I fly an aircraft with Proline 21 kit. This usually tunes the nav to the ILS frequency and on intercepting the final approach track switches automatically to fly the ILS. BUT on certain rare occasions it doesn't! That seems to happen if you miss bits out of the approach, and don't fly what the FMS expects.
The Proline 21 makes it very clear what it is doing, the GPS guidance is on Magenta needles and raw Nav data is green, and messages come up at the appropriate times telling you the ILS will be tuned, and then it puts a dotted needle up for the ILS on the HSI part of the display.
This is seriously integrated kit, and even so the automation has it's limitations. It is essential to keep an eye on what is going on and to fly in a methodical way that means it doesn't matter if, say, it doesn't change to green needles automatically.
Many GA installations are far less integrated and many are total one-offs. That might not be the intention but differences in installations and equipment have this result. Good operating procedures are even more necessary in these cases.
This usually tunes the nav to the ILS frequency and on intercepting the final approach track switches automatically to fly the ILS
How does it handle e.g. the case where you have loaded the ILS frequency, you are being vectored around, you get a vector towards the localiser, but you are not "cleared for the ILS" and, either intentionally by ATC or via the ATCO forgetting about you, you have to fly through the localiser? The box cannot know that you have not been cleared for the approach.
Also there must be some DME figure, beyond which it will ignore the localiser anyway, otherwise it would try to intercept at say 20D if you were getting extended vectoring etc.
There must be various rules which if presented inadvertently are going to confuse it 
As BPF says, there are loads of gotchas in GA systems. For example my autopilot will not properly intercept a GPS track unless the HSI deviation is at least 3 divisions (60%) at the moment NAV is pressed. I had several under-warranty equipment swaps until Honeywell finally admitted it works that way. There are various workarounds, according to how much you want to spook a radar controller wondering where you are going for a number of seconds 
One of them is to "achieve" a large enough HSI deviation by setting 1nm FS or 0.3nm FS sensitivity on the GPS. I fly everywhere with 1nm.
How does it handle e.g. the case where you have loaded the ILS frequency, you are being vectored around, you get a vector towards the localiser, but you are not "cleared for the ILS" and, either intentionally by ATC or via the ATCO forgetting about you, you have to fly through the localiser? The box cannot know that you have not been cleared for the approach.
Also there must be some DME figure, beyond which it will ignore the localiser anyway, otherwise it would try to intercept at say 20D if you were getting extended vectoring etc.>
If you are being vectored by definition you will be commanding the aircraft by use of HDG mode. In this mode the aircraft will fly right through the localizer. As soon as you are cleared to join the localizer, or for the approach, one selects either NAV or APPR, as appropriate.
Falcon has it, we would fly in HDG mode until cleared for the approach. If you have programmed the FMS to intercept the localiser at 50 miles it will do it. But the change to green needles will not happen if there are further waypoints which are not part of the actual approach.
However, the normal plan you would load would be off an arrival/transition/approach all from the database of the FMS. The system expects that, and the start of the actual published approach is one of the rules it uses.There is an option to put vectors as the transition which covers that eventuality.
If the whole thing isn't seamless it puts a discontinuity in the flight plan which means it doesn't automatically sequence past that point unless you intervene..
I am sure that some systems are even more integrated than ours, but the point I was making is that the automation needs to be monitored very carefully, even in a system that has been made for the aircraft. I think everyone has been through a "what's it doing now?" moment.
we would fly in HDG mode until cleared for the approach
If the whole thing isn't seamless it puts a discontinuity in the flight plan which means it doesn't automatically sequence past that point unless you intervene..
A variation of this might be flying a procedural ILS approach, where you set the fix as a DCT and fly the outbound track in the GPS OBS mode (in NAV), but then you want to intercept the localiser on the way in. The "NAV/GPS" switch has to take place after the outboung leg has been flown, and after you switch to HDG. Again, I can't see any way of doing this other than manually. Or is there a GPS with an overlay of the ILS approach which will automatically switch to LOC for the inbound leg? That would be really impressive.
It does that no problem.
A typical one is an ILS-Y for Annecy LFLP, which we would load in the FMS as a Salev 5Y with an AT transition, then an ILS-Y for runway 04
It does it all by GPS and changes to the ILS at the appropriate time.
I think it's more to do with the quality of the databases and the FMS
Peter,
The reason the KFC225 and GNS430W/530W (and GTN) work the way they do where you are prompted to engage approach mode by the GNS430W/530W has to do with the addition of vertical guidance on RNAV (GPS) approaches and how it is implemented. The KFC225 does not support vertical guidance when the navigation source is GPS, it only supports it when the navigation source is an ILS. To fake the KFC225 out on a RNAV (GPS) with vertical guidance, the GNS430W/530W uses a discrete circuit to drive the KFC225 source identification (GPS or VLOC). It fakes it to VLOC, rather than GPS. At the same time, the signal "ILS or GPS approach" is substituted for the normal "ILS energize" and is active on any ILS or GPS approach. At the point in the approach where the ILS or GPS GS/GP normally becomes active, the GNS430W/530W displays a message and the pilot acknowledges it with a PROC "enable autopilot outputs" choice. Immediately after this point, the pilot is expected to engage APR mode on the KFC225. If "prompt" is selected in the configuration, the auto ILS switch function is disabled and the pilot can't enable the feature.
Why did Garmin do this? They did it at the insistence of the FAA to obtain certification for using the KFC225 with the GNS430W/530W. Here is the issue that affected the operation on RNAV (GPS) approaches. With a GPS approach, the navigation data source is GPS. If you engage the autopilot in approach mode, when the GNS430W/530W fakes out the autopilot, the autopilot detects a navigation source change to VLOC and disconnects. This can be overcome, by selecting approach mode a second time, but this was deemed unacceptable by the FAA and the scheme I described was accepted by the FAA. The consequence is that you have to manually assert the autopilot signals via the prompt response on the GNS430W/530W and that auto ILS switch is disabled. I personally don't like anything that auto switches and prefer to make and verify my own choices, so I don't find the solution onerous.
Regards,
John Collins
Thanks for a great explanation, John.
I never knew the KFC225 even knows whether the nav source is NAV or GPS...
I wonder why it need to know it? Is it to modify intercept behaviour?
The APR button only (I thought) increases the lateral sensitivity.
Peter,
The KFC225 uses the GPS CDI source signal in Nav mode to determine if it will use roll steering (GPS) or CDI steering when roll steering data is available. IOW, if GPS CDI source is false and roll steering is true, it will use CDI steering. If GPS CDI source is true and roll steering is false, then CDI steering is used. If both GPS CDI source and Roll Steering are true, then roll steering is used.
It also affects the lateral sensitivity when CDI steering is used (VOR - low sensitivity and GPS - high sensitivity). Vertical GS/GP guidance is only provided when the Nav CDI source is VLOC and the other conditions of an ILS are met.
