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Reinhardt

VNAV & Altitude Constraints

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Goodday Gentleman,

 

Trust you are well. 

 

Maybe this is to my own stupidty ado, but I really need some guidance here. I have been seeing this problem a few times now....

 

Today I approached FAOR on the STV1C arrival for ILS03R. The arrival has a constraint of 8000' at JS017. My 737 just ignored the constraint and tagged along. The Vertical Profile on the descent showed that we are descending perfect and we have no serious deviations. 

 

This is not the first time that I am experiencing this, basically constantly.... am I missing something? It feels to me like this only started recently where it is ignoring any constraints and just descending based on the T/D point it initially established? 

 

Looking forward to your replies.

 

Best Regards,

 

Reinhardt Massyn

 

#Let me just add. The constraint did show up on the Legs, showed 8000" in magenta next to JS017 on the FMC. 

#It automatically started descent by itself when reacing T/D.

#Overhead JS017 I was like 15000" and the vertical deviation was not like 50ft according to the bar on the N/D. 

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Hi,

 

The constraint at JS017 is 8000: the underline is important, it means 8000ft minimum.

So it is perfectly normal that you may pass it at 15000ft (above 8000ft). 
The constraint in the CDU for JS017 must be 210/A8000 in magenta, not 210/8000 (speed max 210kt at JS017 as well).

 

The descent path is calculated taking into account these constraints but if it calculates that it is passing above a minimum altitude constraint, then it will just forget about it and keeps it calculated altitude at this waypoint.

 

Also, it is normal that the aircraft starts descending by herself when reaching the TOD when if you set a lower altitude on the MCP.

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Make sure that the FMA pitch indication in the PFD (third from left to right) says VNAV PTH. If you are in any other mode, even VNAV SPD (which you can be in even if you selected VNAV in the MCP), the PFD will still give you vertical profile indications but the autopilot will ignore the altitude constraints set in the FMC.

 

As Budbud says, 8000 is a minimum, did you bust the altitude before the fix ?

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As has been stated already, it's an "or above" constraint. In the LEGS page, this will be immediately obvious by an A or B, for above or below, e.g. 8000A or 8000B. It's also possible to get a block constraint, such as "between 6000 and 7000", which will be represented in the LEGS page as 6000A 7000B.

 

Something we do for profile awareness - if you have a constraint like the one you mentioned and you want to see where the FMC is actually expecting the aircraft to be, delete the constraint (without EXECuting) and the 8000A will change to a small font estimate of where it'll actually be on profile (around 15,000' it seems). Then erase the change so the constraint remains.

 

Mike Archer

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