DFC90 - GPSS vs. NAV Mode

Hello Avidyne,

beiing a proud owner of a DFC90 in my SR22 for 4 weeks now, I still sometimes feel like a passenger when the A/P does not do what I expect it to do. I must admit I find it hard to learn how the different modes are used – and the DFC90 manual is really not a big help. (Most of the what I learned about how to use the A/P I found in the S-TEC55X manual.

Don't get me wrong here: I think you have a wonderful product (and actually I was responsible for 3 of your DFC90 sales lately and one more of my friends is installing it - now that i told him it's a good investment for his SR22. So, i guess I am not really your enemy :-)

But I think Avidyne should make an investment into a good manual that covers ALL the aspects of the A/P. The baseline: I think the A/P is a very good product - but why is there no good training material ? A simulator would actually be the best thing to have. Too expensive to make? After all I plan to (one day!) fly my SR22 in IMC with your autopilot and my kids in the backseat. To me that is reason enough to get all the details from you.

My main problems circle around the use of the A/P on a typical ILS approach. My airplane is a 2006 Avidyne equipped SR22, with two non-WAAS GNS430s.

The scenario: I fly a typical Standard ILS Approach in Germany, EDMA airport in Bavaria, ILS 25. IAF is WLD VOR. Hold over the VOR. Approach is out of WLD on r-132 to DME 4, right turn to intercept FC 250....

What I have found out is that the A/P will fly directly into the Intermediate Approach in GPSS mode, without crossing the IAF first. But actually i wanted to fly into the hold for practice .... So, to CROSS the VOR into the hold I switched off GPSS and crossed the VOR in HDG mode and made the turn back to the VOR (inbound leg of hold) with HDG too. In the middle of the inbound leg i switched to GPSS and the A/P would nicely fly me out of the hold and into the intermediate approach on r-132.

This is where the "problems" start. Or at least it is not completely clear to me. I have GPSS ON and the A/P will even turn twds the Final. I have the ILS frequency tuned in on Nav 1 and the LSK 1 on the PFD says GPS 1. CDI Mode (430): GPS. The A/P will intercept the localizer but never arm the APP mode or the GS. The GPS (yes, "Auto CDI ON") did not even switch from GPS to VLOC, the Final course was not set.

Is my mistake that I mix up "GPS" (CDI key on navigator) and "GPSS (GPSS key on A/P)?

The A/P will ONLY  switch from GPS to VLOC if I press NAV at the right moment of the intercept (it seems). Only then it will also arm the GS and fly the approach. Why? There is no transition from GPSS to APP automatically? There are some hints in the manual but i dont think it's really clear - at least not to the newcomer.

Now I asked some very experienced COPA members and what they said, in general, was: "FORGET THE GPSS MODE, FLY IN NAV MODE ONLY AND EVERYTHING WILL BE SIMPLE". Beeing a journalist I tried to find out more. The pros keep telling me that i do not need GPSS at all, and that the A/P will fly "just as precisely" with NAV only, and that "GPSS is a thing of the past" which the DFC90 does not need.

Isn't the idea that GPSS will use direct input fro the navigator while NAV will fly the information the HSI displays on the screen. I understand (now) that NAV uses GPS as well when the CDI Key is in "GPS" mode and not VLOC.

All that is very new to me (because I thought the A/P would only fly a GPS flight plan precisely and with nice turns) on GPSS. Now I am beeing told that with the DFC90 the GPSS mode is un-necessary and that I should only fly using NAV - and that it will work better then. But ... why does the A/P HAVE a GPSS mode then? 

It would be great if someone from Avidyne could give me some background information and ESPECIALLY tell me what the best way is to fly a STANDARD ILS PROCEDURE with my setup. WHICH MODE DO I USE AT WHAT POINT IN THE APPROACH? Should I forget about GPSS once I get close to the airport? Should I try HDG+NAV? 

Kind regards,
Alexis






Edited by GYORC - 05 Aug 2013 at 5:16am

Hello, thanks for answering!

1. I never really compared NAV and GPSS enroute, because for me that is really two different things. As your manual states the nav comes directly from the navigator in GPSS mode, while it should come frome the PFD or the PFD's HSI in NAV mode.... I have not tried if the A/P will ANTICIPATE a turn in NAV mode. (Will it?) I HAVE used GPSS or NAV while intercepting an ILS. Result: In GPSS the A/P would intercept the Final course but never switch to VLOC and never arm or intercept the GS. In NAV mode both worked.
If GPSS and NAV are exactly the same - why this difference?

2. Standard ILS procedure means the FULL APRROACH. To the IAF (into the hold), outbound the IAF towards the final, intercepting final

Are you saying that the Autopilot has a NAV and a GPSS key and they do exactly the same???? I could hardly believe that ... Why would somebody build a unit that has two keys that do exactly the same?


Edited by GYORC - 08 Aug 2013 at 4:12pm

Hi Achima, not all of what you write convinces, some does!
I cannot believe that an Avionics Company makes an Autopilot with TWO SWITCHES that do exactly the same. None of this described in the manual, and not even a STANDARD ILS is described, only one with Vectors. Believe me, beeing a former technical (for computer comapnies like Microsoft) and a longtime aviation journalist i can recognize a good manual when i see it.

A GOOD indication for how lousy the manual is is that I talked to (a couple of) Cirrus "standardized" Instructors (and I mean the well known ones) and other "experts" and each of them said something different. Nobody could explain the whole procedure to me or the function of the  keys.

Also: a good manual is one that i UNDERSTAND and that answers my questions. It's really that simple.

YOUR explanation about how to fly the approach is the best so far, I will try that. Although I see other possibilites too, like flying to the IAF with GPSS, switching over to HDG to fly the holding entry, then GPSS to again to fly out the hold into the intermediate approach ...the NAV for the intercept.

