I have just tried it and it absolutely won't have it.
It seems that one has to go through the full correct sequence i.e. fly an intercept to the LOC, press ARM, and allow the AP to capture the LOC and then turn onto it, and then capture the GS.
That is how one normally flies of course, but a few years ago I flew with somebody who claimed it should work, and he seemed to make it work somehow.
Does anybody know the actual required sequence for this product?
S-TEC documents how to do that in their manual. You have to press the GS button twice and it will perform an aggressive manoeuver to capture the GS. More aggressive than I expected 
Nothing in the KFC225 manual about this?
You should be able to just press APR on the KFC 225. The CDI source must be stable on the localizer with the localizer tuned first before you press APR or it will detect a source change. The GS capture logic requires the localizer to be tracking and that the Vertical CDI passes thru center. Normally this is accomplished in altitude hold mode from below, but this is not necessary. You can capture from above or below using VS or pitch, but the path must intercept the CDI center, so if from above, you need to be aggressive in rate of descent as the CDI has to go from "fly down" thru center and slightly past to notice the transition and capture.
S-TEC documents how to do that in their manual. You have to press the GS button twice and it will perform an aggressive manoeuver to capture the GS. More aggressive than I expected.
The Stec logic is different than the King autopilots. When you force the GS tracking on and have a significant deviation from being on center, it has to be aggressive to intercept the vertical course.
I don't recall seeing anything in the manual.
I think NCYankee is right in that it has to see an intercept (i.e. a "zero crossing") on the LOC and then see an intercept on the GS.
It doesn't seem to me that you can just engage it (by pressing AP and then pressing APR, if flying manually) with say 1/4 scale deviation on LOC and/or GS.
That seems like an odd algorithm to me. By definition you are "established" with a half scale deflection and diving through the center requires an unnecessarily aggressive manoeuver and an oscillation. It could just realize that it's within the established criteria and take over.
The S-TEC does it better but it's a very crude autopilot, hardly able to fly an ILS on its own when there is a bit of turbulence. One more month and I'll be done with it
Peter,
In an earlier conversation with tech support, we discussed the algorithm for intercept from above and below. The original analog KFC200 required an actual heading intercept prior to tracking the localizer or it might calculate an intercept beyond the PFAF GS intercept point. The KFC150 and KFC225 didn't have that requirement with respect to the localizer and could be engaged with a centered localizer. However, the GS in all of these units, require the GS indicator to pass thru center, either fly up to fly down (normal) or fly down to fly up (from above) in order to capture and subsequently track the GS.
Interestingly I could not get it to show "APR" by pressing APR.
It was as if the APR mode is disabled if there is a valid LOC and/or GS signal coming in, at the time APR is being pressed.
AFAIK for KFC150 and KFC225 you have to capture localizer first and GS indicator at that moment has to be full scale UP. For me (KFC150) it works (or it used to work until few days ago) like this: NAV (or HDG) engaged, final course set, NAV, APR, localizer capturrd, GS captured and aircraft follows the course and path. However, speed-wise and descent rate-wise I fly approach better than AP with much less oscilation in descent rate and much more consistantly with what's written in approach plates.
Bringing this question back to life… I can’t figure out when to press the APR button on the KFC225. I haven’t flown it for a while, with my plane being in the shop, but in the few months I was flying it, I think I only ever managed a single coupled approach. Every time apart from that, GS capture didn’t happen. I’d pressed the button too early, or too late, or something upset the AP and it cancelled, or… something.
Suppose you’re flying with AP and you have a GPS approach, so you don’t have to intercept anything. When do you press the button?
Yesterday I flew with an Avidyne DFC100 in the SR22. What a treat! To fly a coupled approach you do… nothing at all. At the appropriate point in the approach, it selects approach mode, all the appropriate annunciators light up, and it captures the GS (on LPV approaches). All you have to do is manage power.
