I think the above is wrong. I was told – several times and from competent people – that it all comes from the CDI/PFD.
But since I’m not an engineer i cannot prove that this statement from the people who developed it is right …
Avidyne forums, all over the place, COPA
One other example: DESPITE what the manual says (see quality of manual … ) the DFC90 will fly a DMEarc in NAV mode. The manual explicitely says that this doesn’t work, but it looks like they didn’t tell the autopilot, because most Cirrus pilots know that it does work.
that it all comes from the CDI/PFD
The PFD is only a display device, plus a part- user interface for the autopilot (the DFC90 has no display).
The data to the AP ultimately comes from the GPS/NAV receiver.
It is possible the PFD is acting as an intermediate step e.g. you can have a GPS driving a Sandel 3500 EHSI (via analog or ARINC429) which then drives an autopilot via its analog “FCS” output. The AP is then shielded from what the GPS is doing, and probably won’t know whether you are flying an ILS or LPV.
the DFC90 will fly a DMEarc in NAV mode. The manual explicitely says that this doesn’t work, but it looks like they didn’t tell the autopilot, because most Cirrus pilots know that it does work.
If you have roll steering then it will fly whatever is in the GPS database.
Avidyne forums, all over the place, COPA
Sure, but this is EuroGA 
and nobody is like to spend time reading some US Avidyne forum or pay the $50-60 to join COPA unless they fly or own a Cirrus. Same with the Cessna owners’ forum which is also behind a paywall.
It appears there are people out there (instructors, Avidyne people, etc) who are making simplistic statements which – if as written here – are likely to get some pilot into big trouble. I wonder why they are doing it. Do they not understand the systems, or do they think the audience is too stupid to understand? Some of this stuff is obscure especially by the time you chuck in the additional GNS v. GTN variations but pilots do need to understand how this works if they need proper IFR functionality. The transition from enroute to an IAP is a bad time to find things out.
Sorry – but in this case – i trust Avidyne, COPA and many Cirrus specialists (like Timm Preusser) more than you.
When the GPSS ARINC 429 label is no longer being received after the FAF, the roll steering should command wings level. On a constant wind day, this is sufficient to fool many pilots into letting their autopilot take them to the runway, after all once the wind correction angle is determined, all you need to do is hold that heading. On the after market converters, they will flash the GPS indication when this happens.
Flyer59 wrote:
Then THAT’s what he said ;-) Other S-TECs have a dedicated GPSS button, like the 30+ in my Piper.
He probably said they kept the GPSS “mode” …
The other Stec autopilots don’t have GPSS, they use a separate GPSS converter wired into the heading error input of the autopilot. There are a large number of variations in how these devices are interfaced to autopilots. Some will disable the GPSS when VLOC is selected. I think this is a terrible idea, because if the pilot selects VLOC in anticipation of joining a VOR or ILS approach, the roll steering vanishes.
The only roll steering output is generated from the GPS navigation on the GNS430W. The steering is a precise bank angle value to be used by the autopilot to join and then follow the flightplan as understood by the GPS navigator. Unlike the crude method that traditional Nav/Approach modes use, which are based on CDI deflections and referenced to the course pointer, the roll steering can guide the aircraft with continuously varying bank angle to hold a given course. This makes DME arcs child’s play and the course pointer plays no role in this mode of navigation. The roll steering bank angle is adjusted by the GNS430W to maintain the course centerline on the desired track, so wind correction is almost instant, no S turning.
I would suspect that the difference in Nav verses GPSS has to do with one of them (probably Nav) requires the current selected CDI to be GPS and uses roll steering when it is available whereas the other obtains the roll steering from the last assigned GPS and continues to follow the roll steering regardless of the CDI source selection.
Flyer59 wrote:
i trust Avidyne, COPA and many Cirrus specialists
Frankly, if Avidyne’s enigneers let out such garbage as they did if they say they keep a GPSS button because the “S-Tec people are used to it” when there is NO S-Tec product which has a GPSS button, then it does not inspire confidence.
In the end, it is the manufacturer who needs to know and communicate how their equipment operates and how it is to be operated and not forum posters, no matter what their expertise may be.
When talking to S-Tec folks I did experience a very clear in depth knowledge in what they say and how they communicate. At least this was my impression.
Yeah, that’s why everybody wants an Avidyne autopilot :-)
I clarified the wording about the “key” some posts earlier.
Is that now important if the GPSS feature is a button or feature (2 × NAV). I wrote that because i never flew with the 55, i changed it before the first flight.
The S-TEC30 has a GPSS key, if you buy that feature. I have it in my PIPER (HDG/GPSS)
When the GPSS ARINC 429 label is no longer being received after the FAF, the roll steering should command wings level. On a constant wind day, this is sufficient to fool many pilots into letting their autopilot take them to the runway, after all once the wind correction angle is determined, all you need to do is hold that heading
That is the conclusion I came to also. There is nothing else the AP could possibly do (apart from activating the chute) and it’s what a King one does (selects ROL mode).
Also it does the same in HDG mode, upon loss of Heading Valid (or Gyro Valid) signal.
Interestingly the current vertical mode is maintained so you can fly say an ILS without any lateral steering… would probably use the pedals
If somebody can post a quote from COPA, or Avidyne, where somebody is saying the two buttons do the same, I’d very much like to see it.
i did that, see post above from Avidyne forum!
“You shouldn’t see any difference). GPSS is a legacy holdover for all those pilots who were used to that mode prior to the DFC90. There is no algorithmic difference between the two in the DFC90”
(from an Avidye engineer)
