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PRED ETA-ALT function during climbs.

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Member
Registered: Apr 2013
Posts: 72
Location: YBBN
Hello captains,

A question ref the PRED ETA-ALT function in climbs on the FMC FIX INFO pages.

Example scenario:- Departing EDDF on the SOBRA3P departure to London. A requirement of accepting this SID from ATC is that you cross waypoint RUDOT (~67 miles down track) at/above FL240.

I realise that there are a number of ways to program this constraint in the FMC, but I expected that I should be able to input FL240 in the FIX INFO -> PRED ETA-ALT and I would get a display of a calculated position where I would cross that level on the climb??

Instead, after I input the value (whether sitting on the ground or airborne in the climb), the FL240 I enter against PRED ETA-ALT displays for a few seconds and is then removed (no FMC message accompanies this). If I try putting in any other altitude the same result occurs.

I have confirmed there are no route discontinuities, I have cleared any altitude constraints in the LEGS and set the ALT SEL above FL240.

Can someone clarify what I am doing incorrectly here?

Thanks and blue skies!
SCUD
Moderator
Registered: May 2009
Posts: 5140
Hello Scud,

you wrote you set the MCP to 24000. Did you also set FL240 in the FMC CRZ ALT?


|-|ardy
Member
Registered: Apr 2013
Posts: 72
Location: YBBN
Hey Hardy,

FMC CRZ ALT is set at FL310.

Actually, If I set FL240 in CRZ ALT, I get an `invalid entry' message if I then try and enter enter FL240 in the ETA-ALT. Not sure why, I thought that would have generated a similar result as the PROGRESS PAGE -> TO T/C information.

chrs
Scud
Moderator
Registered: May 2009
Posts: 5140
Well, then ... for further analysis a situ file would be helpful.


|-|
Member
Registered: Apr 2013
Posts: 72
Location: YBBN
Here goes Hardy...

If I have made a dumb mistake please break the news gently!!

cheers
Scud....

https://www.dropbox.com/s/9qn3nsfp7xr9h3o/FIX%20PRED%20ETA-ALT%20test.situ?dl=0
Moderator
Registered: May 2009
Posts: 5140
Thanks for the situ.

Not a dumb mistake :-)

You may experience problems with the calculation of the T/C and T/D when no destination runway or destination airport is entered in the LEGS. This also affects the FIX INFO system as it must distinguish the CLB phase from the DES phase.

To solve this problem, enter ...

a STAR that contains an altitude constraint ...

or an approach ...

or enter the destination airport's ICAO code (e.g. EGLL) in the last LEGS line (this will be automatically replaced when a STAR or approach is entered). To be on the safe side, you may enter an altitude constraint of 2000 feet at EGLL.

Then you'll get a proper T/D and stable FIX INFO data.


Cheers,

|-|ardy
« Last edit by Hardy Heinlin on Fri, 14 Aug 2015 06:28:42 +0000. »
Member
Registered: Apr 2013
Posts: 72
Location: YBBN
Thanks Hardy,

I can tell you I would not have worked that one out on my own.

I would have assumed the FMC `knows' the destination from the ACT RTE -> DEST entry - yet another lesson learned!

Will bear that in mind from now on.

cheers
Scus
Moderator
Registered: May 2009
Posts: 5140
Yes, it knows the destination laterally, but not vertically. It doesn't know where to start the descent until at least one altitude constraint is entered near the destination. The FMC assumes where the descent phase might be, and indicates this by dashes in the speed/altitude lines on the LEGS. Only when an altitude constraint is entered, the dashes will be replaced by predictions (in small font), i.e. the FMC will project a descend path from this altitude constraint backwards up to the intersection of the CRZ ALT where it sets the T/D.


Cheers,

|-|ardy
Member
Registered: May 2009
Posts: 149
Hi - Note that what Hardy suggests is pretty much how the real FMC works from what I've read in Bulfer's user guide. That is full of numerous notes that one should enter the destination airport code at the end of a RTE even if you don't have all the rest of the legs and oceanic clearances yet, precisely to keep the FMC from getting confused - it needs something in there to do its predictions even if they get trimmed up later with more precise information,

Cheers,
Torrence
Member
Registered: Apr 2013
Posts: 72
Location: YBBN
Thanks guys,

I never had an issue with using the PRED ETA-ALT function in the descent phase as by then I (of course) had my arrival route programmed thereby satisfying the FMC's requirements. I just couldn't work out why I could not get it to work in the departure/climb phase.

chrs
Scud.

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