FCTM - Managed descent

[extricate]

Hi there,

Want to gain more knowledge. Pls advise.

I'm reading about managed descent and i have a couple of questions that i would like to ask.

Quote from FCTM:
if the aircraft gets low on the computed descent path, the speed will decrease towards the lower limit of the speed range. When the lower speed limit is reached, the A/THR reverts to SPEED/MACH mode and applies thrust to maintain the descent path at this lower speed. A similar path intercept point n will be displayed on the ND.

When the aircraft gets too low, why would the speed decrease? Won't the A/T bring the speed back up? Is there like a fixed speed at which airbuses fly at when descending? How is the speed decided?

Thanks

[Field In Sight]

The descent path is calculated based on input forecast wind.

To stay on this path the aircraft has the option to speed up or slow down.

If the real wind has a stronger headwind (or less tailwind) then even flying slower will not keep you on the pre-computed descent path. Therefore the aircraft flies at this lower speed but must add thrust to keep you on the path.

FIS

[Microburst2002]

Normally, either pitch AP/FD mode or A/THR will acquire and maintain the speed target. When thrust is fixed, pitch keeps speed. Thrust keeps speed when pitch keeps something else (V/S, G/S).

However, in a managed DES speed is allowed to deviate from target (within a speed band) while thrust is fixed (THR IDLE or THR DES) and pitch maintains the target flight path, which is the computed descent profile.

While inside the speed margin above and below the managed speed target, neither pitch nor thrust care about speed. Only whe pn speed hits a the boundary things change:

If speed reaches upper limit, then pitch will maintain that speed and no longer the profile.

If the speed reaches the lower limit, then thrust will maintain SPEED at the limit, instead of THR IDLE, while pitch will keep maintaining the profile.

In between those cases, speed will "float"

[extricate]

thanks alot guys

[Squawk-7600]

Microburst nailed it :ok: Also be aware that the speed band the FM generates may be greater than ATC or company SOP allowed tolerances. For that reason if the speed decreases in the example you illustrated, you may need to manually select speed to force the speed back up to the target airspeed, even though it was within the target speed band. Not doing so is a sure way to fail your next check ride :eek: Once back at target speed you'll be able to reselect managed descent speed. If repeated selected speed intervention is required, re-enter your destination temperature and a new (hopefully more appropriate) descent profile will be generated.

I'm not sure that was a very good explanation, so I trust that makes sense.