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VNAV chases the snowflake and nosedives when crossing waypoints during descent


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When descending using coupled VNAV, the autopilot sometimes starts chasing the snowflake as it moves downwards (and eventually goes off the bottom of the scale) when getting really close to the waypoint, causing excessive sink rate as the plane tries to maintain what it thinks is the correct vertical path.

 

I haven't been able to do any testing to find the exact circumstances this happens under, but I suspect it only happens with waypoints that must be crossed at an exact altitude. That would make sense from a geometrics point of view. The closer the plane gets to the waypoint, the more any vertical deviations affect the required glide path angle.

 

That is to say, when you're far away, being a bit too high doesn't matter, but when you're really close, being even a little bit too high means you need a much steeper glide to cross the waypoint at the right altitude. The autopilot tries to follow the geometrically correct glide path no matter what, causing the plane to dive as it tries to meet the restriction.

 

The normal glide path is re-established as soon as the plane crosses the waypoint and the FMS targets the next waypoint.

 

I'm using the latest MSFS Marketplace version of the plane with the newest Navigraph cycle, if that makes a difference.

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  • 3 months later...

I hate to bump this old(er) thread, but I've come across the same issue. I've done three STARs into three different airports and at least one time on every arrival, one of the waypoints makes the coupled VNAV (and snowflake) dump to the bottom, thereby nosediving the aircraft, just to instantly jump to the sky as soon as it crosses said waypoint. Eventually is smooths out, kind of like a "cone of confusion" to where I need to disconnect my AP to avoid throwing overspeed flags to my ACARS, and let it do its thing. I can get a video or screenshot(s) if needed.

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  • 2 weeks later...
On 12/1/2021 at 11:47 AM, Raptor05121 said:

I hate to bump this old(er) thread, but I've come across the same issue. I've done three STARs into three different airports and at least one time on every arrival, one of the waypoints makes the coupled VNAV (and snowflake) dump to the bottom, thereby nosediving the aircraft, just to instantly jump to the sky as soon as it crosses said waypoint. Eventually is smooths out, kind of like a "cone of confusion" to where I need to disconnect my AP to avoid throwing overspeed flags to my ACARS, and let it do its thing. I can get a video or screenshot(s) if needed.

I'm still getting the hang of this very impressive sim.  How do you perform a coupled VNAV approach?  I was under the impression that the jet had no automatic VNAV capability.   I've been simply dialing in the altitude for the next waypoint, based on what I'm seeing in the FMS and then controlling rate of descent with vertical speed mode and adjusting throttles as necessary to stay on profile.   What am I missing here?

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i read a thread on here by a real world CRJ pilot, he said, there are some rare CRJs that have factory coupled VNAV. Vast majority are like the regular CRJ, with the VNAV being just advisory. 

 

Its not hard at all to manually do the vertical profile tho. DIR button on the FMC shows you the appx VS you need to hit the next altitude constraint on your SID or STAR, and after 2-4 flights its no problem at all to manage VS, airspeed, LNAV and descend restrictions. Decent amount of work for a single pilot but not overly difficult

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On 12/11/2021 at 10:59 AM, jay jay said:

I'm still getting the hang of this very impressive sim.  How do you perform a coupled VNAV approach?  I was under the impression that the jet had no automatic VNAV capability.   I've been simply dialing in the altitude for the next waypoint, based on what I'm seeing in the FMS and then controlling rate of descent with vertical speed mode and adjusting throttles as necessary to stay on profile.   What am I missing here?

You enable the option in the EFB to have coupled VNAV ...  When you do that you also then get a VNAV button up on the FCP..

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On 12/17/2021 at 7:32 AM, KuntaKinte said:

It's an option. Our 900s do not have it.

Thank you!   I have to ask since it appears you are a RL CRJ pilot (and I apologize for taking this off-topic) - how well does the sim replicate landing a CRJ?   I'm really struggling with getting this thing on the runway without hearing "Sink Rate" and then a pretty big thump.   I'm stabilized in speed (maybe a few knots over as a safety factor) but things rapidly go downhill (no pun intended) once I disconnect the AP.   Any tips you can share?

 

John

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On 12/16/2021 at 10:36 PM, jay jay said:

Didn't think the CRJ had VNAV.   Is that a cheat or do they have this in RL?


900 NextGens and newer 700s have it, if optioned by the airline. The carrier I fly for has it and its a lifesaver. The advisory vnav on the older birds just adds a few more steps, nothing major.

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