View Full Version : Airbus Descent Profile Optimization

9th Jul 2019, 07:40
this question is regarding top of descent calculation,

my previous knowledge is, that starting the descent earlier for a thrust idle descent, saves fuel because the engines throttle down sooner rather than extending the cruise.
(this technique requires lowers speeds to increase glide ratio)

but looking at the airbus literature, the DPO software is extending the cruise phase. id like to understand why.
i am aware of the benefits of DPO at bottom of descent as i always performed in a similar way in selected mode.


9th Jul 2019, 16:28
Because the DPO "reduces margins in descent modes". Without it, you'd go lower early but keep thrust above idle.

he DPO just brings you closer to the OPDES profile, which is the ultimate fuel-efficient profile. Extend the cruise phase, but benefit from FULL idle during the glide.

21st Aug 2019, 02:10
thanks FD that makes sense.

13th Mar 2021, 04:19
Because the DPO "reduces margins in descent modes". Without it, you'd go lower early but keep thrust above idle.

Hi FlightDetent,

Can you please clarify this for me as I'm not sure I fully undestand: On A320 without DPO if you are in managed descent, FMA is showing Thrust idle, if I go and pull OPN DES, the idle thrust will still be exactly the same.
From what I understand the idle thrust does not change and if for example the the wind is as expected the speed will just drop below the speed target to maintain the profile.

A320 equipped with DPO the idle thrust is also the same as aircraft without DPO but the TOD displayed on the ND is the same profile as using OPN DES. Right? :}

13th Mar 2021, 14:31
Is it not true it is more fuel efficient to stay at cruise a bit longer (to ensure staying at least on or slightly above profile) and require occasional use of SB to remain on profile than to start down slightly early (at an open descent profile) and have to occasionally increase thrust to remain on profile?

13th Mar 2021, 14:36
Better to be high and use speed brake than low and use power

Absolutely, within reason, staying too long in the cruise closer in than a normal descent path has you burning more fuel unnecessarily in that phase of flight, wasting your accumulated energy, potential idle descent and requiring excessive speed brake

13th Mar 2021, 16:07

True perhaps. As a matter of fact you could also say that the airplane will never fly fully on idle thrust down the precise pre-calculated profile. Either is low or high.

The trouble with the "slightly above" philosophy, to which I am a keen subcriber if allowed to do my own thing, is if the deviation becomes larger, the recovery options narrow and techniques become unavailable.
To a point where you just cannot do anything. Not a big problem because with the A320 the last anything you could do is around -7500 fpm or.-9000 fpm if in the mach segment (both no gear), provided your colleague uderstands what ECAM is for and what for it is not.

Another vote against the "slightly above" is the ATC side of the equation in 4D airspace. Losing height without the speedbrakes requires speeding up and getting there faster, meaning faster than the orderly flow of other traffic. That causes problems that will be solved at your expensense with extra miles. Even worse by asking to slow down SIGNIFCANTLY while fast and high. Complying will inevitably take you above the profile even more and about to miss that altitude vertical restriction to Approach Sector's TMA airspace resulting in ...
...you do not need to wait for it too long
a polite requiest to increase the vertical descent rate, the good old slow down and go down. Well, no surprise Sherlock, guess who could have prevented from happening in the first place.

Admittedly then, starting a little low is the less demanding option while staying asymptotically below the gliding path is the optimal solution. I am not sure what the ADPO discussed here feels like, whether or not I may have flown one equipped or not. Geesus, it was so long since the last flight I probably could not remember where the ignition keys go.

13th Mar 2021, 16:30
In managed descent, FMA is showing Thrust idle, if I go and pull OPN DES, the idle thrust will still be exactly the same
Right? :}Cheers pin, well, no. Even if that is what the FCOM says, that is a bit of a magenta moment.

The idle thrust is checked on E/WD EIS N1 indication where in-between it needs to say IDLE, on the old CFM the N1 is around 28-34 %. On the other hand what you normally get using DES if the FMS is loaded with winds properly, is a profile that is deliberately flatter (sooner TOD) than the gliding perfection. Once the "idle" VNAV path is attained, A/THR will indeed show THR IDLE but the N1 typically only reduce to 40-42%, for vertical rate around -2800 fpm.

Only when pulling OP DES as you suggest, the A/THR indication stays the same except now the idle command is finally sent to the FADECs, N1 drops and the airplane dives for -3200 / -4000 fpm in the Mach segment.

Guess this fills the blanks.

My belief is it is a feature not a bug, in line with the strategic logic pointed out in the post above. The ADOP seem to reduce the magintude of this behaviour, and its a fancy buzzword for "perhaps we had been overly conservative with the margins".

Remeber when early on type I could get aggravated a lot by not getting IDLE thrust when asked for and indicated on the FMA. That would had been CFM/DAC, perhaps on the other installations it was not that pronounced. Surely I learned not to care that much about the concept.
Yet it still upsets me when above the profile in DES and reaching (but not yet) the fast edge of the managed speed range, the HAL would not set IDLE but insist on keeping those 38-42 N1.

Whenever the colleague in such situation reaches for the speedbrake - a sensible choice if only looking at the PFD - well, what do I say. Eversince The Burn I am not allowed the Su’Kal scream.

