GI275, GTN750 vertical guidance works on 1st landing but not subsequent landings (pattern work)

[FJC]

Visual guidance is not working consistently for me.

 

Here is the equipment:

 

GI275 AI (installed a few months ago)

GI275 HSI (installed a few months back)

GTN750 GPS (installed years ago)

KFC150 Autopilot (original equipement)

 

Here is what is going on:

I’m observing that for the first landing of the day, vertical and lateral guidance works as expected by selecting a visual approach in the typical way (today it was Visual 7L at KLVK). Last week, 25R (first landing displayed vertical guidance) However, subsequent landings aren’t displaying vertical guidance (lateral guidance is fine). The GI276 indicates visual approach on display (no change in display). I’ve even tried to select the visual approach in the air, thinking that maybe the requirement is to set it up for every landing and loading/activating to no avail. I’m flying the pattern consistently enough to where this should not be causing this issue. I’ve not tried anything beyond about 4 or 5 landings. If I happen to try 10+ landings, I have no guess what the result would be and whether the vertical guidance would display.

I contacted Garmin on this. Their suggestion was to install terrain database. I was skeptical because I observed vertical guidance w/o any terrain data installed many times before and with my King HSI previously. My GI275’s are newly installed as of October and my avionics guy left the terrain data uninstalled. To pacify Garmin, I installed a valIid terrain db to both units.  My testing today did not improve anything and confirmed my previous suspicions that the terrain db wasn’t going to improve anything.

I need to go back to Garmin but thought I’d ask here as well. I often find/receive better info than from the OEM. I realize they need to ask everyone “is it plugged in and powered up”  to start everyone from a known point but they could not explain to my satisfaction why terrain data was needed for vertical guidance.

Prior to updating to GI275’s I was able to use the vertical guidance on my King HSI with much more success than now which is another data point.

What would lead to inconsistent behavior identified above? My understanding is the the GTN750 does the calculation using airport data and either it will do it providing there are no obstacles with 3NM or it won’t.

My next thoughts were to test as follows:

1. Power up, do 1 landing
2. Pull circuit breaker during taxi back, power up and do 1 landing
3. Pull circuit breaker during taxi back, power up and do 1 landing
4. Pull circuit breaker during taxi back, power up and do 3 landings leaving the unit on

Will do LNAV approaches later on after this issue is solved

 

Assuming the condition exists after test 3, I’m not sure what to do next. This is looking very much like a software bug. I’ve seen this situation for the last 4 to 5 fly days (it probably existed for longer than that…I just was focused on debuting other items). This is going to be very difficult to reproduce and figure out how to work around it and force the vertical guidance to consistently display.

Any help or suggestions on what to try to solve this problem would be helpful. The tech with Garmin was professional, but I’m not sure he had knowledge or access to a good knowledge base. I hope this is something related to configuration or installation because it would make me VERY nervous if indeed this is some sort of software bug. There is nothing unique about my equipment or setup at least I do not think so. Thank you.

 

Freddy

 

 

 

[midlifeflyer]

I don't understand. Are you saying that you are flying to various airports in the course of a day and are getting vertical landing guidance for only the first. Or are you talking about not getting vertical guidance for a series of takeoffs and landings without ever getting any further away than the traffic pattern? Or something else?

Edited January 9, 2022 by midlifeflyer

[FJC]

This inconsistency as described was observed at KVLK in the traffic pattern (only seems to work once after system is booted). Probably need to fly around and get more data points.

[kortopates]

there is a waypoint that is 3 miles out from the threshold that i believe you need to cross to activate the visual approach.
it’s not designed for use in the pattern.

Every real instrument procedure goes through an arming sequence before it will give guidance going past final. this is to prevent it from sequencing prematurely past final; for example when you’re being vectored down wind abeam the airport. i believe the visual approaches work the same way, but honestly i have never tried to use it in the fashion you describe. I believe it’s intended to be intercepted outside of the final approach segment just like all others.

You do need to re-activate it for each circuit.

Sent from my iPhone using Tapatalk

[PeteMc]

Also a little unclear of exactly what's going on....   But if you're talking about the "Visual" button that pops up on the display, you need to have a Route in so the GTN knows you're arriving at an Airport.  Also,you may need to be X miles away from the airport for the GTN to realize it needs to show you that button.  The Default (I believe) is 10 miles, but you can change that in the Settings. 

NOTE: I don't ever use that function if I'm in the pattern, so I don't actually remember if the button shows up anytime you are within those X miles or if you're approaching from a greater distance and get to the Visual Button to show up.

But regardless it the Visual button showing up, you should be able to go to the Procedures, select Approach for the Airport and scroll down to the bottom of the list.  All of the Visual Approach options are listed there.  So if you select AND ACTIVATE one of those, you should get the full visual guidance. 

