Visual Approach Glideslope Feature

[dresoccer4 118]

Hello there. I have some questions about the visual approach glidesope feature. I tried looking this information up but had no success.

I'm wondering why some airports have vertical guidance on visual approaches and others do not? What is the requirement for GTN 750 to provide a glideslope for a specific runway? Is there a way to look up if a runway has a visual approach glideslope ahead of time, eg. outside of the sim? It would be handy to know for route planning.

For example:
1) At KAXX, the GTN 750 does not supply a glideslope with the visual approach feature for either runway, however there is an LNAV+V approach for runway 17.
2) Just a few miles away at KSKX, the GTN 750 does supply a 3 degree glideslope for the visual approach feature on runway 22 and 31, even though there are no GPS approaches for either runway. 

What is the logic behind when then GTN 750 supplies the glideslope for a specific runway and when it does not?
Thank you!

P.S. Separate issue but I've also noticed that the GTN 750 will sometimes not provide a glideslope for a specific runway when first powering up. But after clicking away and back a couple times it will then provide it. Mysterious.

[RXP 2,338]

Hi,

This is most likely better answered and detailed in the Garmin GTN Pilot's Guide installed alongside the GTN trainer.

23 minutes ago, dresoccer4 said:

GTN 750 will sometimes not provide a glideslope for a specific runway when first powering up. But after clicking away and back a couple times it will then provide it

Can you please further explain what do you mean with "click away and back"? Clicking where exactly?

[dresoccer4 118]

16 hours ago, RXP said:

Hi,

This is most likely better answered and detailed in the Garmin GTN Pilot's Guide installed alongside the GTN trainer.

Can you please further explain what do you mean with "click away and back"? Clicking where exactly?

Thank you, I found the description in the pilot's guide. It says:

The availability of vertical guidance advisories for visual approaches is dependent on terrain and obstacle obstructions along the approach path. If no known obstructions are within the approach path, vertical guidance is provided to a maximum distance of 28 NM from the runway. If there are known obstructions further than 3 NM, but within the 28 NM maximum distance from the runway along the approach, vertical guidance is limited to the approach path portion after crossing the known obstructions. This is indicated by the shortened magenta line on the map after loading the approach. If obstructions are within 3 NM to the runway, along the approach path, advisory vertical guidance is not provided. Lateral guidance is always provided for visual approaches.

So according to this, all visual approaches should have an accompanied glide path as long as there isn't an obstacle within 3NM of the runway. Very interesting! However I've come across many runways with no obstacles that also have no glide path. Continuing my example from previous post, runway 17 at KAXX has a long straight-in approach with no obstacles according to the sectional and approach charts. However the GTN 750 does not supply a glide path, just lateral navigation. I tried to post a picture but I can't seem to find the upload feature on the forum.

Which circles back around to my original question about how the GTN 750 for Xplane decides if it creates a glide path for a specific runway 🧐

As for the other issue I mentioned, when first loading up the GTN 750 and attempting to load a visual approach, many times it will say "NO VERTICAL GUIDANCE". If you then click cancel, and try to load the visual approach again, it may now provide a glide path. Sometimes you need to do this a couple times for it to work. 

Edited May 19, 2020 by RXP

[DavidP 1,408]

The GTN does not decide - it is all coded in the database like any other instrument approach - which then is dependent on the local Air Services authority. It’s probably easier just to accept some will be available and others not.

[dresoccer4 118]

15 hours ago, DavidP said:

The GTN does not decide - it is all coded in the database like any other instrument approach - which then is dependent on the local Air Services authority. It’s probably easier just to accept some will be available and others not.

I wouldn't say it's "like any other instrument approach" because those are published elsewhere and accessible ahead of time for flight planning purposes. Where this approach method appears more arbitrary and inaccessible for flight planning unless you have the GTN in front of you. Do you see what I mean? As I pointed out  the given instructions aren't entirely accurate. Thus the attempt to understand why some runways have the feature and some do not. It's good to be able to look up and understand approaches capabilities ahead of time. As a pilot for the last 15 years I've learned the more knowledge the better in these circumstances. Sometimes the hard way 😉.

Is this database you mentioned available elsewhere to be be accessible for procedures lookup?

I'm hoping this will help others that have the same question as well.

[RXP 2,338]

In an installation manual:

"Vertical guidance is provided for those runways with VGSI information for distances up to 4.0NM from the runway [...]  If the terrain impact point is less than 4NM from the runway and there is no VGSI data available, vertical guidance is not provided for that approach"

It sounds like it is DB dependent indeed. I'd suggest you post this question to Garmin's support and let us know what they suggest?

[DavidP 1,408]

I'm an IFR instructor and own an M20M with a GTN 750/650. If you select a procedure for a given airport you will see the normal selections for RNAV etc including "Visual" for the appropriate runways if it is available.

The database I refer to is the AIRAC cycle that is updated monthly. 

Edited May 19, 2020 by DavidP

[dresoccer4 118]

1 hour ago, RXP said:

In an installation manual:

"Vertical guidance is provided for those runways with VGSI information for distances up to 4.0NM from the runway [...]  If the terrain impact point is less than 4NM from the runway and there is no VGSI data available, vertical guidance is not provided for that approach"

It sounds like it is DB dependent indeed. I'd suggest you post this question to Garmin's support and let us know what they suggest?

Sure thing