ILS appr and auto CDI to VLOC

[pierredavidbelanger 3]

I tried an ILS appr with the GTN750 in the DHC6-300 Twin Otter (it has a kind of custom AP).

I load the appr into the flight plan and activate the first leg. I see the GTN750 standby the right freq for the appr.

Then I let the AP flight to the first waypoint using the GPS mode. While the AP do its job, I adjust the ALT to be at the exact height to catch the GS. 

Then I arm the AP to catch the GS a little before reaching the GS waypoint and wait until I am sure I am right into the GS localizer range.

Now, I expected the GTN to automatically (automagicaly I guess ?) switch the CDI to VLOC and swap the NAV freq. But it did nothing!

I had to manually swap the freq and switch the CDI. Just at this moment, the AP catch the GS and start its descent.

While I was struggling with this, I did not see immediately , but I had a MSG in the GTN that says something like "Cannot reach a vertical waypoint", I wonder if this is linked to the "not so automatic switch" I experienced.

So I guess my questions are:

1) is the GTN is able (supposed) to switch automatically from GPS to VLOC (along with the right freq in the active NAV)?

2) And if so, does it depend on the kind of AP in the aircraft ?

Thank you!

[DavidP 1,412]

The correct ILS frequency needs to be selected in the active window, that will not change automatically.

[RXP 2,338]

1 hour ago, pierredavidbelanger said:

2) And if so, does it depend on the kind of AP in the aircraft ?

 

Hi, the GTN won't depend ont the aircraft A/P. 

There are a few options available in the AFMS settings which may alter the way the GTN 'behave' in regard to the A/P though, and this is further documented in the RXP GTN User's Manual.

[Bert Pieke 7,585]

The correct procedure for most airplanes is using NAV on the autopilot to navigate to the final approach heading, having set the heading bug to that heading ahead of time.

Then:

- Switch autopilot to HDG mode

- Switch the GTN to VLOC mode

- Activate the ILS frequency in the NAV1 radio (in the GTN)

- Verify that the glideslope indicator is active on the HSI

- Switch the autopilot back to NAV mode

That is it!

[pierredavidbelanger 3]

I guess I should have RTFM first 😉

So, as DavidP said, the freq must be changed manually. Also, the GTN can behave diferently for some A/P. Also, Bert Pieke, what you describe seems not what the GTN 725/750 Pilot's Guide (Rev R) says.

The section 6.11 ILS Approaches says:

Quote

Precision ILS approaches can be performed with the built-in VLOC (VOR/localizer/glideslope) receivers. The GPS receiver can be used for guidance prior to reaching the final approach fix, but once there, the proper frequency must be selected on the VLOC window (right side of screen) and the CDI output set to “VLOC.”

The GTN 7XX can be set to automatically switch the external CDI output from GPS to VLOC as you intercept the final approach course (if used with a KAP140/KFC225 autopilot, you will need to manually enable outputs. [...]). When the ILS approach is activated and the correct ILS frequency is active in the VLOC window, the GTN 7XX automatically switches to VLOC [...]

The automatic switch from GPS to VLOC is not immediate, but instead occurs gradually to prevent abrupt CDI changes when coupled to an autopilot. [...]

Those 3 paragraphs answer all my questions 🙂 !

Also, the section 16.4.1 CDI Scale Selection explains where one can configure this automatic behavior.

This manual is fantastically written, I will try to read it first prior to ask questions like this in the future 🙂

[Bert Pieke 7,585]

4 hours ago, pierredavidbelanger said:

 Also, Bert Pieke, what you describe seems not what the GTN 725/750 Pilot's Guide (Rev R) says.

The section 6.11 ILS Approaches says:

Those 3 paragraphs answer all my questions 🙂 !

Also, the section 16.4.1 CDI Scale Selection explains where one can configure this automatic behavior.

This manual is fantastically written, I will try to read it first prior to ask questions like this in the future 🙂

The procedure I outlined is universally used...

