VNAV altitude constraints

[JP59]

Hello,

I noticed when I have an altitude constraint like for example 13000B or 13000A the aircraft crosses the waypoint at 13000. In some cases it is normal because there is other constraints on the route, or performance limitations. But in many descent cases, when the constraint is the first, the aircraft could cross the waypoint above 13000, but it doesn't. Example 13000A at first waypoint and 13000 at waypoint 20NM after. Why does the aircraft crosses the first waypoint at 13000 ? Is it normal ?

[Hardy Heinlin]

Hello,

it picks the first "A" or "exact" constraint for the idle descent path target. A "B" constraint is never used as an idle descent path target (source: Bulfer, Gifford, and others), your predictions in the LEGS will remain dashed. If the first constraint is an "A", the FMC uses that altitude as a target and based upon that it computes a prediction of the descent path and T/D.

The idle descent is over after the first altitude constraint (AFDS may change to SPD | | VNAV PTH). From there on, "B" constraints are considered as well, also "A", and "AB" windows.


Regards,

|-|ardy

[JP59]

Thank you for this explanation Hardy. I was surprised to see the FMC passing an "A" waypoint at the exact altitude during a descent. I though : "Why didn't it computes a higher altitude at the "A" waypoint, in order to move the TOD a little bit further" ? If it is a normal comportment of the FMC, it's ok for me.

[Hardy Heinlin]

The problem is that the path is dynamic. If it would take the next hard constraint after a preceeding "A" constraint, and that same "A" is close to the path, you may get trouble if the head/tail wind changes (or other performance parameters for that matter). The higher the groundspeed, the shallower the idle path (the earlier the T/D). Finally, if the preceeding "A" would hit the path, that would automatically become the target descent waypoint. It may end up in a flip-flop effect, causing chaos in the path prediction and guidance. Of course, this is only if the preceeding "A" is higher than the following hard altitude.


Regards,

|-|ardy