GTN visual approach with KFC 150

[Costa Leite]

Dear all,

On our M20J Allegro we have upgraded the panel to a GTN750 in 2017, but we kept all original KFC 150 system: ADI KI-256 & HSI KI-525A & APcomputer KC-192. We don’t have any GPSS unit neither ALT pre-select KAS-297B. 

Our GTN750 still has the MainSoftware V6.2 that came with the unit but now we would like to go for the GTN SoftwareUpgrade V6.70 in order to benefit from the last functions like Visual Approach, Vnav, etc.

But I am not totally sure if the GTN will send GS information to the KFC150 from the 3°glide path of VisualApp? Does anyone have this configuration and already tried the GTN VisualApproach with KFC150? Do the Autopilot fly the glide path on APP mode?

So far, on our Allegro with the GTN V6.2, while on RNP Approaches, the KFC receives the GP information and shows it on HSI GS and on ADI FD, but the AP will only fly the Localizer and we need to command the descent with the Up/Down switch.

Can anyone share its knowledge on this? Anyway, I think we will move forward with the upgrade and post here the feedback afterwards. 

Cheers,

Fredi

(the picture below is from our panel to only show the current configuration GTN & KFC150)

Edited June 17, 2020 by Costa Leite

[amillet]

It works just like a GPS LP.  Follows the glide slope of the visual approach 

[donkaye]

It sounds like your 750 wasn't wired correctly.  You should be able to fly LPV and visual approaches on the KFC 150 just like you fly an ILS approach.  In the wiring there is a choice for LOC or LOC/GPS when it comes to capturing the glide path.  Sounds like yours was wired for LOC and not the one that recognizes both.

I waited to post this until I spoke with my avionics guys today while handling another matter associated with the new GTN 750Xi.  They saved me from having to replace the new 750Xi after ATC kept telling me I came in broken and unreadable.  Turns out the mic setting was set too low.  Went flying and confirmed all was good.

[carusoam]

@Costa Leite looks like your answe may have just arrived.... (see above)

Best regards,

-a-

[StevenL757]

13 hours ago, donkaye said:

It sounds like your 750 wasn't wired correctly.  You should be able to fly LPV and visual approaches on the KFC 150 just like you fly an ILS approach.  In the wiring there is a choice for LOC or LOC/GPS when it comes to capturing the glide path.  Sounds like yours was wired for LOC and not the one that recognizes both.

I waited to post this until I spoke with my avionics guys today while handling another matter associated with the new GTN 750Xi.  They saved me from having to replace the new 750Xi after ATC kept telling me I came in broken and unreadable.  Turns out the mic setting was set too low.  Went flying and confirmed all was good.

Do I need to raise this with my shop, or should they know this already?

(realizing your crystal ball is probably in the shop getting polished)

[donkaye]

3 minutes ago, StevenL757 said:

Do I need to raise this with my shop, or should they know this already?

(realizing your crystal ball is probably in the shop getting polished)

Anyone who's installed a 750 should know this, but it wouldn't hurt to ask them tactfully if they might have hooked it up wrong because you are not able to capture the GP with your GPS.

[StevenL757]

Just now, donkaye said:

Anyone who's installed a 750 should know this, but it wouldn't hurt to ask them tactfully if they might have hooked it up wrong because you are not able to capture the GP with your GPS.

Got it, thanks...will do.

[khedrei]

I've tried a visual approach on my gtn and it did lateral only. I didnt think it would do the virtical. 

 

Note: the runway I tried it on had no ther available approaches. 

[donkaye]

2 minutes ago, khedrei said:

I've tried a visual approach on my gtn and it did lateral only. I didnt think it would do the virtical. 

 

Note: the runway I tried it on had no ther available approaches. 

If there is an obstacle obstructing the 3° slope then no glideslope will be provided, only lateral guidance.  If you're not in mountainous territory, most airports in the US provide a GP.  Not sure about other countries.

[Costa Leite]

Many thanks Donkaye for the explanation! I will trace this up and post the follow up. On next week I schedule the GTN Software Upgrade and I will be able to check everything. 

