A320...DES mode Vs Selected Speed

[kuwait340]

hello...

if you are descending on managed speed and DES (managed) mode ...let's assume that the managed speed is 280kts...

the ATC ask you to increase the speed to 320kts...

i would go in this case to pull the speed knob and rotate to 320 kts..

in this case...are the altitude constraintes are still respected by the DES mode.

i.e is the profile on the DES mode will be adjusted for the new selected speed 320kts...or will it be based on the old managed speed 280kts...

or do i need to go open descent to go lower than the profile ...to cater for the speed reduction distance..


thanks..

and appreciate if some one give the reference in the FCOM.

[Right Way Up]

Alt constraints will be respected by DES mode, and if they cannot be made the FMGC will tell you. Profile is based on selected speed but will assume the next speed change planned in the FMGC i.e. speed 250 at 10,000.

[OPEN DES]

RWU is spot on.

Your descent path is still assuming 280 knots. Flying selected 320 knots at this stage will have you flying the DES profile with a bit of thrust.
If you have been cleared to an intermediate level and level off for a bit, you could do the following to optimise:
-maintain selected speed
-force the aircraft into the cruise phase again by inserting the present level on the PROG page (or just enter '1')
-modify the speeds on the DES page to 'current mach'/320
The descent profile is recalculated now and you could do everything managed.

Now the reverse scenario:
Managed descent speed 280kts, ATC reduces you to 250 or 220 a long way out, back to selected speed initially. Descent profile still assumes 280kt. Aircraft in descent phase.
-make a vertical revision at any descent waypoint and replace the standard 250/10000 by e.g. 220/'present level or higher'
Now your profile is recalculated and assumes the 220 kts. You could do everyting managed again.


However you will always have selected speed and OPEN DES/VS at your disposal! I usually just use the 3 times table and some allowances for wind etc. and forget about the FMGS profile.

S.

[kuwait340]

excellent replys...thanks guys.

but gents...when you said that all alt cstr will be respected...

let's say you are descending and there is a way point where you have to cross at or below 13000 feet and at an exact speed of 250 kts...

when you select the speed to 320 kts...is the at or below 13000 feet will be respected ?

and what about the speed (the extra) 40 kts (from 280 managed to selected 320) let's say 10 nm before that waypoint ..if i manage the speed ...do i need to use some speed brake to correct the situation...?

thanks

[FlightDetent]

RWU is spot on. Your descent path is still assuming 280 knots. :uhoh: Once I pull speed, the SPD value on MCDU changes from managed to the selected one. I always assumed, that this indicates the profile have been re-calculated. Also, 15 NM after T/D, I select SPD 220 kt, all of a sudden FMGS indicates 4500 ft too high on PROG page ? True or false? Honeywell pegasus 2+ softeware?

K340: It is very likely that you would get a "SPD ERROR AT xxxx" message. At higher altitudes THR IDLE descent is not based on idle thrust :ugh, so you can always drop below with OP DES, and deccelerate to match the constraint with no airbrakes. OTOH, I find that below approx FL200 THR IDLE descent is based on true idle thrust, so a speedbrake (or better descent planning) is unavoidable.

[kuwait340]

i have just found this in FCOM4

4.02.30 P7

Descent Speed Profile:

if the pilot revert tp SELECTED speed/MACH mode during descent, the profile is NOT modified and the aircraft flies the same profile at the FCU selected speed/mach value.

that means...it is better to drop below the profile couple of hundreds of feet to cater for the extra knots when starting the desceleration.

if not...if you like to continue on the DES mode...speed brakes might be required for the speed reduction.

[Dream Land]

Correct, to change the profile you will need to change the cost index, my un-expert opinion is that you will not get a speed error at a constraint by pulling "selected speed" unless the constraint is a speed constraint. Cheers, D.L.

