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G3X/GTN/ILS Approaches

cderk

Well Known Member
Having a strange issue and wondering if anyone else has experienced something similar...

When doing an ILS approach, the localizer locks on and then once established the autopilot disengages and the green course deviation needle disappears.

Trying to diagnose if this is a software issue or perhaps my antenna. Glideslope is still working, so I’m leaning away from it being an antenna issue. I’ve got the archer NAV antenna, and its worked fine in the past. Only thing that has changed is that the wingtip came off for paint, but I’ve taken it off and verified that everything is still connected. I’ve also removed the paint in the dimples on the end of wing to make sure that the screws are making contact with the wing for a good ground.

Strange thing is that even when I lose the course deviation needle, the GTN650 still shows ID for the LOC frequency and I can hear the Morse code.

Thanks
Charlie
 
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You might want to try flying an ILS at a different airport just to see if it behaves the same way all the time.

There was one airport that I used for ILS practice were something similar happened to me. It's been a while so I don't remember exactly. My recollection is that, I'd lose NAV lock right as I was intercepting the final approach course.

The first time it happened to me, I couldn't quite figure out what was going on. I tried the approach again and the same thing happened. Except on the 2nd try, I was watching things more closely and was able to re-engage the ILS by hitting NAV on the AP controller.

Flying an ILS at other airports worked properly. My guess was that there was something in the database that wasn't quite right for that one airport.
 
You might want to try flying an ILS at a different airport just to see if it behaves the same way all the time.
Thanks, I was thinking that too, so I’ve flown 3 different ILS approaches at two different airports. Same results on all of them
 
are you loading an ILS approach in the 650 or just tuning the ILS freq? If doing the former, I would try the latter and see what happens. There could be a setting issue related to the switch from GPS to VLOC once established.

Larry
 
are you loading an ILS approach in the 650 or just tuning the ILS freq? If doing the former, I would try the latter and see what happens. There could be a setting issue related to the switch from GPS to VLOC once established.

Larry

If flying the procedure (650) and activated, then it should automatically switch from GPS to VLOC as the GS is intercepted.

Just make sure your AP is operating on the external GPS (650) or the G3X will NOT fly the approach. I am pretty sure this is it. Loading the procedure on the 650 calls up the plate and shows on the G3X, then if you don't look at the source, it will act just as it was on the 650 but stops at the ILS.
 
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You can also get this problem if you have duplicate fixes at the LOC intercept point ie going Direct to fix ABCDE and the approach has loaded another ABCDE after that in the flight plan.
When the aircraft gets to ABCDE, the autopilot says ... ! can't go to the next point ..because I was just there .... and drops out.
you can fix that by going direct to the fix loaded by the approach ..before you get there!
 
You can also get this problem if you have duplicate fixes at the LOC intercept point ie going Direct to fix ABCDE and the approach has loaded another ABCDE after that in the flight plan.
When the aircraft gets to ABCDE, the autopilot says ... ! can't go to the next point ..because I was just there .... and drops out.
you can fix that by going direct to the fix loaded by the approach ..before you get there!

Unless the GTN is doing something wrong, I cant understand how that is possible.

It went something like this:
1. I called approach and told them I wanted the ILS/LOC 24 at KABE
2. He told me to fly direct to MUDRE at or above 3000
3. I selected AND activated the approach with the initial fix being MUDRE
4. Autopilot is in NAV mode and then I chose VNAV direct to MUDRE
5. On my way to MUDRE, he cleared me for the approach, so I hit APR on the autopilot
6. So now, I’m still in NAV mode, APR is armed, the ILS is tuned and ID’d on the GTN (i can hear the Morse code).
7. When I get to MUDRE, the GTN switches to LOC mode, the needle on the G3x changes from magenta (GPS) to green (LOC) and the autopilot turns inbound.
8. Within a few seconds, the autopilot disconnects and is now in ROLL mode and the green course deviation bar disappears.
 
You have to capture the VNAV waypoint prior to the FAF, you might try the same app but using ALT select rather than VNAV and see if it works.

From the pilot guide:

With the G3X Touch and GTN combination, the VNAV flight director mode is designed to be used with vertical guidance for a descent from cruise into an approach or on an arrival. The G3X Touch flight director will typically be in ALT mode prior to intercepting the glideslope or glidepath. This should help clarify the question of when to arm GP or GS mode during an approach. While you can arm APPR any time after you have captured your last VNAV descent leg, if you are unsure of the timing of the last VNAV descent, you can always arm GP or GS mode after you cross the waypoint prior to the FAF.
 
