jeehell wrote: ↑Thu May 25, 2023 10:18 am
To match a constraint by increasing V/S you would NEVER first slow down then accelerate to increase the V/S, this does not work like that. The aicraft adds thrust once the VDEV is roughly higher than 250 ft, in which case it's only way to get back on profile is using SPEED mode and 1000fpm target.
Hi JL. sorry but i have to argue that, hoping you´re not gonna hate me.
What i mean is not related to match a constraint. It´s more related to some changes of the FMGS management which where already correct in former builds. The changes you made meanwhile may affect also the mismatch in computations or in matching constraints, because we exspecting a specific reaction if the FMGS as we where trained by the type rating course.
When i made my type rating course, my Instructor showed us, how you schould manage the decent exactly that way in the real aircraft. And at that time, Jeehell FMGS acted the same way, what it does not anymore since build 56 or so. The reason why especially this behavior was discussed in the type rating course, was to understand why well trained A320 Pilots never ever have to use V/S mode in descent, which is really really dangerous if you are not aware, that in V/S mode, the ac does not have a fail save to get into over- or even underspeed.
That´s the reason why in reallife, in case you may end up too high and too fast at the final descent point, which would end in a rejected landing or go arround, you stay in managed or open decent mode and use the FMGS system logic instead to get a much higher rate of decent for a short time, when the ac tries to catch up the managed speed. In fact the real ac usually increase the rate of decent at first to keep a specific speed instead of adding power to the engines.
Have that said, this does not mean that under no circumstances the ac never increase thrust, of course. It´s just that it would first increase the vertical speed as i have discribed.
Even if you are not need to catch up the descent profile, because you are way above the profile, the real aircraft uses primary slight changes of the decent rate to keep the ac within the managed speed range. Only if that would not have enough effect, i. ex. if you level off, the ac add thrust.
Don´t get me wrong, please. This is only meant in short words, and i know that in fact it´s much more differential and complex. But actually it seems the ac is nailed to specific rate of decent and allways increase engine power to control the speed while it decents and that´s just not how the real aircraft does. Usually the engines stay at idle in an continous descent and don´t need thrust unless the decent isn´t decrased.
I give you an example. In case you will not have the allowance by ATC to decent when you reach your FMGS computed TOD, it is common procedure to change into selected speed mode and slow down the ac instead. This can be done down to Green dot speed. Then, once you get the allowance for your decent, you put the ac as usaul in managed decent mode and change the speed back to manged speed. The real aircraft will then go into a high rate of decent in order to increase the speed. It may also add some more thrust to the engine aswell, but that is usually not nessecary.
What i try to say is, that Jeehell FMGS doesn´t even think about a significant higher rate of decent. Instead it allways add thrust ONLY and thats´s not right.
Another example: If the ac is in managed decent mode and let´s say keeps the managed speed exactly but your VDEV is way too high, let´s say 5.000 or 10.000 ft. It schould be possible to increase the rate of decent by using the speedbrakes, to reduce the VDEV. But if i do this in Jeehell it add power to the engine to compensate the increase of the descent rate. And this is definetly not right.