Of course your method is much better. You are in Bavaria, right? Is your Cessna green? ;-)

I want to know from Avidyne: What is the difference between using the GPSS key and the NAV key. OF COURSE one difference is that the A/P will NOT activate the approach and GS - while NAV does! YOu have to fly in NAV to have the automatic switchover from GPS to VLOC in the GNS430.

My personal opinion: In a completely over regulated flying world in which I need an engineer, a witness and probably a notary if i want to change the landing light bulb - the manual for an IFR (!) autopilot should cover each and every aspect of it, have illustrations, tables, graphics and explaining ALL TECHNICAL BACKGROUND. We are not talking about a coffee maker - the target group of this product is one that is interested in technology. And it's a product that can save your life - or kill you. And the people who don't want to read it, will not anyway.


Edited by GYORC - 08 Aug 2013 at 1:15pm

@GYORC

I am following this link with some surprise.

My ac is equipped with 530/430 non waas, a kfc200 and an Aspen which will allow for gpss steering. Since the Aspen I no longer fly in Nav mode but allways in heading mode with gpss.

The main difference is that it will automatically change course whenyou arrive at a waypoint. In nav mode it does not do this. Having said that i wunder why it would not do that with the Aspen...

Now when arriving at an ILS I will be in hdg/gpss and the garmins in gps.. It will go to the iaf and intercept .. At that point .. The garmins will automatically change to vloc. This is the part Were I loose you. I believe the Garmin itaelf makes the switch and not that it does this by a signal from the ap?

All I need to do is monitor wether the switch was made and check or switch the ap to approach mode.

I will have to read that a couple of times, before i will (maybe) understand it. Now there SEEMS to be a difference between GPSS and NAV again while I was just trying to accept that there should be "no difference in behaviour" between GPSS and NAV, see first reply.

And you think the manual makes all this clear?

Haha... I could have done that ;-) Very cool. 12 years ago, on my IFR checkride, the examiner did not realize that I had a DIRECT to the airport in the GPS which showed me the bearing on the NDB approach. Or maybe he saw it and didn't care ...

I have to further test all this (probably spend € 1000 on fuel before I fully understand it)
My SR22 doesn't have the WAAS 430s, and all I am really interested in is flying ILS and GPS approach procedures safely.

But, again, when I intercepted the LOC with GPSS the system did NOT switch from GPSS to AV automatically, nor did it set the Final Course as it should. It only did all this in "NAV".
 

I really don't care which unit is "responsible" (i do not intend to sue my autpilot ;-)) ... I just said it DOESN'T WORK.

Before I make anymore big mouthed statements I'll check if the Approach was really activated. BUT: If I activate the approach then it will always start at the IAF, right (or fly to the IAF first, correct?) So I would have to use the "SUSP/OBS" method to fly through the holding, correct?.., Then GPSS to the Final and then everything should work automatically, yes?

GNS is set to CDI Auto, by the way ...


Edited by GYORC - 09 Aug 2013 at 6:53am

Thanks for the tipps.. I will try all that next time!

Of course I know that Avidyne did not design the 430. But somebody should tell you how that whole system REALLY works, and since the GNS430 manual existed years before the DFC90 in this case Avidyne should tell people how it really works, with every detail and every possible scenario too. O mean it is a RETROFIT for an avidyne equipped Cirrus with GNS430!

A DFC90 module for FSX or Xplane would be easy to do. Avidyne equipped aircraft for FSX already exist.You can have the Space Shuttle's Avionics on the net ...

My opinion is different: The DFC90 is a RETROFIT for exactly this avionics suite in the Cirrus, and therefore it has to be explained how it will work/react in this environment. If I don't understand how it works, then the manual is bad.

It might be that aircraft come with 12 page manuals. But I do not have to like that. And I don't. In 1940 Chuck Yeager was introduced to a fighter aircraft by an instructor: "You close the canopy. At 70 mph you take-off and raise the flaps at 100. Landing works the other way round". We could have left it at that level, right?

I don't even say that everything has to be in the manual. But maybe it would be appropriate to give the pilot access to the necessary information?

If you don't want it - don't read it! :-) I do want it, and I know why.

Ahh... and about the possible update of Garmin's software: Thats what they invented updates for manuals for. Is done all the time. 


Edited by GYORC - 09 Aug 2013 at 12:03pm

Of course you don't have to agree. But as i said before: Avidyne advertises this system as a retrofit for Avidyne + Garmin GNS430 equipped SR22s. They even write "takes full advantage of the 430's automatic switching ...." and so on (wording from memory).

So you think it is asking too much if i want a manual (website, call, smoke signals) that explains in detail how the A/P will work?

I like the autopilot as much as you do. I even helped Avidyne sell three of them to friends (actively!). But a good technical description is the minimum i could ask for.

Yes, I will only use NAV for ILS, just as you described. I was in Heringsdorf a week ago!



Edited by GYORC - 15 Aug 2013 at 2:26pm

Dear Jake

at the moment i am on vacation in Greece, 1800 km away from my plane :-(

I thank you very much for your kind reply and if you want i am more than willing to help you improve the manual a bit.

Believe me, it's hard for me to critisize other writer's work, especially since I'm an aviation journalist, book author and former magazine editor.

If i can help, i will and for free. Just give me a little time to think about my specific questions and about how the manual could maybe be improved a bit!

you'll hear from me soon!
Alexis von Croy

sent from iphone

Looking.  Hopefully answer is delivered to you on the same call that should resolve your roll servo question from a different thread.

Thanks Bill.    Not sure when the next rev of the Pilot Guide will get pumped out but I did put your note in that folder.