GTN 650Xi Auto Switch and GFC 500 ROL reversion

[PT20J]

It's easy two see how the GTN works because Garmin built a nice simulator, but Garmin has never built sims for its autopilots and the documentation is not always clear. So, I did a bunch of tests today, and found that the logic is actually pretty simple. For all these tests, I started in NAV mode on a GPS track to an IF just prior to the FAF on an ILS or LOC approach.

Auto Switch: With GTN Auto Switch enabled, the GTN will fly via GPS to the IF, roll steer to the final approach course, and once the active waypoint changes to the FAF, switch the CDI to VLOC. It doesn't matter whether the loaded approach is a ILS or LOC -- it works the same.

GFC ROL reversion: If GFC APR is pressed (which arms LOC and GS) before auto switch, the GFC will capture the localizer after the Auto Switch without reverting to ROL. If APR is not armed, it will revert to ROL when the CDI switches. This means that Auto Switch will only work without ROL reversion on an ILS. 

Auto Switch disabled: With APR armed, the GFC will revert to ROL if the CDI is switched to VLOC far enough from the localizer course to end up with full deflection after the switch.

I leave it as an exercise for someone else to determine how far you can be offset from the localizer and have the manual CDI switch not cause a ROL reversion .

EDIT: From the documentation, I believe that it is probable that a manual CDI switch from GPS to VLOC will not cause the GFC 500 to revert to ROL if the CDI ends up being a half scale deflection or less on the localizer, but I haven't tested this. It does not revert if the CDI ends up centered.

 

[ArtVandelay]

It's easy two see how the GTN works because Garmin built a nice simulator, but Garmin has never built sims for its autopilots and the documentation is not always clear. So, I did a bunch of tests today, and found that the logic is actually pretty simple. For all these tests, I started in NAV mode on a GPS track to an IF just prior to the FAF on an ILS or LOC approach.
Auto Switch: With GTN Auto Switch enabled, the GTN will fly via GPS to the IF, roll steer to the final approach course, and once the active waypoint changes to the FAF, switch the CDI to VLOC. It doesn't matter whether the loaded approach is a ILS or LOC -- it works the same.
GFC ROL reversion: If GFC APR is pressed (which arms LOC and GS) before auto switch, the GFC will capture the localizer after the Auto Switch without reverting to ROL. If APR is not armed, it will revert to ROL when the CDI switches. This means that Auto Switch will only work without ROL reversion on an ILS. 
Auto Switch disabled: With APR armed, the GFC will revert to ROL if the CDI is switched to VLOC far enough from the localizer course to end up with full deflection after the switch.
I leave it as an exercise for someone else to determine how far you can be offset from the localizer and have the manual CDI switch not cause a ROL reversion .
EDIT: From the documentation, I believe that it is probable that a manual CDI switch from GPS to VLOC will not cause the GFC 500 to revert to ROL if the CDI ends up being a half scale deflection or less on the localizer, but I haven't tested this. It does not revert if the CDI ends up centered.
 

I guess I have auto switch enabled as mine switches automatically, HOWEVER mine was flying the complete approach like you did. What will it do if you’re being vectored; will it auto switch if in heading mode, APR is armed?

[PT20J]

12 minutes ago, ArtVandelay said:

I guess I have auto switch enabled as mine switches automatically, HOWEVER mine was flying the complete approach like you did. What will it do if you’re being vectored; will it auto switch if in heading mode, APR is armed?

Yes, that’s the way I most often do an approach. 

[Marc_B]

13 hours ago, ArtVandelay said:

What will it do if you’re being vectored; will it auto switch if in heading mode, APR is armed?

If ATC has you vectored and is telling you they are gonna have to fly you through the localizer for spacing, make sure LOC is not armed (APR). If it’s armed it will auto switch and capture the LOC turning inbound.  If you have APR armed, just press the APR button again and it will turn off.

Edit: @ArtVandelay if you're being vectored, I take it that you're in HDG mode and a CDI switch won't matter.  However if you're in GPS mode coming in a feeder route, then you need to switch to HDG mode prior to the CDI switch or GPS turn inbound...GPS + CDI switch = mode reversion.

Edited March 19, 2024 by Marc_B

[ArtVandelay]

If ATC has you vectored and is telling you they are gonna have to fly you through the localizer for spacing, make sure LOC is not armed (APR). If it’s armed it will auto switch and capture the LOC turning inbound.  If you have APR armed, just press the APR button again and it will turn off.
Edit: [mention=11849]ArtVandelay[/mention] if you're being vectored, I take it that you're in HDG mode and a CDI switch won't matter.  However if you're in GPS mode coming in a feeder route, then you need to switch to HDG mode prior to the CDI switch or GPS turn inbound...GPS + CDI switch = mode reversion.

Im assuming a classic ILS, Mooney turn to 270°, intercept the localizer, cleared for the approach. If in HDG mode, as you pass through the ILS Id expect it to switch from HDG to APR, if APR is armed.

[Marc_B]

7 hours ago, ArtVandelay said:

If in HDG mode, as you pass through the ILS Id expect it to switch from HDG to APR, if APR is armed.

I think that would depend on when you pressed APR.  When I let the AP do it's thing, I find that it switches CDI when the waypoint sequences to the leg leading to the FAF and about 1 dot outside of course; as soon as the CDI switches LOC becomes active.