FMC "A"

[Guest wwe2261]

When flying and using the FMS I notice that some of the altitude assignments for the waypoints are suffixed with an "A" I can't seem to find what this means, maybe I missed it but any help would be appreciatedJeff

[Guest tmetzinger]

>When flying and using the FMS I notice that some of the>altitude assignments for the waypoints are suffixed with an>"A" >>I can't seem to find what this means, maybe I missed it but>any help would be appreciated>>JeffThe A suffix means "At or Above", the B suffix means "At or Below", these are called soft constraintsThis is used when constructing a VNAV path. I'm not sure exactly how the PMDG code does it, but the way I expect it to work is that the computer should calculate the path to the first "hard" constraint (i.e. 8000) and then check to see if any earlier soft constraints (i.e. 12000B) conflict with the path. If no conflicts, the path is used. If there is a conflict, the path gets adjusted to meet the soft constraint.I'm not sure this is 100% correctly modelled in the PMDG, but I'll do some testing this week to see if I find any discrepancies.

[Guest glenb]

TimThat sounds reasonable. With all the descsuion about descents I was hoping a programmer would explain how the computer determines the descent path.

[Guest]

Soft constraints are often part of SIDs and STARs, usually linked to a specific speed for passing a waypoint.For example on passing waypoint DENUT aircraft should be at FL250 or above, this is written on the charts.Then at another waypoint they must be at FL70 or above and at 190 knots.

[Guest meyomyx]

Interestingly (or not),I've noticed recently that with a "B" constraint, the TOD point has been calculated and displayed some way PAST that point. Remove the "B" and presto, the TOD is at an appropriate point.Not sure whether this is a consistent quirk or otherwise.