[MD-11X] Constraint AT OR BELOW

[Guest FrancoisH]

Hello there,Again a question about this fabulous plane...I set into my flight plan a -220 on a waypoint and checked in lateral revision that it show AT OR BELOW FL220 on the waypoint...When I selected my altitude on the FCP, it show "Profile to 10.000" and started the descend as it should but on the waypoint with the AT OR BELOW constraint, it intercepted FL220 and maintained it until the waypoint... as if I set a "AT" constraint... wouldn't be normal operation to this type of constraint to continue the descent below as you request it ? This constraint it to avoid the Profile to be ABOVE this altitude on this waypoint, not to level at it ?Regards.

[3-2-1-Now 126]

Hi,I'm off on another test flight now - I'll try it here and see what happens!Best regards,Robin.

[3-2-1-Now 126]

Hi,I tested this, and it is working correctly. :)What exactly was the routing as displayed in the F-PLN page?Best regards,Robin.

[Guest FrancoisH]

You mean what by "working correctly" Robin ?I mean that when you descend over your "AT OR BELOW" waypoint, if you set below 220, the plane should NOT make a step at FL220 but continue the descent down to the next constraint or to the best level... This constraint is just here to force the profile to descent early or to be levelled before this point (true ?)Why the Profile mode then inserted a step at FL220 over this point ? The next constraint was in any case making this one being fulfilled, even if not set... so the plane should have continue the descent trough this without a step at FL220But it does this step.Noticed this constraint came from the STAR (from navigraph). Maybe the problem is there.Regards.

[Guest FrancoisH]

I made many try around this issue and wrote it to PMDG support as following : Hello there,Maybe I don't understand very well the way the FMS handle "AT OR BELOW" constraint but... here what I made. From EBCI to LFMN, yesterday, I was FL390, I set at LFMN VEVAR5C arrival (0810 AIRAC cycle). It get a constraint at -FL220 over GAPDO which is normal as long as it show BELOW FL220 on this point on the charts...I myself set another constraint, a bit after this point FL110 over MOTUR. If I look at the F-PLAN, I see that if I remove the -FL220, it will cross at FL210 GAPDO... I then set agagin the AT OR BELOW over GAPDO and I select 11000 on my FCP...PFD Show Profile to 11000... Descent initiate normally but... it show as well a step at FL220... ND vertical profile deviation bug started to go down, showing me above the profile... and then the plane kept FL220 over GAPDO rather than following the profile to 11.000... So if the constraint into the FMS Work correctly, it calculate the right profile... it anyway make the A/P making a step at FL220, what I should NOT do as long it's AT OR BELOW restriction.Isn't that a bug ?Regards.Making another flight on Paris, I got the same issue again with an AT OR BELOW constraint.Regards.

[emvaos 13]

Francois,Wanted to reply on this one first time around. The FMS will initialy completely disregard "soft" constraints but through a series of solver iterations see if the constraint is satisifed and only then will a "soft" restriction become "hard". Note also that the path is being built from the destination upwards, so what you see at FL220 really depends on what happens below. Decelerations comprise a major factor. Decelerations combined with in between (non collocated) altitude restrictions may lead to this or that result based on maximum economy and the performance library. Finally note that this FMS model allows for non-geometric paths (i.e. intermediate mini cruise) after a decel if this turns out to be the most economical profile. In general if it does so then it IS so.Vangelis===================================== E M V Precision Manuals Development Group www.precisionmanuals.com=====================================

[Guest FrancoisH]

Ok I think I do understand... I still just do not understand then why the pink profile losange still descend below my "AT OR BELOW" constraint and then request high DRAG to get back on the descent afterwards.Regards.