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Can KFC225 be engaged *on* the ILS?

If it’s working for a LOC/GS ILS but not for any RNP, take a look at the GTN installation manual, figure E-31 for Bendix/King autopilot interconnect.

Note 7 says (paraphrased) “refer to section for the correct GPS select configuration settings which for the KFC 225 must be set to ‘prompt’”.

If you then look at table 5-14 in it discloses (by implication) the surprising fact that for the KFC 225 (and the KAP 140) this setting affects the autopilot outputs from the GTN.

There could be something else going on as well, but this setting is right there in the config screen and the manual is very clear that it must be set to “Prompt”, so worth checking IMHO.

Denham, Elstree, United Kingdom

Somewhere here should be the answer. There is a “buttonology” requirement arising out of the STC application, although that is for LPV, not for RNP (GPS approaches) generally which just use a normal NAV mode.

The only way I know of to “legally” make the KFC225 treat ILS and LPV identically is to drive the KFC225 from an SN3500, analog roll streering interface, with the SN3500 getting ARINC429 roll steering from the navigator as usual. The SN3500 STC covers this – unless your installer wants to cause trouble by reading between the lines. One day I will do this…

Shoreham EGKA, United Kingdom

So my guess is that the GTN is not installed correctly for flying a vertical path using GPS. There is an extra discrete that is used, the setup is supposed to use prompt mode, and if ILS Energize is used from the GTN, it won’t work. There is a separate signal used called “GPS or ILS approach” If installed correctly, the prompt message occurs first, then one presses PROC and Enter on the GTN, and finally one presses APR on the KFC 225. The extra discrete is used for driving the GPS select in the AP and it fakes being “not GPS” so that the KFC 225 thinks that it is flying an ILS using conventional signal source from a LOC and GS receiver.

If you don’t have prompt mode setup, the AP will kick out of APR mode when it sees GPS get changed to ILS. If wired correctly, after the AP kicks out, you can re-engage APR a second time and it might work, but that depends on it being wired correctly.

KUZA, United States

Thanks everyone. I did a flight to day and got it to work. But honestly it isn’t obvious. There are two “magic things”:

- when the GTN750 thinks it can drive a coupled approach, it pops up a little soft button saying “Enable Appr output”. You have to press this
- the KFC225 requires you to be in HDG mode when you press APR. Typically flying on GPS you’re in NAV mode – doesn’t work.

So the sequence is:

- when you are close to the last IF before the FAF, sync the heading bug to the current heading
- select HDG
- wait for the message from the GTN 750, and press the button
- NOW press APR

If you get all this right, it annunicates “GP Arm” and then as you intercept the GS, switches to “GP” and flies the glide path.


Last Edited by johnh at 28 May 18:03
LFMD, France

Excellent news. FWIW that’s the same sequence I use with a GTN650+KAP140 combo.

Denham, Elstree, United Kingdom
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