[Rmag]

Just a few more things to consider when you are trying to fly a GPS glide path...

1.  Make sure your KFC 150 is in APR mode.

2.  Make sure you are below the glide path when your approach starts so that you intercept the GP.  If you are at or above the GP it will not capture it.

To the OP, your KI 256’s flight director should capture and fly vertical component.  I agree, something is not right with your system.

[moosebreath]

Item 2 above is incorrect.  The KFC 150 will capture from above or below the glideslope.  The manual states this, I have tested it on the bench, and demonstrated it in the airplane.  

What catches many people is that you must fly through the glideslope slightly, from above or below, for it to capture.  The electronics use the change from above to below, or the reverse, i.e. a “zero crossing” to initiate the capture.  If you can carefully fly exactly to the glideslope and never cross it the unit will not capture from above or below.  

I believe this is the source of the “only from below” myth as the most common way to capture is by flying through the glideslope slightly in level flight.  Coming from above you are likely to be sensitive to not going below the glideslope and never capture.

The best way to prove this is either believe the clear statement in the manual or demonstrate it with the autopilot on the bench.  It is easy to show in flight on a calm day too.

[Rmag]

So how do you intercept the glide slope if you are above it and on runway heading?

Every time I’ve done this the glide slope marker is indeed depicted on the VDI if that is what you are referring to, but it’s all the way at the bottom.  In this case, the plane will never start down because the glide slope is missed.

Unless your saying your just a little bit over it and it captures because your close, I’ve seen that work...  but that’s gonna be hit or miss depending on your timing.

[moosebreath]

If you are above the glide slope and on runway heading you must fly down THROUGH the glide slope to capture it for an approach.  When you are above the glide slope the marker will indeed be at the bottom of the VDI.  As soon as you descend sufficiently so that the glideslope needle crosses the center of the VDI  the autopilot will capture the glideslope.  

The same applies if you are below the glideslope initially.  However in that case as you fly level at runway heading you will eventually fly through the glideslope and capture.  If you fly level ABOVE the glideslope it will never capture as the electronics will never see a zero crossing.  I am not aware of any General Aviation autopilot that will fly down from above the glideslope and then follow it down.

[Rmag]

11 hours ago, moosebreath said:

If you fly level ABOVE the glideslope it will never capture as the electronics will never see a zero crossing.  I am not aware of any General Aviation autopilot that will fly down from above the glideslope and then follow it down.

Ok I was confused why you said my post was wrong until I went and re-read it!  I wrote,  “If you are at or above the GP it will not capture it.”  If you are “at” the glide slope it will certainly capture, so I was wrong there and honestly have no clue why I wrote that.  I was intending to relay to be below not above.  We agree if you are above, it won’t capture.

 

[Marauder]

Ok I was confused why you said my post was wrong until I went and re-read it!  I wrote,  “If you are at or above the GP it will not capture it.”  If you are “at” the glide slope it will certainly capture, so I was wrong there and honestly have no clue why I wrote that.  I was intending to relay to be below not above.  We agree if you are above, it won’t capture.
 

Dan - with these visual approaches, do they consider the 3 nm waypoint the FAF for GS intercept? Or does it even matter where the GS is intercepted as long as you are at or below the correct altitude?



Sent from my iPad using Tapatalk Pro

[Rmag]

 

41 minutes ago, Marauder said:

Dan - with these visual approaches, do they consider the 3 nm waypoint the FAF for GS intercept? Or does it even matter where the GS is intercepted as long as you are at or below the correct altitude?

 

The 3 nm is just a target Approx 900- ft at a 3° slope.  It doesn’t matter where you intercept as long as you are at or below.  In fact you can see the ball on the VDI even if you are not on runway heading because it is a calculation. For example be on a 45° intercept angle to the visual approach a couple miles away and activate the visual approach.  You can see the VDI and see the glide path start moving down VDI even if you aren’t lined up on runway heading yet.

