VNAV on GFC500 Autopilot not working - suggestions?

[201Steve]

I just did some training last weekend and worked with a guy that has the same system in his airplane. (Big shout out to @mike_elliott and his guys for a great session). One thing that has still yet to be resolved is the VNAV function on my autopilot. We simply could not get the VNAV to arm on the autopilot head.

 

Quick Reference of my setup:

-Dual G5's

-GTN650 xi

-GFC500 Autopilot

 

-I have "VNAV Enabled" turned ON in the GTN box

-I have set the lowest altitude on the G5 Altitude bug

-I have entered all the parameters for the descent profile (altitude, fix, descent rate, etc)

 

Once all of this is done, all that should be left to do is arm the VNAV on the autopilot head, but the button will not arm. Can you think of anything that may be preventing this from working properly? Setting issue? Software issue? Hardware/wiring issue? I have put out the same query to the installing shop as well. Curious what feedback comes.

 

Thanks for your feedback! Have a good day

[kortopates]

Was the default descent angle or angle you set, sufficient enough to not need a TOD to commence till a minute out from when you where trying to activate it? If for example your descent angle was set to 3 degrees but in order to get down to the lower altitude required 4.5 degrees from where you were, the VNAV will not activate. If these conditions are not met, you'll not see a TOD timer on the VNAV page to indicate its going to work. An another example its not hard to set up a descent where the needed TOD is actually behind you based on the descent angle which will prevent it from activating.

Perhaps a simpler way to demonstrate this is to set up a VNAV direct descent to a waypoint or offset from airport to come down to traffic pattern altitude. Set it up well in advance and you should see it all come together with the TOD countdown, and TOD fix in space shown on the MAP etc. While the TOD timer is counting down, you'll see VNAV in white (Armed) on the G5, then turn Green (active) once the descent starts. 

Edited August 15, 2022 by kortopates

[201Steve]

Hmm. I want to say we were 50 miles out at 4,000 trying to set it up to get down to 3,000 2-miles before the IAF. I believe we set it at 3 degree. Should have been plenty of time before TOD for it to arm. I see where you're going, though, and will take it into consideration on the next fiddle faddle test flight. I thought I was doing something wrong, but my instructor (only difference is his has GI275's) could not figure it out either. Has same setup and is proficient with it.

[Ulysse]

Did you have Nav mode enabled on the gfc500? If on hdg mode, vnav will not arm

[201Steve]

Yes, but another good suggestion. 

 

[mike_elliott]

Thanks Stephen, Mooney Pros, Inc. appreciates your dedication to becoming proficient on your magnificent panel. Rob and I will continue to try to resolve why this is happening to you of course.

 

[201Steve]

28 minutes ago, mike_elliott said:

Thanks Stephen, Mooney Pros, Inc. appreciates your dedication to becoming proficient on your magnificent panel. Rob and I will continue to try to resolve why this is happening to you of course.

 

Thanks Mike, appreciate that. I have reached out to the Garmin shop about it, they have extended an invitation for them to look at it, but it's an hour from me, I work, and so I figured I would query the group for anything we may have missed.

[PT20J]

If the GTN 650Xi is set up correctly, you should see TOD and BOD markers along your course on the GTN map. You should see a VNAV vertical deviation indicator on the G5. You should get a voice annunciation “Vertical Track” one minute before the TOD point. 

There are several conditions that will prevent the AFCS from accepting VNAV mode. Also, it won’t work if the TOD has been passed.

Skip

[Jim Peace]

20 hours ago, 201Steve said:

Thanks Mike, appreciate that. I have reached out to the Garmin shop about it, they have extended an invitation for them to look at it, but it's an hour from me, I work, and so I figured I would query the group for anything we may have missed.

is your GTN in VNAV calc or VNAV?  there is a difference.  Which shop does Garmin recommend that is an hour from you? 