[FJC]

After the first time visual approach load/activation, repeated selection load/activation results in no vertical guidance display (lateral guidance always displays correctly). This is all happening around the same airport. The airplane visual approach icon is displayed the entire time at the lower left. If I did not have a route already input, I'd never see the visual guidance on the first try as system would not know what to do and would not produce any lateral guidance which is functioning correctly EVERY time.

I'm doing everything I know to do but still cannot obtain visual guidance time after time. Should work is the operative word!

[kortopates]

Are you going back out to where the visual approach waypoint is each time - which is about 3 miles from the threshold?


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[midlifeflyer]

1 hour ago, kortopates said:

Are you going back out to where the visual approach waypoint is each time - which is about 3 miles from the threshold?


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We appear to be asking the same question and are thinking the same thing - that the system is working correctly but  @FJC is not going to the "sweet spot" for engaging the calculated vertical path. 
 

@FJC, it almost sounds like you are expecting something like an electronic glideslope that is generated by something completely independent of the airplane and doesn't care where you are coming from. Just intercept it and you got it. That's not how this works. The visual glidepath is a calculation based on inputs from you and your airplane. It creates a top of descent waypoint some distance from the threshold. Unless you fly 737 sized traffic patterns, it's doubtful you  are getting to a location where it would engage.

[FJC]

It sounds like 3NM minimal distance from threshold is needed otherwise I wont see vertical guidance but lateral guidance is always going to be present? Correct?

 

[kortopates]

It sounds like 3NM minimal distance from threshold is needed otherwise I wont see vertical guidance but lateral guidance is always going to be present? Correct?
 

Yes!


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[RobertGary1]

I’m surprised it ever worked without the terrain database because it does affect it. One airport I’m based at is down in a bowl and the system will say it won’t work due to terrain. 

[kortopates]

I’m surprised it ever worked without the terrain database because it does affect it. One airport I’m based at is down in a bowl and the system will say it won’t work due to terrain. 

I don’t know for sure, but i believe the descent angles are pre-canned, not dynamic, and probably based on VGSI - at least that how i would do it.


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[donkaye]

4 hours ago, kortopates said:

I don’t know for sure, but i believe the descent angles are pre-canned, not dynamic, and probably based on VGSI - at least that how i would do it.


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4 hours ago, kortopates said:

I don’t know for sure, but i believe the descent angles are pre-canned, not dynamic, and probably based on VGSI - at least that how i would do it.


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The Manual for the 750 states that a requirement for the Visual Approach is a valid Terrain Database.

[FJC]

Clarification: I always have had a valid terrain DB installed on GTN750. Garmin asked that I install same onto GI275's.

[FJC]

More data:

Today I flew patterns at KLVK (all within 2NM)...the 1st one, visual approach worked flawlessly which is pretty typical of first try of the day. The 2nd did not (i repeatedly loaded and activated the 7L visual approach to no avail). I reset the GTN750 by pulling the circuit and rebooting which resulted in the 3rd approach working flawlessly (after inputing the visual approach). Why would the approach not work on the 2nd try but on the 3rd when GTN750 was rebooted?

Thoughts? Theories? I think I need to capture video of the GTN 750 and and GI275/HSI for more clues. Thank you.

 

Freddy

 

[carusoam]

Let’s see if @KLudwick knows anyone who may be aware of this inside of big G? (Quirky GTN behavior with vertical guidance and visual approaches(?))

Best regards,

-a-

[gacoon]

51 minutes ago, FJC said:

More data:

Today I flew patterns at KLVK (all within 2NM)...the 1st one, visual approach worked flawlessly which is pretty typical of first try of the day. The 2nd did not (i repeatedly loaded and activated the 7L visual approach to no avail). I reset the GTN750 by pulling the circuit and rebooting which resulted in the 3rd approach working flawlessly (after inputing the visual approach). Why would the approach not work on the 2nd try but on the 3rd when GTN750 was rebooted?

Thoughts? Theories? I think I need to capture video of the GTN 750 and and GI275/HSI for more clues. Thank you.

 

Freddy

 

After you complete the first approach and go missed try this:

1.  Initiate direct to the airport.

2. Select approach, PROC , visual, rwy

3. Go downwind and try again.

if it doesnt work you probably didnt go downwind far enough to engage system logic, keep trying until you find out how far you need to go, as suggested start with the 3 mile gate

Probably need to use this process for each approach.

[skykrawler]

Maybe your software needs updating.

