extending the speedbrakes in this situation only adds power without changing the ROD or speed.
As far as the difference between open descend versus V/S:
You stated you were descending at green dot speed, and I assume you have selected speed at green dot. In open descend, giving you a descend rate of about 800FPM. You can't really use the speedbrakes, because of the increase in VLS. If you select V/S nothing relating to the aerodynamics with the airplane changes, the speed and V/S will remain the same. If you increase the ROD by selecting a higher V/S, your speed will increase, and you could have achieved the excact same thing in open descend by selecting that higher speed. Open descend would IMO be the better mode, because if through variations in wind during the descend, your speed momentarily drops below the selected speed, in open descend the autopilot will correct that by increasing the ROD, and in V/S the autothrust will correct that by adding power.


2)
Speed (or Mach) on the FMA means the autothrust is responsible for maintaining the speed, and the autopilot is responsible for maintaining the vertical path. Possible modes for this situation are ALT, V/S, GS. In open descend the thrust is idle, in open climb the thrust is at climb power, and for both of those the autopilot pitches to keep the speed at the managed or selected speed bug.
If you climb in V/S and the speed is higher that the selected/managed speed, the autothrust will reduce power to decelerate to the selected speed. If the speed is lower, the autothrust will increase power up to climb power, to try to get to the selected speed, but it might not be able to if a high V/S is selected.
If you descend in V/S, and the speed is below the selected speed/managed, the autothrust will add up to climb power to accelerate to the selected speed. If the speed is higher than the selected/managed speed, the autothrust will reduce the thrust to try to decelerate to the selected speed, but it might not be able to if a high V/S is selected.
(not an all inclusive list of possible modes, and explanation based on AP/AT on with PL in the climb detent)

vilas

OP DES is a fixed thrust(idle) mode it will pitch up and down to maintain speed and resulting ROD has to be accepted. To increase ROD you can use speed brakes keeping the desired speed above the increased VLS. Nothing more can be done unless descent speed is increased. SPEED/VS is fixed VS mode at desired speed which is maintained by variable thrust. You have the option of selecting higher VS till the thrust reaches idle. Any higher VS will result in speed increase. Let's compare the two modes, say you are in OP DES with thrust idle(obviously) and now you pull VS it will open at present VS and present idle thrust. There will be no advantage of selecting higher ROD without resultant speed increase. However in OP DES you had the option of increasing ROD with some speed brake that is not there in SPEED/VS because use of speed brake will bring the thrust out of idle to maintain the speed.

FlightDetent

It's a fine virtue to verify own thoughts and not just trust the instincts at face value.

Yet, sometimes, you simply are right and the other guy is not.

FullWings

As a general point common between most makes of aircraft, I find the use of speedbrake with V/S a) requires more attention thus increasing workload and b) is, for most situations, unnecessary as OP DES or LVL CHG will do the job nicely and prioritise speed, leaving RoD under your control, as opposed to V/S which does the opposite.

Observing from the monitoring seat, V/S + SB on anything but a completely smooth and calm day requires constant adjustment of the SB. Fine if you have the capacity to get it right but often it loads up the monitoring pilot as well as speed wanders down to (or below) flap speed.

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.

VariablePitchP

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)

sonicbum

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.

Fursty Ferret

You kind of identity the problem and the solution yourself in your first paragraph: "VLS shoots up".

Speed brakes are not a switch. Extend them fractionally, let the aircraft respond, and then slowly and smoothly extend as needed. Once established in the descent you can get close to full deflection (or half with autopilot engaged).
VLS is proportional to load factor so similar care is required during the level off.

sonicbum

I am afraid it is not VLS increases only with speedbrakes extension. On the other hand V alpha prot is G dependant and will increase in turns.

fdr

Care to put that on a T-shirt?

k.swiss

Gentlemen, ladies, thank you all kindly for your responses. Thank you for the motivation and also great advise to consider and take onboard! I will endeavor to read each one.

Cheers!

To summarise, regardless of auto-thrust mode, use of speed-brake will result in an effect if the aircraft is beyond its speed target. (In the scenario of descent)

However to be noted that in the case of SPEED mode if the auto-thrust can not maintain a target, due to excessive V/S then reaching VLS or overspeed could occur.

FlightDetent

Many times, as you will clearly see around here, people verbalise things differently. But actually do the same thing.

Surely you remember the class-room exercise where teams of 4 need to write SOP for boiling an egg (*).

Indeed, dirty wing performs worse. Fixed V/S mode targets the selected value, A/THR matches the bug. Misconfugure the three and something has to give. That you already knew.

Because you made it to the seat not by divine error but by being qualified with awareness which way is up, forward and how gravity works.

​​​​​So perhaps the right thing to ask on the never-ending thorny path to proficiency is not what the instructor said but rather why he said it.

(*) in case you did not, you still can see where that is going which proves my point.

{I will be collecting the T-shirt orders from mid-month}

k.swiss

Never did the egg experiment but makes sense to teach it! Thanks again all. T-Shirt orders incoming.

Fursty Ferret

You're absolutely right and I humbly apologise for the screw-up.

vilas

To cut the long story short, for a given speed OP DES with available speed brake results in max ROD. However OP DES is not a recommended mode on the final approach and whatever the vertical displacement, on final approach it must be managed with SPEED/VS. If not possible a GA is the only option.

grrowler

If you’re within limits, maintain OP DES and the speed, and hang some flap out. The aircraft will no longer be flying best L/D speed and your ROD will increase.

As mentioned, this would only be appropriate prior to commencing approach, and as always go around or discontinue the approach if you aren’t ready.

taa211

hi there.
i have a question about speed brake and Vls.
if using full speed brake without Auto Pilot, how much increasing Vls?
i’m looking forward to your reply.
Thanks.