TOP of descent, and descent monitoring. (A320)

[Microburst2002]

The airplane gets high, while maintaining selected speed, but honestly I'm not sure. It is not tidy to descend that way. I pull for OPEN DES with the speed selected, in those cases, and use speedbrake if necessary.

[macdo]

If you have SELECTED Speed and Managed DES the a/c will maintain the trajectory of the MANAGED DES profile in the FMGC. It will also attempt to satisfy your Selected Speed demand as best it can. If you are asking it to slow down, it will bring the engines to idle (if not already there) and attempt to give you any pitch up it can to reduce the speed back to the Selected Target. The only pitch up opportunity that will develop is when the FMGC finds itself slightly under its computed profile, possibly because the predicted winds are different to reality. It does work, but not very well. You can intervene and deploy 1/2 speedbrake until close to the Selected Speed target.
If you select a higer speed target, the FMGC will add thrust (SPEED will show in the FMA) to accelerate to the target.
As Microbusrt2002 says, this is untidy usage of the automatics and can easily lead to a 'whats it doing now?'. As a general rule don't mix SELECTED and MANAGED, although like all rules, there are times when it doesn't apply.

[dream747]

So am I right to conclude that profile still has priority over speed when a lower speed is selected while in Des mode? In the sense that it will pitch up to try its best to maintain the selected slower speed at idle but not at the expense of deviating from the descent profile?

I went through the books and I can't find anywhere where it states how the aircraft will deal with selected speed (lower than the managed target speed) while in Des mode on profile. It only explains the situation when you select a higher speed.

Thanks for the clarification guys.

[Ollie Onion]

Dream,

don't know if it is written anywhere but from my experience the speed is secondary to the profile. It will do the best it can to pitch up to reduce speed but as soon as it deviates from the profile in pitches down and lets the speed increase again until it is back or under profile.

[macdo]

DREAM
I think the following ref. from our FCOM explains it:

DSC – AIRCRAFT SYSTEMS
DSC-22_30 – AUTO FLIGHT - FLIGHT GUIDANCE
DSC-22_30-10 – General

The AP and FD pitch modes can control a target SPD/MACH or a vertical trajectory, and the A/THR
mode can control a fixed thrust or a target SPD/MACH. However, the AP/FD and the A/THR cannot
both control a target SPD/MACH and trajectory simultaneously.
Therefore the AP/FD pitch modes and A/THR mode are coordinated as follows:
- If an AP/FD pitch mode controls a vertical trajectory, the A/THR mode controls the target SPD/MACH.
- If an AP/FD pitch mode controls a target SPD or MACH, the A/THR mode controls the thrust.
- If no AP/FD pitch mode is engaged, the A/THR mode reverts to controlling the SPD/MACH mode.
In other words, the selection of an AP/FD pitch mode determines which mode the A/THR controls

There is a table afterwards but I can't copy and past it in.
In any case, you are quite correct.

[Microburst2002]

You know... I'm not sure who is primary. If pitch follows the profile, this should be maintained at the expense of higher speed, but speed should be primary to profile, in my opinion.

Question is what mode will ATHR have active in such scenario, very slow selected speed and steep profile with DES? THR IDLE or SPEED (with teh engines in idle)?

If is THR IDLE, then speed is primary. That's what happens in my opinion, but as I say, I always use OP DES in such cases. If it was, SPEED, however, then profile would be primary.

[Natstrackalpha]

Not only but also,

Constant power.

See what your Green dot is at level flight. Note the N1. T`will be about 10% above GW...the same power setting will normally be good for S speed.when Flap 1 is selected. when glide becomes activ, select flaps 2 and gear.
keep this power setting and allow the a/c to slow. .About one dot below glide select flaps 3 then select Flaps Full as you intercept the glideslope at the Vapp target.
Usually only small amounts of thrust changes are necessary but if you need it then adjust thrust sooner rather than later, obviously.

[dream747]

I had the opportunity on my previous flights to try out what we've discussed here so far. Apparently, a lower speed has priority over the profile in managed descent. What I did was select a speed of 230 knots and the aircraft simply pitches up to reduce the speed and continues deviating to get high on the profile, just as it would in an open descent.

[Microburst2002]

dream 747 thanks for your experiment

the principle of the 320's AFS is that always someone is in charge of speed, except in DES mode when speed is allowed to float about the target, within the magenta margins.

If with thrust idle a/thr mode on pitch maintained the profile, then "nobody" would be in charge of speed.

I assume that THR IDLE was green in your FMA, right?

[dream747]

Microburst, yes, THR IDLE was on the FMA throughout. On that particular day ATC needed us to reach a waypoint at a certain time, which we could only do so if we reduce our speed considerably. However we were only told after our original TOD point based on the FMGS descent speed. We went for managed descent (since we were still on FPR), and selected 230 knots. The aircraft simply pitched up and deviated from the profile while continuing it's descent on 230 knots. Eventually the 'More Drag' message came up on the FMGS. That's the whole story.

I think you have summed it up perfectly. Thanks for the interesting and fruitful discussion guys.