[201Steve]

4 hours ago, Jim Peace said:

is your GTN in VNAV calc or VNAV?  there is a difference.  Which shop does Garmin recommend that is an hour from you? 

VNAV calc is just a tool in utilities, is it not? I’m operating the VNAV through the flight plan, adding values, and using VNAV direct

[Jim Peace]

2 hours ago, 201Steve said:

VNAV calc is just a tool in utilities, is it not? I’m operating the VNAV through the flight plan, adding values, and using VNAV direct

If the box is set up as true VNAV it is my understanding that you will not see VNAV calc in tools. Mine is set up in VNAV calc and I do not get top of descent stuff or the vertical guidance enroute  except on approach. 
 

there is a YouTube video on this and the two ways the GTN can be set up. It may take digging deep into your install manual. 

[donkaye]

On 8/15/2022 at 7:41 AM, 201Steve said:

I just did some training last weekend and worked with a guy that has the same system in his airplane. (Big shout out to @mike_elliott and his guys for a great session). One thing that has still yet to be resolved is the VNAV function on my autopilot. We simply could not get the VNAV to arm on the autopilot head.

 

Quick Reference of my setup:

-Dual G5's

-GTN650 xi

-GFC500 Autopilot

 

-I have "VNAV Enabled" turned ON in the GTN box

-I have set the lowest altitude on the G5 Altitude bug

-I have entered all the parameters for the descent profile (altitude, fix, descent rate, etc)

 

Once all of this is done, all that should be left to do is arm the VNAV on the autopilot head, but the button will not arm. Can you think of anything that may be preventing this from working properly? Setting issue? Software issue? Hardware/wiring issue? I have put out the same query to the installing shop as well. Curious what feedback comes.

 

Thanks for your feedback! Have a good day

Do you have a Baro-corrected altitude source, one of the requirements for VNAV.  I've had students who wondered why their VNAV did't work and they didn't have the Baro-corrected altitude source.

[201Steve]

3 minutes ago, donkaye said:

Do you have a Baro-corrected altitude source

The g5 is hooked up the the pitot static system and reads dead on with the analog primary, but I guess the question I would have from your suggestion, how would the box know one way or the other? Perhaps a setting telling it so… but it would t make sense for it to track vertical on a precision approach but not for VNaV

[donkaye]

4 hours ago, 201Steve said:

VNAV calc is just a tool in utilities, is it not? I’m operating the VNAV through the flight plan, adding values, and using VNAV direct

No such thing as VNAV calc.  It is a separate software routine and is called VCALC.  Also, it is one or the other and is set up at the time of installation.  Your choice.  Also VNAV won't work with VNAV direct, as the descent begins immediately from the present position in that case.

[201Steve]

1 minute ago, donkaye said:

Also VNAV won't work with VNAV direct, as the descent begins immediately from the present position. in that case.

Hmm. That makes sense. Thought we tried with and without but cannot recall. Hard to recall the steps (or missteps) when you push every button in the system trying every variation 

[201Steve]

@donkaye what would you use vnav direct for, then? I assume it would just give you a vertical cdi to hand fly should you choose?

[donkaye]

11 minutes ago, 201Steve said:

@donkaye what would you use vnav direct for, then? I assume it would just give you a vertical cdi to hand fly should you choose?

I personally never use it, and you should be careful when using it because you might have a mountain in the way of your descent.  However if you want to immediately start your descent for whatever reason, you would set the altitude to which you want to descend and the GPS will construct a descent profile to arrive at that altitude from your present position.

[donkaye]

I'm actually not sure what a Baro-corrected altitude means in the context of VNAV.  I'm thinking it may require an air data computer.  I tried to Google it with no satisfactory results as related to VNAV.  Maybe someone here knows, or I think I'll check with my Avionics shop or as a last resort, Garmin.