Yes, you CAN configure an automatic GPS/VLOC switch on the GTN, but this is not true for all airplanes..

Sometimes manual is safer..

[Moonpilot 1]

Vloc... connected to auto pilot... auto pilot tracks with 2 scale deflection, is there a way to improve data ref on X Plane 11 for auto pilot to track more acurately?    Currently using Realty XP GNS 530 and stock Cessna 172 on X Plane 11.40...  GNS tracks correctly using GPS until final approach fix when switching on VLOC, auto pilot tracks with 2 scale deflection on the ILS.... can hand fly and center needle. 

[RXP 2,338]

2 minutes ago, Moonpilot said:

auto pilot tracks with 2 scale deflection

Hi, can you please clarify what do you mean with this?

Edited July 23, 2019 by RXP

[Moonpilot 1]

I would like to see when connected to the auto pilot using Xplane 11.40 and the stock Cessna 172 XP with Realty XP GNS 530 plug in, after crossing the IAF, and switching to the VLOC mode with the correct identified Nav frequency tuned in for the ILS approach, the plane does not center when connected to the GNS 530 using the VLOC CID key, and tracks two dots deflection, I would like to see the auto pilot connected to GNS 530 center the needle, is there a setting to center this needle when tracking.... I was in correct when I mentioned Data Ref... should have asked if there is a correction... I have the latest upgrade .   

[RXP 2,338]

I still don't get it, I'm sorry.

Do you mean when you change CDI mode from GPS to VLOC, the needle which was in the center in GPS mode, all of a sudden, shifts to 2 dots deflection when receiving the ILS signal? Which in turn will make the A/P coupled to the ILS to bank the aircraft to reach the needle and fly it center again?

Or do you mean the aircraft A/P continues to fly the ILS as expected, that is no sudden turn to reach a deflected needle, but all along, the needle stays deflected to 2 dots, as if you're flying a parallel track?

[Moonpilot 1]

Yes per your first paragraph...........  In the GPS mode, the GNS flies directly to the faf with the CDI centered on the localizer, as soon as the Vloc mode is selected the plane continues to fly the approach slightly to the right of needle, approx 2 dot deflection, the auto pilot should correct and center the needle, at least this is the way it works in physical plane.   I expected with little or no winds the needle would center while operating with the auto pilot..... if hand flying I understand it is up to the operator... thank you for your help.   

[RXP 2,338]

Is this 2 dots (nearly full deflection) or 2 ticks ?!?!?

Is this happening at every airport/approaches or one in particular?

If at many airport, can you compare one with negligible MagVar and one with lots of Magyar to cross-check if this is a factor?

Is this custom scenery or stock scenery? (there might be an ILS/RWY angle discrepancy).

 

[Moonpilot 1]

When I fly again I will check out the actual defection... I have only been practicing at my home airport..... I have flown many non precision approaches at other airports.... I will try other airports and see if I still have the same deflection.... and will let you know...... I just got my Realty XP GNS 530 hooked up in my  Sim running two computers, one for the visual and one for the instruments, with Air Manager pusing to Air Player... everything is finally working including bezel, auto pilot, and the instruments..... I did by the way also fly the approach on the stock Xplane 11.40 Cessna 172 not using Air Manager with the same results...... I kind of wondered if it could be the airport.... I will try some other locations.... thank you again.. 

[DavidP 1,412]

5 hours ago, Bert Pieke said:

Sometimes manual is safer..

I agree. I find the auto changeover a bit "flukey" in the Trainer, and as such am not willing to try it in real life.

Manual changeover - you know it will work.

[Moonpilot 1]

I never in real life use the auto pilot to fly the approah after leaving the faf....... but thought it would be nice to have the feature on the sim.... I only have a two axis auto pilot, still have to fly the GS by hand, it is safer and easier to fly inbound manually... and be ready for the MAP.... I don't have the luxury of owning a Garmin to navigate by, steam gauges and basic Nav Coms with dual GS and HSI... still have a working ADF too....