Oei Airbus Operations

[Iakklat]

Ok just a quick question.Can someone please explain what, if any, the managed speed target will do differently if you experience an engine failure in the Alt Crz mode as opposed to the Alt mode?
Sure i can recite the Fcom 4 explanation but it still is as clear as mud to me:ugh:

[FlapsOne]

Alt Crz = Managed target speed becomes the higher of engine-out cruise Mach number or speed, or current speed.

Alt = You will have selected, not managed speed won't you?

[idg]

Flaps One
No I don't think so. If in ALT mode we could still be using managed speed.

Everyone I ask seems to think that the FMGS will give 'Green Dot' whenever there is an engine failure, but as FCOM 4 describes this is far from the case! The a/c might begin to drift back to GD even with the EO CRZ SPD showing on the PFD since there is unlikely to be enough thrust at altitude, but of course that was not the question!

If the a/c is in ALT mode (or actually in V/S zero as we would be the case when accelerating at EO ACCEL ALT) the FMGS will take into account any speed constraint ahead if still using managed NAV. In this way when very heavy, for instance, we might never get to GD (this dependant on if we are still in FLX/MCT not TOGA!).

[FlapsOne]

What has EO Accel Alt got to do with a question about ALTCRZ vs ALT?

[idg]

Because the FMGS uses the same principles when engine out in ALT as it does when V/S zero. In early mod states of the FCU/FMGC after a delay V/S zero would revert automatically to show ALT on the FMA. Doesn't appear to do this now.

[TyroPicard]

ALT CRZ implies CRZ phase, FlapsOne is correct, and in practice that means no change from current speed unless you are flying slower than EO CRZ SPD. Not Green Dot 'cos the other engine(s) would reduce thrust, which is normally undesirable at this point.

ALT implies CLB or DES phase: target speed is set to EO CRZ SPD at the FCU altitude (i.e. current speed is not an option). Always subject to limit speed.

Cheers, TP