A330 ALT versus V/S 0

[subria023]

Hi all,

I just started an A330 course and, while studying the system, I have a question regarding the function of the ALT and Push-to-level off V/S button on the FCU. My understanding is that both buttons have the same function as to perform an immediate level off at the desired altitude but my question is:

When an immediate level off is performed by ALT button, the ALT mode appears on the FMA and the altitude is maintained. When there is any disturbance to the flight such as updraft or turbulence and the a/c deviate from the altitude then the ALT mode will resume the a/c back to the altitude being held. But if the level off is performed by pushing the V/S button, the V/S +0 appears on the FMA. In this case, will the a/c resume its altitude after the disturbance?

I tried to look for the explaination in the FCOM but still could not find the answer. Any comment will be appreciated.

Subria

[ahramin]

In this case, will the a/c resume its altitude after the disturbance?

No.

[junebug172]

V/S is just V/S and nothing more. Just as with airspeed, it doesn't care about altitude. It just tries to hold 0.

[subria023]

Thank you very much !

[Stuck_in_an_ATR]

Out of curiosity (I fly the '320 and we don't have this button but I had it on some previous types)

When pushing ALT during climb:

- does the AP capture the altitude at the moment when the buton was pushed (ie. overshoots and then descends), or,

- does it level off (as in VS=0), and then tracks current Alt?

Cheers

S.

[ironbutt57]

yes push the vs button "push to level" is indeed on the 320

[ahramin]

In a climb pushing ALT pb will level off at the altitude it predicts it can level off at and then maintain that altitude. The prediction and the aircraft don't always work out the same though so you can see some overshoot and return with the ALT button.

Also there is some different behaviour in certain cases when the autopilot is engaged after the alt button is pushed. DSC-22_30-70-70.

[nitpicker330]

Here's a tip.

If you need the Aircraft to level off quickly then push ALT.
Selecting VS 0 brings about the level off much slower…..

Was the same in the 777