[Airbus] Managed Descent Logic
 

Good Afternoon,

The following has been answered in some sort but many years ago (decades) probably with different FMGS software version. Can someone help me with the following:

1. During descent in MANAGED mode (THR DES+DES) if the speed is then SELECTED (SPD+DES) will the constraints be respected by the vertical guidance? For example 12,000FT by XX waypoint.
2. Same scenario, but now the constraint is 200KTS/12,000FT will the constraint be respected with enough time to slow down if the speed is SELECTED? (How do you know when to slow down?)

TIA.

It depends on the software. If you have the version with the latched descent then the path takes priority over the speed. All altitude constraints will be respected. If you have the version that does not latch the profile then selecting a speed may result in the aircraft flying above the profile in order to maintain the speed. This can result in at-or-below constraints not being respected.

For your question 2, the same applies but you need to estimate when to slow down. With the profile latched the profile will be flown assuming a deceleration and so it should be apparent by it levelling at the constraint or shallowing the descent that it is expecting the deceleration to commence. You also have the magenta spots that indicate deceleration points but I can't remember off the top of my head if they remain visible in selected speed.

Cheers not heard the term "latched" for us but know it is for the A350, will read into it!

As explained above, a new version was introduced a while ago. With the new version, the descent path as computed by the FMGS takes priority in order to make all the altitude constraints (as here in the US) and the speed is allowed to drift away. Vertical restrictions and descent path has priority over speed, this is called latched descent. The VDEV has a small magenta box along the altitude display to simbolize the vertical path is now latched or has priority.
Actually in real world ops you can see this mode and works beautifully, now one has to solve the issue of ATC asking to maintain a lower speed as well. For this we are gonna use the speedbrake for example.

The traditional method had it that speed has priority over the vertical constraints. If a constraint was predicted to be missed due to a shallower descent path, than crew had to use speedbrake while the engines were at IDLE and DES mode was active. If the crew forgot/not deployed the speedbrakes, at some point the PFD and FMGS wold pop up a more drag message to warn the crew to use whatever they could to save the vertical profile with that selected speed.

If you're Boeing literate, then "latched" is generally similar to VNAV PTH.

Thank you, your username is very scary! I'm scared to even write a reply to you knowing your past, good to see you have transitioned into Aviation. (I think..)

Appreciate the reply, out of interest do you find this information in the FCOM/AUTOFLIGHT/MANAGEMENT part or is it in the FMGS specific part, because can not seem to find the chapter. I want to know if the aircraft I will fly (still doing the TR) has the new latched mode or is the existing software..

It's in the FCOM,

DSC-22_30-40-60-30

It depends on both FMGC manufacturer and Software standard. It will only be in your FCOM if you have the particular SB embodied. If you know whether you have Honeywell/Thales and the software (should be something like H3I15 or S7AI12) some help may be available. See Autoflight-> General -> FMGC Standard.

The “latched” function refers to the Descent Profile Optimisation (DPO). Not all aircraft have it (refer to FCOM), the NEOs should come with it but it is available for retrofit on the CEOs.

You can have the latched function without DPO. Our CEO have the function without DPO and the descent profile is too conservative.

A common misconception, but not true. In our fleet DPO and the latch are completely separate. If an aircraft has DPO then it doesn’t latch and vice versa.

Thanks for this info, I've literally being flying for some years without any idea of what that symbol meant. But the question I have is does the FMS choose when to larch and when not to latch? Or is it the case that once the FMS is updated with the latch feature it will always be in latched mode during a managed descent.

Thanks

It latches once it gets to within some threshold from the profile. Exactly what the threshold is is unclear sometimes it won't latch until very close to profile and other times it will capture it from some vertical distance away. If you are 1000' above profile and in selected speed you can typically get it to NOT latch if you don't want it to. If you manage the speed then the FMGC will use the speed window to dive down and pick up the profile. In selected speed and unlatched the FMGC will respect the speed which may allow you to keep above the profile and stay unlatched (if that's what you want).

What would be the point of going down in DES and selected speed? I’ve seen people do this and asked them why. Nobody’s been able to give me a sensible answer yet…

Given speed control from ATC and altitude constraints on the STAR.

Er, you want to get home quicker and also make your altitude constraints.

Given that the DPO is essentially less conservative than the traditional descent profile has anyone found that it usually ends up being too optimistic and leaves you high of profile? Our newer Airbus aircraft seem to have this tendency and hence I stay below the profile at the start of the descent, those who manage right from TOD end up above. I have no idea if our newer aircraft have the DPO function but I'm assuming so as I see marked difference in the aircrafts ability to follow the profile versus older aircraft.

I find our DPO aircraft are ok. Is probably a good idea to make sure you have the latest descent winds loaded in the FMS so it’s got good data. You can tell an aircraft has DPO from the QRH config summary.

Generally I see in the latest part of the approach it needs a bit of help, especially when you need to decelerate from 250 to green dot. Usually some speedbrakes is needed, even with winds loaded.
My idea is that either is due to idle/perf factor probably wrong or because initially was developed for NEO aircrafts then retrofitted also to CEOs. On NEOs works quite well on CEOs needs some help… But again it’s my theory. Honestly I like a lot.

Yes it does for the CEO. I think the reason behind is that the CEO may need a different and lower IDLE PERF number, it is a matter of tweaking till getting it right. For this crews must comment this to maintenance and OCC, so they have valuable inputs and can keep on playing till getting it right.