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You have to capture the VNAV waypoint prior to the FAF, you might try the same app but using ALT select rather than VNAV and see if it works.

From the pilot guide:

With the G3X Touch and GTN combination, the VNAV flight director mode is designed to be used with vertical guidance for a descent from cruise into an approach or on an arrival. The G3X Touch flight director will typically be in ALT mode prior to intercepting the glideslope or glidepath. This should help clarify the question of when to arm GP or GS mode during an approach. While you can arm APPR any time after you have captured your last VNAV descent leg, if you are unsure of the timing of the last VNAV descent, you can always arm GP or GS mode after you cross the waypoint prior to the FAF.

Walt, I’m not having an issue with VNAV. VNAV works great. The LOC mode is what’s not working properly. On this particular approach, i did the VNAV capture about 15 miles out, and it worked - I Crossed the final approach fix exactly where i was supposed to.
 
Walt, I’m not having an issue with VNAV. VNAV works great. The LOC mode is what’s not working properly. On this particular approach, i did the VNAV capture about 15 miles out, and it worked - I Crossed the final approach fix exactly where i was supposed to.

You missed the key words here:
you can always arm GP or GS mode after you cross the waypoint prior to the FAF.

You can't VNAV to the FAF and then expect it to capture the GP.
 
You missed the key words here:
you can always arm GP or GS mode after you cross the waypoint prior to the FAF.

You can't VNAV to the FAF and then expect it to capture the GP.

I’m not VNAV-ing to the FAF. And I’m not having issues with the Glidepath. I’m VNAV-ing to the IAF, with APR in standby. It gets to the IAF, turns inbound, SWITCHES to LOC automatically, works fine for a few seconds and then the LOC CDI needle disappears. Glidepath works fine
 
I’m not VNAV-ing to the FAF. And I’m not having issues with the Glidepath. I’m VNAV-ing to the IAF, with APR in standby. It gets to the IAF, turns inbound, SWITCHES to LOC automatically, works fine for a few seconds and then the LOC CDI needle disappears. Glidepath works fine

My bad, didn't look at the plate till now, I was thinking MUDRE was the FAF.
 
Unless the GTN is doing something wrong, I cant understand how that is possible.

It went something like this:
1. I called approach and told them I wanted the ILS/LOC 24 at KABE
2. He told me to fly direct to MUDRE at or above 3000
3. I selected AND activated the approach with the initial fix being MUDRE
4. Autopilot is in NAV mode and then I chose VNAV direct to MUDRE
5. On my way to MUDRE, he cleared me for the approach, so I hit APR on the autopilot
6. So now, I’m still in NAV mode, APR is armed, the ILS is tuned and ID’d on the GTN (i can hear the Morse code).
7. When I get to MUDRE, the GTN switches to LOC mode, the needle on the G3x changes from magenta (GPS) to green (LOC) and the autopilot turns inbound.
8. Within a few seconds, the autopilot disconnects and is now in ROLL mode and the green course deviation bar disappears.

Hello Charlie,

Your item 8. above says "the autopilot disconnects". Did you mean to say that the autopilot remained connected (servos engaged), but the flight director changed the lateral mode from LOC to ROL?

Page 353 of the current Rev. M G3X Touch Pilot Guide provides 2 reasons why the lateral mode may transition to roll hold mode when flying an ILS.

Have you looked at your flight data log? It is likely that the reason for the lateral mode change can be seen in that data.

Steve
 
Hello Charlie,

Your item 8. above says "the autopilot disconnects". Did you mean to say that the autopilot remained connected (servos engaged), but the flight director changed the lateral mode from LOC to ROL?

Page 353 of the current Rev. M G3X Touch Pilot Guide provides 2 reasons why the lateral mode may transition to roll hold mode when flying an ILS.

Have you looked at your flight data log? It is likely that the reason for the lateral mode change can be seen in that data.

Steve

Steve - yes, sorry... it does switch to roll mode. I’ve sent the logs to G3Xpert to see if they can make sense out of it
 
LOC AP Reversion

Steve - yes, sorry... it does switch to roll mode. I’ve sent the logs to G3Xpert to see if they can make sense out of it

Good Morning Charlie,

We have your logs from this flight. We will take a look at them this morning and get back with you.

Thanks,

Justin
 
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