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Certain SIDs/STARs Breaking VNAV

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Hello,

I've been experiencing a strange issue with certain SIDs/STARs that I've never seen before. When flying KSMF-KLAX in the 737-700, my route was FTHIL2 FRA REBRG IRNMN1, departing KSMF RWY 16L and flying the RNAVZ 24R approach at KLAX. However, after activating the route and the entering performance values, VNAV data is not computed. Waypoints on the legs page look like "---/------" where a computed speed/altitude value should be. If I try and take off, I get a "VNAV disconnect" error. However, when I switch STARs and load the RDHOT.DIRBY1 arrival, VNAV data is computed as expected. This has happened on a few other SID/STAR combinations but I can't recall any at the moment.

I have created a small video that demonstrates the issue. Initially, I enter my desired route and performance data. VNAV data is not computed, so I switch to the DIRBY1 SID to test. VNAV data is then computed, so I attempt to switch back to the IRMN1 which still does not work (sorry for the quality, it was a quick upload): 

 

 

I am using the latest Navigraph cycle (1711), Prepar3D v4.1, NGX v1.20.8465, Windows 10.

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I also noticed that the VNAV data works fine when I load the REBRG.IRMN1 arrival in the 737-800, however, I cannot load the NURAY.SUUTR2 STAR at KSMF without breaking VNAV in the -800.

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This bug also shows up at EGLL, and the problem there isn't the arrival but the approach transition.  You didn't mention if it works okay without selecting the approach or approach transition, landing North or South?  I looked at the sidstar file and I don't see anything in the SUUTR that might be the problem.

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Hmm, just tried the 737-700 WL with the identical route 

FTHIL2 FRA REBRG IRNMN1

KSMF 16L

KLAX 24R RNAVZ

Filled the discontinuity between FRA and REBRG and set the transition to DAHJR and my VNAV profile with a FL360 cruise filled in.

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5 hours ago, softreset said:

Hmm, just tried the 737-700 WL with the identical route 

FTHIL2 FRA REBRG IRNMN1

KSMF 16L

KLAX 24R RNAVZ

Filled the discontinuity between FRA and REBRG and set the transition to DAHJR and my VNAV profile with a FL360 cruise filled in.

Did VNAV work? What flight sim are you running and what OS?

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i tried it too. loaded up fine also, although it did seem to take a while to calculate. using fsx-se and win7

then i tried it with your weight values and the problem shows up. at 107 ZFW and CI 80, your calculated econ descent speed is like .800/330 knots. this is way too fast to make both the speed and altitude restrictions at the entry of the star. that ambiguity is what causes the vnav path to not calculate. either try a lower cost index or set your vnav descent to .78/280 and it should work fine!

cheers,-andy crosby

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1 hour ago, adf said:

Did VNAV work? What flight sim are you running and what OS?

Yep, worked just fine.

FSX:SE / Windows 10 Pro Creator's Update

Used the default PMDG livery

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35 minutes ago, spesimen said:

i tried it too. loaded up fine also, although it did seem to take a while to calculate. using fsx-se and win7

then i tried it with your weight values and the problem shows up. at 107 ZFW and CI 80, your calculated econ descent speed is like .800/330 knots. this is way too fast to make both the speed and altitude restrictions at the entry of the star. that ambiguity is what causes the vnav path to not calculate. either try a lower cost index or set your vnav descent to .78/280 and it should work fine!

cheers,-andy crosby

Interesting, didn't occur to me that the CI might be too aggressive. I'll try something a little more conservative. 

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when i first tried it i had ZFW 116 and CI 50 and it loaded up. 

keep in mind that with that star and settings that you used, you are basically saying you want to meet the crossing restriction of FL290 or below at 330 knots (at CROWY), and then immediately decelerate to meet the 280/FL270 only one waypoint later at MUPTT, it is only like 12nm and just not enough time for it to slow down by 50 knots and meet the altitude restriction at the same time. if a star has a speed limit like that i will usually just force my descent speed to match it as this seems to eliminate any guesswork.

good luck!

cheers,-andy crosby

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1 minute ago, spesimen said:

when i first tried it i had ZFW 116 and CI 50 and it loaded up. 

keep in mind that with that star and settings that you used, you are basically saying you want to meet the crossing restriction of FL290 or below at 330 knots (at CROWY), and then immediately decelerate to meet the 280/FL270 only one waypoint later at MUPTT, it is only like 12nm and just not enough time for it to slow down by 50 knots and meet the altitude restriction at the same time. if a star has a speed limit like that i will usually just force my descent speed to match it as this seems to eliminate any guesswork.

good luck!

cheers,-andy crosby

Alrighty, thanks for the heads up! Ran it with a CI of 60 and it worked!

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