[201Steve]

@donkaye While I've got your ear, I've got an easy one for you. As it pertains to an ILS approach and the auto-switch feature in the GTN from GPS to LOC, is there anyway for the box to tell the G5 HSI what the ILS final approach course should be? In a real world scenario, I am flying a DME arc with GPS guidance to intercept the localizer. Just before reaching intercept, the GTN box auto switches from GPS guidance to VLOC, BUT the course on the HSI pointer is set for something random, I assume perhaps the last course that you used. Is there anyway for the box to know the ILS course so that when it switches from GPS to VLOC, it also sets the course pointer on the HSI to the final approach course for the ILS? I can't find anything in the Pilots guide about it.

[donkaye]

1 hour ago, 201Steve said:

@donkaye While I've got your ear, I've got an easy one for you. As it pertains to an ILS approach and the auto-switch feature in the GTN from GPS to LOC, is there anyway for the box to tell the G5 HSI what the ILS final approach course should be? In a real world scenario, I am flying a DME arc with GPS guidance to intercept the localizer. Just before reaching intercept, the GTN box auto switches from GPS guidance to VLOC, BUT the course on the HSI pointer is set for something random, I assume perhaps the last course that you used. Is there anyway for the box to know the ILS course so that when it switches from GPS to VLOC, it also sets the course pointer on the HSI to the final approach course for the ILS? I can't find anything in the Pilots guide about it.

The CDI should auto slew to the correct course for ILS and LOC approaches.  It will not auto slew for VOR approaches.

[Jim Peace]

19 hours ago, donkaye said:

No such thing as VNAV calc.  It is a separate software routine and is called VCALC.  Also, it is one or the other and is set up at the time of installation.  Your choice.  Also VNAV won't work with VNAV direct, as the descent begins immediately from the present position in that case.

This is the answer right here correct?  He may have Vcalc set up in the box vs VNAV. 
 

 

[kortopates]

[mention=7354]donkaye[/mention] what would you use vnav direct for, then? I assume it would just give you a vertical cdi to hand fly should you choose?

It’s the ideal method for descending VFR to an airport traffic pattern. You can also specify an offset like a mile or two before and the angle or rate you want come down at.
I use it to come slowly from many miles away descending from upper teens.

An ADC would give you Baro and i believe add a Wind vector display to your G5. It’s also used to automatically sequence past heading to an altitude leg used on departures and missed approaches.

Sent from my iPhone using Tapatalk

[donkaye]

An example of how NOT to use it:  Coming from Stockton, I would not use VNAV Direct to start the descent from say the Altamont Pass to VPEMB, the visual reporting point ATC wants you to be over when going to San Jose.  You would definitely hit the East Bay Hills Mountains.  Instead, based on my preset descent angle, I set a VNAV constraint the forces the descent to start when there will be no intersection of the airplane with the East Bay Hills.

[201Steve]

6 hours ago, donkaye said:

The CDI should auto slew to the correct course for ILS and LOC approaches.  It will not auto slew for VOR approaches.

Reading this page regarding the auto-slew, I wonder if it will not auto-slew if you haven't enabled APR mode on the autopilot head previous to the switch between GPS and LOC. Something to consider. I was waiting until intercepting final, before the FAF, to arm approach mode. It would be odd though, that it would auto switch from GPS guidance to LOC guidance in NAV mode, but only provide auto-slew in APR mode.

[Ragsf15e]

33 minutes ago, 201Steve said:

Reading this page regarding the auto-slew, I wonder if it will not auto-slew if you haven't enabled APR mode on the autopilot head previous to the switch between GPS and LOC. Something to consider. I was waiting until intercepting final, before the FAF, to arm approach mode. It would be odd though, that it would auto switch from GPS guidance to LOC guidance in NAV mode, but only provide auto-slew in APR mode.

I realize there are differences, but the G1000 i fly sets the ILS course when you load the approach (unless you’re using that cdi).  So if you’re in a gps mode it’s set.  I would expect it to be the same?  What about when you activate the approach?

BL - the AP mode shouldn’t matter, the GPS should set the cdi when the approach is loaded or activated.