DFC90 user manual chapter 3-4:
If the Primary Nav LSK on the PFD is not set to a GPS source (GPS1 or GPS2), then the “GPSS” autopilot mode annunciator is lit in yellow indicating you have one source driving the CDI depiction and another source driving the autopilot.
and
Nav mode is entered by pressing the “NAV” button on the autopilot control panel. The system will light up the control panel button in green and track the lateral profile provided by the navigation source that is selected in the Primary Nav line select key (LSK) on the Avidyne PFD or the CDI Navigation Source Select button along the bottom edge of the Aspen PFD. This could be GPS or it could be VLOC.
So in short: GPSS and NAV are only the same if the GNS/GTN is in GPS mode. What is different from other autopilots is that the DFC90 will use ARINC roll steering even when it is in NAV mode, other autopilots only use CDI deviation in that mode.
Yes, so I only fly NAV mode on the autopilot, even when flying a GPS Flight Plan. Everything is the same, precision, intercept … And if you are in NAV mode you do not have to switch from GPSS to NAV for the ILS … one thing less to forget
I am not familiar with these boxes in that detail (actually, from flying with others, neither are many GNS owners too) but surely if you dial an ILS frequency on the NAV receiver there is no way to get lateral guidance from anything other than the LOC receiver. GPS doesn’t come into it at all (unless you completely separately set it up in OBS mode to display the extended runway centreline, as I do, but you won’t be getting any control signals from that).
What is possible is that the GPS transmits the lateral guidance using ARINC429, even in LOC (or VOR) tracking. There is no reason why this can’t be done, and it would the GPS the opportunity to provide a better lateral guidance to the autopilot. Does this actually happen?
The GS is always analog (GPS/LPV or ILS) and there is no ARINC429 mode in GA avionics.
If you have a Flightplan in the 430 that ends with an ILS approach then the waypoints of that approach (IAF, FAF) are in the GPS flightplan too. In NAV mode the A/P can follow this GPS flight plan until you are ready to intercept the (radio signal) of the localizer. That’s when you switch the GNS to VLOC. But the Autopilot can stay in NAV mode all the time.
Is that what you mean, Peter?
But the Autopilot can stay in NAV mode all the time.
Is that what you mean, Peter?
Yes, probably 
Mine (KFC225) is flown in HDG mode and then you press APR when approaching the LOC (if cleared for the approach )
I suppose one could fly it towards the LOC in NAV mode instead of HDG, but one would have to be tracking a VOR and not GPS (because, ahem, ILS needs the “GPS/NAV” switch in “NAV” position so GPS guidance is not possible) but I have never tried that and it would be a pretty contrived procedure, no pun intended…
If/when I get a new GPS I will re-educate myself, though it is prob99 going to be 2 x IFD540 and feeding the SN3500 (via ARINC429) which then drives the AP transparently, so the AP is not aware of the navigation source. That’s what I have now, but only in GPS mode. For ILS (“NAV”), guidance is still direct from the radio to the AP.
FYI, the GNS430/530 work differently than the W or the GTN unit with respect to providing roll steering via ARINC 429 during an ILS. The WAAS units suspend transmitting the roll steering label after passing the FAF on an ILS, the legacy units continue to provide it to the MAP.
If you get such a Waypoint from an RNAV transition while flying a “vectored ILS”, how do you handle it, for example with the GNS430?
In the US, we teach not to use VTF, always load the full procedure. Controllers will say they are going to vector you to final and then at the last second, clear the aircraft to join the approach at a waypoint. Direct to a fix is not a vector and requires programming. If you load the entire procedure, all the waypoints are available for joining the approach while at the same time, activating the IF to FAF leg or the leg from the FAF to the MAP provides the same guidance as VTF. Selecting VTF may also be easily accomplished at any point in the procedure, including the approach clearance when you are given a heading to join the final.
Loading the full procedure provides the pilot the option of selecting any waypoint to join the approach, including the IF and any step down. The GNS and GTN will not offer an IF as a fix to join the approach in the load approach dialog, unless it is also an IAF. This can quickly confuse the pilot if they are not prepared. Also, with these GPS, one may not choose direct to the FAF as this results in a nasty message that the approach has been cancelled.
NCYankee…. that’s a good way to do it. I have to make a note! You can always select another point and make a direct …
(But we were talking about the RNAV waypoints, right?, which are not in the ILS min flight plan …) So you’d have to check the RNAV chart and make a direct to that point if you get. (Do i understand this? :-))
Peter: I am waiting for the IFD440 to replace my non-WAAS 430s. For a time I thought about either buying 2 WAAS 430s (and seeling the non WAAS units)… but I think the IFD440 will be great, especially since I have the Avidyne glass cockpit
I don’t know what was on your approach plate, but many ILS have fixes on them that can be navigated to by a GPS. At my airport, KUZA, we have two ILS approach procedures, one (ILS or LOC Z RWY2) that uses ground based navigation aids for joining the procedure and the other (ILS or LOC Y RWY2) that uses a terminal arrival area using RNAV only waypoints. The latter procedure requires GPS unless vectored to final. Both procedures have an IF that one can navigate to with RNAV capability.
So in short: GPSS and NAV are only the same if the GNS/GTN is in GPS mode. What is different from other autopilots is that the DFC90 will use ARINC roll steering even when it is in NAV mode, other autopilots only use CDI deviation in that mode.
The KFC225 uses ARINC 429 roll steering when in Nav mode and the CDI source is GPS. Other after market units used with GPSS converters have similar capability, although the autopilot remains in HDG mode. Some GPSS converters require a CDI source input, which can block using GPSS when the source is VLOC. I personally remove the VLOC signal so that the source is not considered. Aspen supports this mode of operation, but the G500/600 doesn’t.
Unfortunately when Garmin upgraded their GNS430 to W, they eliminated transmitting the foll steering label on passing the FAF for an ILS approach. They were not consistent as they don’t do this for a VOR approach or a LOC back-course approach. The original GNS430 and GNS480 doesn’t drop roll steering guidance under the same circumstances. I have always thought this was a mistake for Garmin to suspend the transmission inside the FAF as there are many lower priced autopilots that don’t support a GS tracking feature and could use the GPS based GPSS steering for a better result than using the CDI tracking, particularly in strong crosswind situations. When the W or GTN are coupled to an autopilot, at the FAF when the GPS suspends the roll steering label, the GPSS converter commands wings level. Some pilots have never noticed this and think erroneously they are still getting lateral guidance.