[1964-M20E]

I received a similar letter about my ADSB OUT.  First answer the letter.   They will give you time to get it right.   First go online and download the files which indicate the issue.  second get with your avionics shop to discuss the problem and solution. 

[Costa Leite]

Many thanks to all comments on this topic.
I updated the GTN software last week and already tried the visual approach a few times, but our KFC150 only capture the CDI and not the VDI even though I see the VDI on the HSI. I am aware of the necessity to fly through the glideslope slightly, from above or below, for the AP to capture the VDI, but I couldn’t fly the approach with AP engaged on APP mode through the GS like on an ILS at any of the trials. 

As I have the same problem when trying others GPS RNP oficial approaches, I am thinking that maybe our wiring is wrong... Donkaie, can you please be more specific of this wiring problem? Were is this wiring option? At he GTN or at HSI? Any pictures or diagram to share about this? Is this a simple wiring change that I can do by my own, or does it need any configuration on the GTN to be changed?

[donkaye]

I asked my avionics tech about your issue before I gave my last response.  Your avionics guy should be able to check your wiring.  It should only be a one wire change if that is the problem.  It should be done by an avionics person.

[Costa Leite]

Ok. Thanks for the advice Donkaye. I will let this maintenance for our avionics guy. But it is real strange that the KFC150 captures the LOC&GS of an ILS but not of an GPS Approach. So I guess the missed wiring is only about GPS wire, right?

[Warren]

I re-read your problem but am still not clear on your setup or performance.  Here is my feedback for what it is worth.

***Edit—the comments below apply to a KI 209A, but you are using the KI 525A so disregard this section
1.  Software configurations.  Can you capture and fly an ILS or LPV on autopilot with your setup? If so, the wiring to the autopilot is complete and providing lateral and vertical guidance.  I would look at software configs to resolve.

2.  Wiring.  There are not separate wires to the autopilot for GPS or ILS guidance — 7 wires total:  2 wires for lateral guidance, 2 wires for GS flag, 2 wires for vertical guidance and 1 wire for ILS energize (there are two additional wires to the indicator for the lateral flag).  The same signals that drive the indicator for GS flag and GS are tied to the autopilot.  If your indicator is giving vertical guidance, your autopilot should be as well - assuming no broken or missing wires.  Easy for a tech to check as there are not many wires to look at.

 

Good luck

Edited July 15, 2020 by Warren

[Schinderhannes]

All Garmin boxes provide lateral and vertical guidance output on 2 different connectors. P1001 is the main connector and provides the signal for GPS and VOR. P1004 solely provides output from the VOR/Glideslope receiver. I would check if the the vertical guidance for the autopilot is connected to P1004 as this scenario would provide vertical input to the autopilot when flying an ILS approach, but not when flying a GPS approach.  In most installations P1004 has only one set of wires for the NAV audio. If possible I would try to take a look at the back side of the unit and check how many wires are connected to P1004. 

 

[Costa Leite]

Thanks Warren and Schinderhannes for your comments.

I am precisely looking for thins kind of information / advice. 
I confirm that the AP can fly correctly ILS approaches and follow the GS. But cannot fly the GS of GPS approaches with vertical guidance. This already happened before the recent GTN software upgrade, but now after the software upgrade it is more noticeable because the AP cannot fly the GPS Visual Approaches and it should!

It is really strange that the AP can fly ILS approaches and capture the ILS GS, but cannot fly the GS of a GPS approach even that the GS is correctly shown on HSI KI-525A...
As far as I can understand, the wiring from GTN to HSI should be OK because the HSI shows the GS on both GPS and ILS approaches. On the other hand the wiring from HSI to AP should be OK for ILS but not for GPS... or maybe it’s correct and It is the wiring from GTN to AP that is wrong..? I don’t even know if there is any wiring direct from GTN to AP...!?

Edited July 15, 2020 by Costa Leite

[carusoam]

Costa was there a mention of what software version your GTN is using?

The software was updated to add these skills along the way...

It sounds like you may be able to get a more recent version of your GTN’s software...

Best regards,

-a-