[FlightDetent]

Correct, to change the profile you will need to change the cost index Really? :confused: My MCDU changes with SEL SPD, not sure about the yo-yo. FCOM reference from K340 is spot on, yet I think the software works differently. :uhoh:

[OPEN DES]

that means...it is better to drop below the profile couple of hundreds of feet to cater for the extra knots when starting the desceleration
No in the situation you describe: flying 320 kt selected when the managed descent profile is still based on 280 kt. It is better to forget about the whole managed descent profile or force the managed profile to be recalculated with a new managed speed of 320kt.
option 1:
-disregard the FMGS profile, it has no significance any more. You are low on the 'true' profile.
-select VS -1000 fpm or -500 fpm whatever makes ATC happy in your area
-work out your own descent and pull for OPEN DES (3 times table does magic plus a few miles for decel)
option 2:
-recalculate the profile. DL suggested to reinsert the cost-index. This does force a recalculation, same does: selecting re-direct to the TO waypoint, re-inserting QNH, temp, wind -changing or re-inserting any constraint.
However the profile is recalculated STILL assuming the MANAGED speed target of 280kt even though you are flying selected speed at this stage.
The only way to recalculate the profile properly with your 320kts is to leave the DESCENT phase temporarily to allow for a new managed speed insertion on the DES page.
You can leave the descent phase by entering the cruise phase again in case of an intermediate level-off. As described in my post above.
Option 1 is much easier. We are pilots after all.
Hope this helps

[OPEN DES]

My MCDU changes with SEL SPD, not sure about the yo-yo


yoyo assumes managed speed still.

[Dream Land]

OPEN DES, I think I have it a bit wrong, so, to make things clear as mud, let's say that on our current flight plan using CI 35, somewhere on the descent portion of the flight, the prediction is that we will cross the XYZ fix at FL180, now as Kuwait has suggested, during the Managed Descent (280 ish) I decide to pull selected speed of 320KTS, I am under the impression that I will still cross XYZ fix at FL180, do I have this wrong? :confused:

[OPEN DES]

Hi Dreamland

Yes you will still make good the contraint FL180 at XYZ.

However the flown profile will be the original 280kts profile. Flying 320 kts selected will have you flying with quite a bit of thrust. IMHO better to forget the profile and go for V/S initially to shallow descent and then go for OPEN DES.

Re-inserting cost index or anything will not change the descent profile to match the selected speed. It will only recalibrate the profile using actual OAT and actual w/v but still assuming the 280kts since this is the managed speed target.

Apologies if I am not clear, but English is not my native tongue.

S

[Dream Land]

OPEN DES, thanks, understand your point, just wanted to make sure I haven't misunderstood all these years. :}

[kuwait340]

OPEN DES...you are 100% correct...

by pulling V/S to shallow the descent in order not lower than the computd profile then pulling OPEN DES ....is correct if you have normal descent path with out any constraints.

But...if you have a speed constraint over a waypoint (let's say to cross a waypoint at a speed of 250kts)...i think it is better to drop below the profile rather than selecting V/S to shallow the descent...because you need some more distance for the speed reduction...as the FMGC was calculating a speed reduction from 280 to 250 not from 320 to 250.

and as for the ALT constraint....no problem at all...it will be respected either with selected speed or managed as long as the DES mode is engaged.

thanks ...
please feel free to comment

[OPEN DES]

But...if you have a speed constraint over a waypoint (let's say to cross a waypoint at a speed of 250kts)...i think it is better to drop below the profile rather than selecting V/S to shallow the descent...because you need some more distance for the speed reduction...as the FMGC was calculating a speed reduction from 280 to 250 not from 320 to 250

Yes you will need more distance to reduce. However whether to drop down or not immediately would depend on how high you are.

Remember that the managed descent profile at high levels has quite some residual thrust. Going just for OPEN DES instead of DES will make quite a difference already. As pointed out by FD.

[Wingswinger]

...which is why experienced 320 series pilots will go 10 miles beyond ToD as calculated by the FMGC before commencing a descent in OP DES thus keeping the thrust at idle all the way to the OM. Provided there are no altitude constraints to get in the way and the track miles to touchdown are known, that is.