FMC question

[reflic]

I checked out the tutorials, but i didn't see (or i couldn't find) this particular question. Why is it when my STAR has a waypoint that say is 14,000A. I know this is the symbol for Above, but if i execute this procedure, on any VNAV decent, the aircraft will never meet the restriction, one time it passed the way point 10,000 ft above the way point, and the VNAV path sky rocketed down to like 6,000 ft/min. If i do a 14,000B everything works as it should. 

[mmerelles]

31 minutes ago, reflic said:

I checked out the tutorials, but i didn't see (or i couldn't find) this particular question. Why is it when my STAR has a waypoint that say is 14,000A. I know this is the symbol for Above, but if i execute this procedure, on any VNAV decent, the aircraft will never meet the restriction, one time it passed the way point 10,000 ft above the way point, and the VNAV path sky rocketed down to like 6,000 ft/min. If i do a 14,000B everything works as it should. 

mmm not sure i am understanding what you are asking for. If the waypoint says 14000A the aircraft can cross that waypoint at any altitude above 14000. At which altitude? the altitude that the FMC computes is best to pass above 14000 for that waypoint and comply next waypoints ahead.

Maybe you should provide an entire route explame to understand better your point. All the route including runways, sid, route, star, arrival procedure, etc. So we can double check what you are asking and eventually try to replicate if there is a potential bug

[reflic]

1 hour ago, mmerelles said:

mmm not sure i am understanding what you are asking for. If the waypoint says 14000A the aircraft can cross that waypoint at any altitude above 14000. At which altitude? the altitude that the FMC computes is best to pass above 14000 for that waypoint and comply next waypoints ahead.

Maybe you should provide an entire route explame to understand better your point. All the route including runways, sid, route, star, arrival procedure, etc. So we can double check what you are asking and eventually try to replicate if there is a potential bug

Well maybe I should ask first, what does the aircraft want to do if a way point has a restriction that is a Above symbol. For example let's say we're decending from 31,000 feet and I Donno waypoint A has a restriction of 16,000A, and the next way point is a 14,000 restriction 15 miles away how would the plane theoretically handle that? I'm not really sure how the terminology is for A and B. Except I know it's above and below 

Edited April 27, 2016 by reflic

[mmerelles]

First of all, the aircraft is more efficient, faster and consumes less fuel at higher altitudes. So, for descends, the FMC will try to keep the aircraft at higher altitude as much as possible computing a valid descend profile that does not violate any procedure constrain, neither aircraft/commercial descend rates.

XXXXXA means the aircraft must cross that waypoint at that altitude or above. It means, you are not authorized to descend more than XXXXX. But the aircraft can cross much more higher actually if required.

XXXXXB means  the aircraft must cross that waypoint at that altitude or below. It means by the time the aircraft arrives to that waypoint it must be already at that altitude or even below if required.

XXXXX (in bold) is a hard specific restriction, the aircraft must cross that waypoint at that specific altitude, not lower, not higher.

 

That being said, if you are 31000 and next WPT is 16000A the FMC knows it can not cross  that waypoint below 16000 but any altitude above. By the rule of higher if possible is better, the FMC will cross higher than 16000. How much higher? depending on waypoint restrictions ahead. The farther away next restrictions are, the higher the FMC will cross because it has no need to descend that much yet.

 

By contrary if you are 31000 and next waypoint-1 says 16000A and next waypoint-2 is pretty close to waypoint-1 having a hard strict 15000 restriction, then the FMC will cross waypoint-1 quite close to 16000 because it has to continue descending to 15000 in a short distance to meet waypoint-2

 

Hope you have the idea.

 

note: please note during descend  the aircraft may be flying undesirable higher than the vnav profile due to lack of DRAG. Watch your FMC messages and watch your VNAV descend profile on the ND. The FMC will let you know how you are regarding the target profile.

[reflic]

Wow thanks for that awesome explanation!