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GTN visual approach with KFC 150


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Hi Carusoam

When we bought GTN750, the unit came with MainSoftware V6.2. And it was already something wrong because since first day on GPS approaches we saw GS indication on HSI but he AP would not capture and fly the glide path... only the LOC. But the AP could fly ILS approaches fully coupled with LOC and GS.

After the recent GTN SoftwareUpgrade to V6.70, the problem persist, and it is also noticeable on “GTN Visual Approaches” which are GPS approaches self computed by GTN with GP output, that is clearly visible on HSI as well, but the AP cannot fly it, although it can still fly fully coupled an GS of an ILS approach. 

Edited by Costa Leite
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Ok. I had a chance to fly yesterday and confirm that the KFC150 will capture and fly the visual approach (I had never thought to even try it before).

1.  The current software for the GTN is 6.7 so this is not your issue.

2.  You can capture and fly an ILS approach so there should not be a wiring issue.  In the attached diagram, you can see that there is an additional GPS connection to some autopilots but the KFC 150 uses ILS signal line for both the GPS and ILS approaches.  There are only 7 wires.  This does not agree with what Don states from his technician and I am hoping it is confusion as to which type of autopilot and the associated differences in wiring.

3.  For an ILS approach, you set the GPS to CDI mode and select the Approach mode on the KFC 150.  For a GPS approach, you need to make sure the GTN is in GPS mode and the KFC 150 is in approach mode.

If this still does not work, I would assume it is a setup in the GTN software.

Good luck with your troubleshooting.

628D86B5-AA5D-43BD-BC1B-172D4B33E50B.png

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On 7/3/2020 at 7:33 PM, moosebreath said:

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.

Likewise, the Kings use closure rate to the lateral final approach course to make the lateral capture. You may not get capture if you are already perfectly on the lateral path when you press APCH.  It expects things to occur the way they would if you were flying an analog ILS. You would dial in the final approach course, press APCH, the annunciator would indicate "ARMED", and you would use the heading bug to fly into the approach course where it would capture, as if you were vectored in or were flying the inbound procedure turn. 

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  • 4 weeks later...

Dear all,

I had a chat with Garmin Avionics Europe, and here is the reply:

If the autopilot will fly an ILS without any problem, but will not fly the glideslope of any GPS approach, then this may indicate a wiring problem. There is an "ILS Energise" line from the GTN to the autopilot, which is meant to confirm that your Nav Radio is tuned to an ILS, which is what allows the autopilot to go into approach mode and fly the vertical element of an approach. The autopilot does not know about GPS approaches, so this is the only way it can allow approach mode to work. The solution to this is to connect the ILS/GPS energise pin on the GTN (Connector P1001, Pin 56) to the ILS energise pin on the autopilot. (please consult the manual of the unit/your installer for the exact pin) At the moment the connection is probably from the GTN ILS only pin (P1004, Pin 29) Once the ILS/GPS connection is made, this will trick the autopilot into thinking you are tuned to an ILS frequency when actually you are flying a GPS approach. That will allow the autopilot to go properly into approach mode and fly the vertical element of that GPS approach. Please let me know if this resolves the problem.

So, it seems that Donkaye and Schinderhannes advices are correct, and I will have a look into this when possible.

Many thanks to all for your help. I will post my feedback when I get this issue sorted out. 

 

 

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  • 7 months later...

Dear Mooniacs

After a long time, we finally switched the pins from AP to GTN to the correct position, and now the problem is solved!

Due to Pandemic we couldn't have done this service before, but now our Allegro is performing RNP LPV app fully automatically with the KFC150 AP.

Thanks to all that shared their knowledge!

Cheers,

Fredi

P.S: attached is a video of the aircraft performing a GTN Visual Approach to runway16, during a crosswing day. Sorry for the video quality, but it is only to show the AP going from HDG to APP mode, and them capturing GS fully automatically into a runway without any IFR procedures :-)

 

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Nice work Fredi!

The GS shows up on the HSI... as you meet it from below.

The FD bars lead the way...

GS slope capture is a mild light bulb coming on...

The old BK displays are working pretty well.

The X-wind is mildly strong...

Pilot still gets to operate the throttle...   :)

Thanks for sharing all the details...

Its a nice busy minute and a half.

PP observations while following Fredi’s scan...

Go MS!

Best regards,

-a-

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