[Jim Peace]

If I select the visual approach on my 650 I get vertical guidance on my G5 for all approaches I do when working the pattern. Even if I turn base inside 3 miles. I only have to select it once for as long as I work the pattern. 
 

i have seen where the vertical guidance appears a bit later than I would like but it does eventually show up. 

[carusoam]

Let’s see if Freddy is around today… @FJC

See Jim’s note above regarding his G650 and G5 guidance….

Best regards,

-a-

[FJC]

Hi @Jim Peace,

What you describe would be GREAT if it worked that way for me but it does not. There is something going on...just cannot pinpoint what it is.

I submitted further feedback to Garmin. Let's see what they come back with.

Freddy

[PT20J]

I tried the following on the GTN 650Xi simulator. Easier since I can fly the pattern at 350 kts.

1. Position airplane at KLVK.

2. Clear flight plan

3. Select PROC, KLVK, Visual 25L

4. GTN changes mode annunciation from TERM to VISUAL and enters flight plan V3NM -> RW25L

5. I flew multiple patterns at 3000’ and it stayed in VISUAL with visual track segment displayed on final. 

So, it seemed to work. The only thing I couldn’t do was land each circuit.

Skip

 

 

 

 

 

 

 

[SydV]

On 1/8/2022 at 9:33 PM, FJC said:

Visual guidance is not working consistently for me.

 

Here is the equipment:

 

GI275 AI (installed a few months ago)

GI275 HSI (installed a few months back)

GTN750 GPS (installed years ago)

KFC150 Autopilot (original equipement)

 

Here is what is going on:

I’m observing that for the first landing of the day, vertical and lateral guidance works as expected by selecting a visual approach in the typical way (today it was Visual 7L at KLVK). Last week, 25R (first landing displayed vertical guidance) However, subsequent landings aren’t displaying vertical guidance (lateral guidance is fine). The GI276 indicates visual approach on display (no change in display). I’ve even tried to select the visual approach in the air, thinking that maybe the requirement is to set it up for every landing and loading/activating to no avail. I’m flying the pattern consistently enough to where this should not be causing this issue. I’ve not tried anything beyond about 4 or 5 landings. If I happen to try 10+ landings, I have no guess what the result would be and whether the vertical guidance would display.

I contacted Garmin on this. Their suggestion was to install terrain database. I was skeptical because I observed vertical guidance w/o any terrain data installed many times before and with my King HSI previously. My GI275’s are newly installed as of October and my avionics guy left the terrain data uninstalled. To pacify Garmin, I installed a valIid terrain db to both units.  My testing today did not improve anything and confirmed my previous suspicions that the terrain db wasn’t going to improve anything.

I need to go back to Garmin but thought I’d ask here as well. I often find/receive better info than from the OEM. I realize they need to ask everyone “is it plugged in and powered up”  to start everyone from a known point but they could not explain to my satisfaction why terrain data was needed for vertical guidance.

Prior to updating to GI275’s I was able to use the vertical guidance on my King HSI with much more success than now which is another data point.

What would lead to inconsistent behavior identified above? My understanding is the the GTN750 does the calculation using airport data and either it will do it providing there are no obstacles with 3NM or it won’t.

My next thoughts were to test as follows:

1. Power up, do 1 landing
2. Pull circuit breaker during taxi back, power up and do 1 landing
3. Pull circuit breaker during taxi back, power up and do 1 landing
4. Pull circuit breaker during taxi back, power up and do 3 landings leaving the unit on

Will do LNAV approaches later on after this issue is solved

 

Assuming the condition exists after test 3, I’m not sure what to do next. This is looking very much like a software bug. I’ve seen this situation for the last 4 to 5 fly days (it probably existed for longer than that…I just was focused on debuting other items). This is going to be very difficult to reproduce and figure out how to work around it and force the vertical guidance to consistently display.

Any help or suggestions on what to try to solve this problem would be helpful. The tech with Garmin was professional, but I’m not sure he had knowledge or access to a good knowledge base. I hope this is something related to configuration or installation because it would make me VERY nervous if indeed this is some sort of software bug. There is nothing unique about my equipment or setup at least I do not think so. Thank you.

 

Freddy

 

 

 

Freddy - if you’re still having this problem, make sure on the Navigation Page, you hit the CDI button until the GPS is illuminated in the boxes below.  If it says VLOC in Green you won’t get the vertical guidance on the visual approaches

[FJC]

I've basically forgotten about it...it is one of those things I do not track much.

Just to clarify...What I cited above was ALWAYS in GPS mode....I would not expect superimposed visual guidance for anything but in GPS mode. At the time, most of what I used had GPS approaches...

 

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Airplane: 1997 Ovation, 280HP, Garmin GTN750, GI275, factory air

Aviation Reviews: https://www.toointeresting.com/wordpress/?page_id=174