Garmin ILS approach - transition from GPS to VLOC

I am no avionics specialist and I am sure you get a decent reply soon enough, but a while ago I did this project where I designed the whole installation (including the wiring) and got the FAA approval for it.

It quickly became obvious that there are umpteen ways to install the more complex kit, according to how much effort you want to throw at rewiring half the aircraft and in particular how many relays you want to install and wire up.

In general, most NAV receivers output a signal when an ILS frequency is selected on the NAV radio, and this signal can be used to drive a relay to switch over the "NAV/GPS" function automatically. Whether one chooses to implement this might depend on what else will be affected.

My GTN650 does the swap automatically. As long as the approach is activated and the correct localiser frequency is selected, it switches over.

Chapter and verse from the Pilot's guide (section 6.11):

The GTN 6XX 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 6XX automatically switches within 1.2 NM left or right of the final approach course. This switch can take place anywhere from 2.0 to 15.0 NM from the FAF.

If you turn onto the localiser too close to the FAF (common when vectored at Biggin), you need to activate it manually - otherwise it just works.

According to the manual, there is also some work involved in activating a separate autopilot output for KAP/KFC - seems you get a special button for this.

Section 15.4.2 of the Pilot's guide details how to enable auto CDI switching.

Hi Steve,

there are a few conditions that have to be met. First of all, you have to enable the automatic GPS/VLOC switch in the settings of the Garmin device.

Then you have to load an approach, i.e. some ILS or LOC for the given runway. Next you have to activate the approach (e.g. "vectors to final").

Once it gets close enough (as jwoolard described), it will both perform the switch to VLOC and set the course pointer on your HSI/EFIS to the localizer course.

Works great but make sure you always double check what it is doing. A Cirrus pilot killed himself in Zurich some years ago (partially) because he was so used to the Garmin setting the course pointer that he didn't realize it didn't do it in his specific case. He concluded the system was faulty and started to perform some high bank VFR in IMC manoeuvers that eventually got him killed.

I am very suprised they specifically exclude the KFC225, which has an ARINC (digital data stream) roll steering input, and the GTN boxes contain both the NAV and the GPS receivers, so I wonder why anything needs to be switched on the autopilot?

Hi Achimha

A Cirrus pilot killed himself in Zurich some years ago (partially) because he was so used to the Garmin setting the course pointer that he didn't realize it didn't do it in his specific case.

Do you have a link to the accident report? sounds like it should be required reading for anyone using this feature.

I am very suprised they specifically exclude the KFC225, which has an ARINC (digital data stream) roll steering input, and the GTN boxes contain both the NAV and the GPS receivers, so I wonder why anything needs to be switched on the autopilot?

As far as I can tell, this switches between GPSS roll commands (on the ARINC bus) and analogue deviation outputs. The full sequence (making an educated guess here from the various manuals) is that pressing the button switches the autopilot to a wings level or hold bank mode, and enables the analogue outputs. You then need to enable APPR mode on the autopilot manually.

Here's the accident report. It is very well written (at least the German language version) and definitely worth reading.

A Cirrus pilot killed himself in Zurich some years ago

My IR instructor used to call the CDI/GPS button on the 430 "the killer switch". I hadn't realised just how prescient that was.