14th Mar 2021, 03:41
In a managed descent, the FMA is the power request of the FMGC to the FADEC, (i.e. - above profile THR IDLE displayed FMA.) The EWD is the actual power command by the FADEC - (right?) So even though we see the FMA change from SPEED to THR IDLE, we may not see a corresponding change on the EWD to IDLE because the FADEC may be commanding a repressurization/modulated idle segment at that time and thus a higher than expected N1.

14th Mar 2021, 08:02
FlightDetent and CaptainMongo, Thank you for your inputs.

I can't believe I never noticed that the idle power was different in Managed Descent and Open Descent after so many years.. :}
I could wait for my next flight but I'm too impatient so I will just ask: If we are in Managed descent and above the profile, let's say the speed target is 280kt but because we are high the speed will go on the upper side of the bracket: The idle thrust will still remain the same or it will drop to the same idle as ''Open Descent Idle''?


15th Mar 2021, 04:54
As decribed nicely above the FMA “Thrust Idle” and ED “Engine Idle” are different. Yes it is possible to see engines at Idle ( as displayed on ED ) during managed descent if conditions require it.

Not sure if that answered your question? I followed the bit about being low on profile with speed target of 280kts. However became lost with “because we are high the speed will go on the upper side of the bracket”?

15th Mar 2021, 05:25
Someone correct me, on the A330 you would get THR DES | DES | NAV | 1FD2 first and only later show THR IDLE | DES | NAV | 1FD2, once idle thrust is TRULY commanded.

@CM: I'd take a great reservation to your description that FADEC makes it's own mind against the instructions from A/THR, but not having the book at hand will just suggest you run, run, quick because I can throw it fast, precise and deadly :) But yes, different connections among the stakeholders for the same result: Sometimes it's not the real idle.

@pinteam You need to have a bit of an eye for N1 to spot it, (*) Does not happen all the time, on many occasions you do get the full plain IDLE straight away. CaptMongo is pointing in the right direction. If we are in Managed descent and below the profile, let's say the speed target is 280kt but because we are high the speed will go on the upper side of the bracket: The idle thrust will still remain the same or it will drop to the same idle as ''Open Descent Idle''? Uh, come on, high or low you're asking?

What I remeber is that if you are above the profile and speeds starts to increase towards the upper magenta bracket, - provided this fake idle is active - nothing happens and IAS goes to target +20 easily, with thrust on. Only once the limit of managed range is hit, and vertical trajecotry control will yield to IAS control, the ATH/R calls for the proper idle. Nothing changes on the FMA at that moment, but you'll clearly see the N1 commad, blue inner, rings illuminate and once attained IDLE will appear on E/WD, flashing at first.

The A330 FMA makes it easily understandable.

15th Mar 2021, 07:27
Guys sorry! Of course I meant : When above the profile! xD.
Thank you for comments!

16th Mar 2021, 13:54
I have been flying long enough to have several books thrown at me, currently the queue is quite short you shouldn’t have to wait.:)

Why is it then that when we see the FMA on a managed descent change from MACH or SPEED to THR IDLE, sometimes we do not see a corresponding change to the E/WD display to IDLE?

On some occasions when I have not seen the E/WD change to IDLE as described above on a managed descent, I pull OPN DES and then see the E/WD display IDLE. There must be something else going on here.

16th Mar 2021, 16:09
In my experience, the managed descent profile on the A320 is too conservative. It arrives at IAF-altitude too early, meaning too much before IAF, and calculates typically with a speed of only ~145kt at IAF instead of 185kt (which is what you typically fly at IAF: CONF 1, ready to set CONF 2).

To fly a more efficient descent, you basically have to use OPEN DES to start descent behind the TOD of the managed descent profile, which means you loose the function of the speed bug bracket of managed DES (variations of wind compensated with variations of descent speed), which is a pity.

So an optimized, more realistic descent profile calculation seems a good thing to me.

28th Jul 2021, 14:15
Hello Guys,

Yesterday for the first time I flew an A321 Neo equipped with DPO. And something bugged me. When we were in Idle descent using managed descent. FMA was saying Thrust Idle but not on the EWD.. Then I just pull Open Descent and the engines lost about 2% N1 and went finally really to Idle. Honestly it looks like they just reduced slightly the idle thrust versus Airbus Without DPO but I thought the idle should have been exactly the same as Open Descent as the FCOM states: With DPO, along the idle segment, without altitude constraint, the descent profile computed by the FMS is the same as the one flown in OPEN DES mode. Therefore, reverting in OPEN DES mode during the descent does not change the descent rate.

29th Jul 2021, 08:56
Actually the FMA always does not show what's behind the scene. I had posted earlier from the Sochi accident that when THR CLB/ OP CLB engages in FMA aircraft actually acts initially through Speed VS of +8000ft/mt. Only when the thrust goes past a certain N1 then it goes to THR CLB and OP CLB. Similarly in DES mode the thrust is not really idle but retains some thrust to prevent large deviation if anti ice is used. I had also noticed contradictory statement in FCOM "The aircraft may deviate from the DES profile while DES mode is engaged if:‐
Unexpected wind conditions are encountered
‐Anti-icing is turned on‐
The lateral flight plan is changed"
and FCTM says "The idle segment assumes a given managed speed flown with idle thrust plus a small amount of thrust. This gives some flexibility to keep the aircraft on the descent path if engine anti-ice is used or if winds vary. This explains THR DES on the FMA". Perhaps it meant not fully caters but partly. However according to me there's no THR DES in A320/21. OP DES should always be idle except perhaps the repressurization segment.