Without the RH knob working, the GPS is next to useless. If you swap the units, assuming the ARINC 429 is used to interface to your PFD, at a minimum you will have to reconfigure the tag that identifies the unit as LNAV 1 and 2 in the respective systems.
Yes, thank you. It does work when you go back and forth several times it will eventually jump to the next letter .. but that’s not what I need when i fly IFR.
I already wrote the seller and the shop. They can find a way to fix this, or i will give back one or both units (although it feels like the seller don’t want to do that). But anyway: I ordered the shop to check the units before I bought them and they gave me a “go ahead”. But i have pretty strong arguments to support my view, I even think they are convincing ;-)
Some pictures about the above:
Flyer59 wrote:
I then asked Avidyne. Answer: we ONLY put the GPSS key becasue the DFC90 is a retrofit for the S-TEC 55 and the pilots are “used to it”.
Err what? That would mean to me that this guy does not know what he is talking about.
The 55x does NOT have a GPSS button. You achieve GPSS by pressing NAV twice.
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” …
A year ago I asked some questions on the Avidyne forum about GPSS/NAV, and this was the answer from Avidyne:
Are you observing some behavioral differences with the autopilot in NAV vs GPSS? (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.
And this was an answer from some user called “Achima” (;-)):
__( …) … NAV and GPSS are the same thing. Historically, GPSS was an add on and implemented by putting the AP in HDG mode and having an external unit calculate headings and feed them to the AP. In the DFC90, it’s all done by the AP itself. It’s great that the DFC90 takes roll steering information (i.e. GPSS) in NAV mode and the additional GPSS button is just a convenience for pilots that expect it. ( … ) __
The following is what I have found out, for DFC90.
The NAV button selects the following mode:
ARINC429 lateral input when a GNS source is in GPS mode
CDI lateral input when a GNS source is in VLOC MODE
The GPSS button selects the following mode:
ARINC429 lateral input in all cases
So the two buttons can create very different results, which will also depend on the type of nav source. For example, as NCYankee says, a GNS non-W box will output ARINC429 all the way down to the runway, whereas a W box stops as the FAF. So using GPSS with a W box will cause loss of lateral nav when the FAF is reached.
One Q I have is this: what happens with the AP in GPSS mode, with a W GPS, when you reach the FAF? It loses the lateral navigation data! Does it just hang there, e.g. in ROL mode (wings level)?
I can see why some people say “use NAV all the time” because it avoid the above Q and saves them explaining the other stuff, especially if they don’t understand it themselves. But to say NAV and GPSS are in any way identical in functionality is wrong and probably dangerous.
Well, I could now repeat, again, what the Avidyne engineers say themselves and write on their website. But I won’t :-)
Only this much: You can fly all IFR approaches in NAV mode, and it works. And that’s actually how most Cirrus pilots (who have more experience with it than me) do it.
It’s a big difference in what airplane the DFC90 is installed. With the Avidyne Entegra EXP5000 PFD, latest version, GPSS is never necessary and never produces a different or better result. It does exactly the same.
Where are their website statements?
Peter’s summary is correct.
NAV and GPSS on the DFC90 are not identical but you can live a happy life without knowing the difference.
