AT OR BELOW constraint ignored ?

[Guest FrancoisH]

Hello,Was flying two days ago from LFPG to LEMD and got in trouble with a descent constraint. I put a limit on a waypoint but the plane still planned descent AFTER this point. Is that the way it is designed ?I thought "AT OR BELOW" was as any other AT constraint making sure the profile will go below the requested altitude... put a "AT" lvl solved my issue.Regards.EDIT// I would add that sometimes, on descent, it make a step at AT OR BELOW constraint which is not something wanted on many procedures... Anyway I manage manually descent on many occasion but just wanted to say about it.

[downscc 1,327]

Francois: The AT OR BELOW works, don't know why it didn't for you. I assume you decreased the FCP ALT to allow it to go down... this type of descent clearance is not unusual where center might give "pilot descretion to FL230 cross ABCD at or below FL330... it is also used on many New England (US) arrivals to get under the corridor traffic.

[Guest FrancoisH]

Yes but the problem IS that it is not always working and the plane make a step then do a steep descent to go back on the profile after the waypoint you set the constraint.

[downscc 1,327]

Let me try to reproduce that, and lets see if I understand you. You are saying that a conditional altitude constraint does not modify the descent profile except for passing that fix?For example: LENDY5 arrival to KJFK has a conditional constraint at HARTY AT OR BELOW FL230 then 45nm later there is a hard constraint at LENDY to cross at FL190. When I fly this, I set the FCP ALT to 19000 and at the TOD she descends in the normal profile to HARTY and crosses at or below FL230 then a very shallow descent to LENDY.This is an example... the aircraft does not return to profile, it stays on profile and the profile includes the conditional constraint. Were it not for the conditional constrait I would not have the shallow 45nm descent from FL230 to FL190.Give me the specifics of your problem statement and lets try to reproduce.

[Guest FrancoisH]

Hello Dan,As you requested, I made extensive tests and finally I post what I was able to reproduce almost each time...I was this time flying from LFBI to LFST (yes, I know, runways are short ;)), Epinal (EPL) 6V STAR, then ILS RWY 23 via Saverne (SAV).As showed on the charts, I put 6500 ft over SAV VOR and a AT OR BELOW FL140 over DME 15 from SAV. If I had not put AT OR BELOW over DME15, I should have been FL130 over this point... I expected the plane descend FL 140 then continue descent as long as the profile require it rather than making a step at FL140 (AT OR BELOW mean for me it can go trough limitation and continue descent).Here are the screens :Thank you for answering me before, I hope you will be able to explain me a bit better something I probably do not understand.Regards.

[downscc 1,327]

She should not have gone into HOLD at FL140 but decended below FL140 in PROF to 6500. This surprises me becuase I have flown conditional constraits that did work (passed below the AT OR BELOW altitude instead of AT it).Please, if you don't mind, run the same arrival using 14000 instead of FL140 and tell me if it does the same thing. No need to capture screen prints. I just want to test the bug to see if it changes behavior.Thanks Francois.

[Guest FrancoisH]

You mean test the same thing with higher transition altitude ?Because I did entered 14000 and not FL140 ;)

[downscc 1,327]

No, disregard. Thanks.

[Guest FrancoisH]

As long as I noticed this, I would ask another question.If I put a constraint on a waypoint descending to my destination, but this Waypoint is far before the real top of descent, is that normal the FMS do not manage to get levelled at this point then resume descent afterward ? Currently, the system only do normal descent to the point the offer a path at 750ft/min afterward until crossing the real profile later but do not offer a "level" at this waypoint.Is that current or is this related to the same issue ? I searched trough documentation (I am so glad I got the printed version ;)) but did not find any clue about it.Regards.

[downscc 1,327]

My (limited) understanding is the altitude constraint below cruise and before TOD will result in a descent phase of flight. In other words, you do not return to cruise after the constraint. The way to fly this is to lower your cruise altitude.

[Guest FrancoisH]

Yes, it is what I does untill now.