So they have two buttons which both do the same thing, but they have different labels?
If an Avidyne “engineer” said that with a straight face…
Yes … they did say that. (Don’t shoot me for it :-))
Peter wrote:
So they have two buttons which both do the same thing, but they have different labels?
That was my experience. And better to stay in NAV as it meant you had auto sequencing into an ILS for example – fly outbound of a procedural ILS in GPS and then CDI autoswitches to green needles when turning inbound.
GFC700 only has HDG, NAV for lateral modes.
Right, Jason – and that’s why i asked them, initially. Because that measn you will fly the GPS track and the ILS in the same mode. Mich safer. On eof the first things my friend from Lufthansa told me: Be careful with the different modes, better stay in NAV. He knows nothing about the Cirrus and Avidyne, but he said that in Business Jets, he also flies, they had problems many times with the “auto switching”, modes, etc.
Yes … they did say that. (Don’t shoot me for it :-))
I am happy to remain staggered 
I don’t see how you could make GPS-to-LOC auto switching reliable without coding the DOC of each particular LOC transmitter, and then it will still break if ATC give you a “wrong” intercept. Is this a record for TLAs in one paragraph?
my friend from Lufthansa
If he is a currently serving ATP, and he was unable to give detail, I would hope he sticks to standard routes
he said that in Business Jets, he also flies, they had problems many times with the “auto switching”
Loads of pilots flying modern gear report that, but nobody seems to know why this happens. I think @NCYankee knows a lot about this.
Flyer59 wrote:
Then I asked an engieer who does seminars about DFC90 and Avidyne Avionics. His answer: “Forget the GPSS key, fly everything in NAV mode” of the A/P … and that’s what beeing taught on all COPA seminars now.
That’s a bit short and hides the actual way how things work in the DFC. If my GNS is set to LOC mode but a route is programmed, putting the DFC into GPSS mode will make it follow that GPS route programmed in the GNS and not use the LOC even though that is what the PFD would display at that time. I use that quite often to setup the NAV while still flying on the GPS. Of course it has danger of confusion which is probably why this instructor tells people to ignore it but the GPSS button does have different semantics and you can make use of them.
Peter wrote:
So they have two buttons which both do the same thing, but they have different labels?
They don’t do the same thing but it’s complicated
If you set up a GPS route and a GPS approach, you can fly the whole thing in NAV.
It IS a bit more complicated, but essentially you can always leave it on NAV
Of course you can do everything in NAV and just pretend the GPSS button is not there. However, you can also work out what the actual differences are and then use the power of this functionality
I always disliked that the mode of the GNS would influence all the other gear and with the GPSS button you can tell your AP “use GPSS even when the GNS is set to LOC”. Quite handy.
If you set up a GPS route and a GPS approach, you can fly the whole thing in NAV.
Indeed.
Of course you can do everything in NAV and just pretend the GPSS button is not there.
But doesn’t that rely on auto switching of the lateral navigation source, from GPS to LOC, at some “appropriate” point?
No wonder so few people understand this…
But telling people to just ignore such and such button is just a recipe for getting into trouble.
I always disliked that the mode of the GNS would influence all the other gear
Possibly, the GNS box stops outputting ARINC429 roll steering commands when you switch it from GPS to VLOC (called “NAV” in the King sphere of influence)? I would be amazed if a GNS provided ARINC429 steering with a localiser source…
I can’t see any difference between using NAV mode and GPSS mode. All the turn anticipation stuff still works. There just doesn’t seem to be any benefit to the extra button press that I’ve been able to detect.
Isn’t the reason there is a GPSS mode on the autopilot simply that it completely bypasses the PFD. So if you have a PFD failure, you can still engage GPSS mode and get lateral guidance on the AP directly from the GNS?
