A320 level off arrow
 

Hello.
Quite bad but even if flying for some years on A320 I miss the level off arrow help on descent. In climb phase the prediction on ND is always quite straightforward, on descent I’m not able to rely much.
let’s say managed descent, managed speed on arrival, all good.
at a certain point ATC gives me a shortcut and I go OP Des and selected speed. Is the level off arrow on ND correct for my current scenario?
and if later on ATC asks me to reduce 210 does it account for my new selected speed?
From what I understand it should be correct unless Approach phase is active. If approach phase is active instead of selected speed it’s predictions are based on the assumption that I will configure so will be in error…. Same applies if Decel point is midway as from passing that point my predictions will be wrong if still in selected (higher) speed.
in other words, is it really helpful on the last stages of descent if using open des?

I'm not rated on the A320 but on the A330. I find the level off arrow rather...fanciful. The only level off arrow I rely on is my brain. I will be at target waypoint in X minutes and I'm needing to be at Z altitude over there so I'm needing Y rate of descent. Works every time.;)

eheh in the end I use the same technique, works perfectly. But sometimes the lazy part of me wonders how to use the Airbus magic and resting a bit my mental maths ;)

The only mode in which you can rely on level off arrow is V/S. All the other modes are "approximations", or in other words, it changes constantly according to actual conditions (current V/S, wind....) and it is not precise. In general, more constraints in FMS for the descent, the vertical calculation is bigger crap :)

The level off arrow will only be very accurate in VS mode if you fly the actual selected or managed speed as the FMS assumes you fly the actual speed and the actual VS. If for example you select 220kt but use a VS-2500 in descent, your actual speed might be around 280kt but the FMS uses 220kt for the predictions and therefore in that case you will reach your target altitude further than the level off arrow. As homebuilt mentioned, A simple calculation using time and VS works much better. Time from the ND or GS. If you fly at 300kt that’s 300/60= 5nm/min. If you have a VS -1500 you will lose 3000 feet every 10 nm.