GTN750 / VNAV Calculations

[smwash02]

I have a GTN750 and I recently upgraded the firmware to the latest and one of my favorite features has been removed / rewritten.

Under utilities there was a VNAV/VCALC button and I had this defaulted for 1000AGL 2NM from destination, but you could also set it to be at a certain altitude for crossing fixes and it'd give you VS required based on current location or time to start an X FPM descent, etc. I can do it in my head of course, but this is easier.

It seems with the new version none of this is configurable anymore, but rather must be a part of a flight plan.  Inconvenient, but alright. The problem is even when I added the altitudes and activate it, it still doesn't ever show the VS required or TOD. I think the issue is my VS target is greyed out. I tinkered with it for 20-30 minutes in flight but couldn't get it to work.

I loaded up the Garmin trainer and was able to get the TOD timer to work and am actually able to adjust the VS target, but it never shows the VS required.

My unit is on 6.71 and the trainer is 6.70.29, so there is some variance. Photo of my panel attached here. Anyone have any guidance on how this should work or does this new feature only work with a Garmin A/P attached?

Edited July 21, 2021 by smwash02

[kortopates]

It still works fine. But the VNAV constraint is added in your flight plan. Go to your flight plan page and add the VNAV constraint altitude to your destination - say for example you want to be 1000' AGL 1 nm before the airport - then input the needed MSL altitude in the Alt field next to the destination airport and specify 1 nm along track Before. Then activate VNAV direct to enable it - but if there are other waypoint ahead, you might want to wait till your closer or given intermediate altitude constraints on your earlier altitude.

Either way you'll get TOD annunciations and requrired VSR to make the altitude constraint ahead.

[smwash02]

4 minutes ago, kortopates said:

It still works fine. But the VNAV constraint is added in your flight plan. Go to your flight plan page and add the VNAV constraint altitude to your destination - say for example you want to be 1000' AGL 1 nm before the airport - then input the needed MSL altitude in the Alt field next to the destination airport and specify 1 nm along track Before. Then activate VNAV direct to enable it - but if there are other waypoint ahead, you might want to wait till your closer or given intermediate altitude constraints on your earlier altitude.

Thanks for the reply. I have set those and it works in the trainer but not on my plane's 750. In the photo above you can see I have an active constraint set but no TOD is shown and my target VS is greyed out -- which it always is on my 750.

[kortopates]

6 minutes ago, smwash02 said:

Thanks for the reply. I have set those and it works in the trainer but not on my plane's 750. In the photo above you can see I have an active constraint set but no TOD is shown and my target VS is greyed out -- which it always is on my 750.

Did you either click VNAV Direct on the Flight plan page or have the INFEQ sequence to the active waypoint? if not, without that nothing happens I believe.  

[smwash02]

I did, approach was loaded and active.

Attaching another one I did recently -- photo of both the FPL and VNAV screen. Was at FL120 and 42 miles out.

[kortopates]

the first photo shows it should be active, but second picture is before it became active.

there are some conditions which disables VNAV such as having any approach procedure active and OBS.


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[Jim Peace]

The box can be set up to do either VNAV or VNAV calc.  It will get switched when doing a major software update etc.

You or a Garmin dealer need to go into the meat of the settings and change it from one to the other....

 

This may give some info:

 

Edited July 23, 2021 by Jim Peace

[smwash02]

1 hour ago, Jim Peace said:

The box can be set up to do either VNAV or VNAV calc.  It will get switched when doing a major software update etc.

You or a Garmin dealer need to go into the meat of the settings and change it from one to the other....

Thanks for this video. He did all the steps I did, but his VS target as you can see in my photos is greyed out. I'll have a dealer look at it to see if there is some bit to be flipped to make that work right. I agree with them the VNAV is more powerful, mine is just not operating properly.

[Rmag]

Do you have a baro-corrected altitude source connected to the GTN 750 like a G500, G5, GI275?  VNAV will not work without it.  If you do not have a baro-corrected source you want your GTN set back to VCALC instead. Also it is possible to set a VNAV constraint that cannot be achieved for example you are at or under the constraint, or you are currently flying at a heading perpendicular to or away from the waypoint. This is likely not your problem since you were at Flight level 120 and 42 nautical miles out.

Edited July 23, 2021 by Rmag

[smwash02]

8 hours ago, Rmag said:

Do you have a baro-corrected altitude source connected to the GTN 750 like a G500, G5, GI275?  VNAV will not work without it.  If you do not have a baro-corrected source you want your GTN set back to VCALC instead. Also it is possible to set a VNAV constraint that cannot be achieved for example you are at or under the constraint, or you are currently flying at a heading perpendicular to or away from the waypoint. This is likely not your problem since you were at Flight level 120 and 42 nautical miles out.

I hadn't thought of that nor did I see that in any of the materials I'd read trying to understand it, but I think this is the missing piece. My altimeter passes the baro correction to the KFC150, but it has no way to pass that on to the Garmin devices. I am waiting for my appointment to get my 275s installed, but it hasn't come up yet. I'll flip it back to VCALC until the time comes.

[kortopates]

Ah yes, you won't be able to take advantage of many of the features, such as flying virtually every departure procedure that has you fly on heading straight out till 400' AGL and then see it automtically sequence to the next leg - sensing you crossed 400 AGL. Without air data you have to manually sequence the flight plan departure leg.  My air data come from my G500, but you'll also get it from the GI-275 and I imagine G5  would also provide it too (but not sure).

[FlyinAggie14]

I've run into this same issue with a GTN650xi and G3X on the way back home from Oshkosh.  I was north 40nm of KIND enroute to I69 at cruising altitude of 9000.  I manually input fixes with altitude constraints as I was approaching Cincinnati that would step me down and keep me below the CVG bravo airspace.  As soon as I added constraints to the flight plan in the 650xi my info block with time till TOD worked and TOD showed on the map but I could tell that it had a steep decent selected due to placement of TOD.  I went into my utilities page and under VNAV the VS target was greyed out as shown in your picture.  I thought that was weird so I kept checking with the same issue until I got closer to the TOD.  At some point the VS target became selectable and I could adjust it from a -1300fpm to -500fpm setting.  After that it re-calculated and set everything up perfectly.  Unfortunately I do not remember how this related to distance/time to original TOD or to the setting of my altitude bug on the G3X other than my decent started about a minute after adjusting the VS target. 

Two theories I'm going to test:
1. Is it tied to distance or time to calculated TOD at the default feet per minute configured?
2. Is it tied to the altitude preselect bug in the G3X.  Maybe VNAV target and required fields do not work if the G3X altitude select bug is at current or higher altitude and the bug must be lowered to calculate?