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Confusing FMS Altitude Calculations

Posted: Sat Dec 28, 2019 2:12 pm
by freeze1058
Hi,

I'm a little confused about the FMS-3000 altitude calculation for legs where no manual constraint has been entered. Cruise level is FL270.
mv_b350_fms_01.jpg
mv_b350_fms_01.jpg (79.38 KiB) Viewed 2526 times
SULED - 9000 in line with manual constraint entered for KLF
TADUV - 9000 in line with manual constraint entered for KLF
MILGU - 3000 (beginning of the STAR)
NOLNI - ----- (part of the STAR)
KLF - Manual constraint at 9000

To my understanding (from Boeing 737-800) the altitudes should display FL270 (cruise level) and then calculate intermediate values down to 9000 at KLF.

How do I need to interpret the given values? Where do I find the actual top of descend point?

Cheers,

Stefan

Re: Confusing FMS Altitude Calculations

Posted: Sat Dec 28, 2019 2:25 pm
by KenG
Did you enter the manual constraint at KLF?

Is it also part of the STAR?

What was your flight plan?

Re: Confusing FMS Altitude Calculations

Posted: Sat Dec 28, 2019 2:59 pm
by matpietsch
Hi Ken,

this happens with every altitude constraint (user or part of a STAR). All the waypoints prior get populated with the same altitude. Strangely enough this does not happen every time. At least to me.

Re: Confusing FMS Altitude Calculations

Posted: Sat Dec 28, 2019 3:29 pm
by KenG
Is it repeatable with a certain flight plan, or totally random?

Re: Confusing FMS Altitude Calculations

Posted: Sat Dec 28, 2019 3:39 pm
by freeze1058
Hi,

my initial flight plan was:

EDDN ERET7K ERETO DCT TABAT T202 MILGU MILG3S EDDB

Didn't have time to retry same or alternate route but will report once tested.

Regards,

Stefan

Re: Confusing FMS Altitude Calculations

Posted: Sat Dec 28, 2019 3:49 pm
by matpietsch
KenG wrote:
Sat Dec 28, 2019 3:29 pm
Is it repeatable with a certain flight plan, or totally random?
Once it happens again I will post the route here.

Re: Confusing FMS Altitude Calculations

Posted: Sat Dec 28, 2019 4:24 pm
by freeze1058
I just re-entered the route posted above without additional constraints:

mv_b350_fmsr_01.jpg
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mv_b350_fmsr_02.jpg
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mv_b350_fmsr_03.jpg
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mv_b350_fmsr_04.jpg
mv_b350_fmsr_04.jpg (78.74 KiB) Viewed 2484 times

Cannot identify much of a logic behind the values:

DN103: Instructs for descend (2250) between the constraints
Main leg: 3000 in line with first approach constraint PIKOV at 3000
MILGU/NOLNI: No values as part of the STAR
KLF...: Unclear value 2487 during transition from STAR to approach.

Re: Confusing FMS Altitude Calculations

Posted: Sat Dec 28, 2019 4:32 pm
by freeze1058
BTW, the auto-calculations only appear after selecting an approach.

What is the real-world procedure? Should approach already be selected during flight plan entry at origin?

Re: Confusing FMS Altitude Calculations

Posted: Sat Dec 28, 2019 4:35 pm
by KenG
Generally, an approach is assigned by approach control, so very rarely would you select an approach on the ground. However, that is still not correct logic for the FMS.

Re: Confusing FMS Altitude Calculations

Posted: Sat Dec 28, 2019 4:54 pm
by matpietsch
KenG wrote:
Sat Dec 28, 2019 3:29 pm
Is it repeatable with a certain flight plan, or totally random?
Hi Ken,

here is my current issue (flight still running).
Route: EDDF -EGKK

SOBR1Y SOBRA Y180 BITBU Y181 DEMUL M624 BUB L608 DENUT L610 TEBRA TEBR1G

TEBR1G Altitude restriction: 14,000 at ABTUM.

This is how the FMC legs page looks like with VNAV already selected:
2019-12-28_17-49-2-898.jpg
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2019-12-28_17-49-5-832.jpg
2019-12-28_17-49-5-832.jpg (421.73 KiB) Viewed 2462 times
2019-12-28_17-49-7-733.jpg
2019-12-28_17-49-7-733.jpg (440.25 KiB) Viewed 2462 times
Is the down arrow at ABTUM actually correct? I don't know the PL21 in detail yet but I would think a down arrow means "at or below".

Re: Confusing FMS Altitude Calculations

Posted: Sat Dec 28, 2019 5:01 pm
by KenG
Arrows are the phase. Above or Below would have an A or B proceeding the altitude.

Re: Confusing FMS Altitude Calculations

Posted: Sat Dec 28, 2019 5:02 pm
by jarek
Down arrow indicates descend leg. SIDs get automatically Up arrow mark, STARs are marked with Down arrow.

Re: Confusing FMS Altitude Calculations

Posted: Sun Dec 29, 2019 3:36 am
by Heronjr
matpietsch wrote:
Sat Dec 28, 2019 2:59 pm
Hi Ken,

this happens with every altitude constraint (user or part of a STAR). All the waypoints prior get populated with the same altitude. Strangely enough this does not happen every time. At least to me.
+1 and for every flightplan.

Re: Confusing FMS Altitude Calculations

Posted: Sun Dec 29, 2019 8:27 am
by kasten.amarant
same problem ive mentioned here

http://milviz.com/forum/viewtopic.php?f=105&t=11467

I didnt see this topic here, otherwise i would have posted here

Re: Confusing FMS Altitude Calculations

Posted: Sun Dec 29, 2019 10:42 am
by matpietsch
kasten.amarant wrote:
Sun Dec 29, 2019 8:27 am
same problem ive mentioned here

http://milviz.com/forum/viewtopic.php?f=105&t=11467

I didnt see this topic here, otherwise i would have posted here
Actually they want an topic for every occurrence. So you did it right :).