The 55X works fine. A number of TB GT owners, who had lots of money, ripped out the KFC225 and put in the 55X.
With an autopilot, the desirable features are, most important on top
The last few are navigator dependent.
The PIT mode closely approximates that, with the identical benefit that the aircraft won’t stall until it reaches its ceiling.
And with IAS mode you get a drawback: you get an AF447-type dependency on the airspeed. Depending on the navigator/PFD design, you can lose a large chunk of your panel if airdata is lost.
And with IAS mode you get a drawback: you get an AF447-type dependency on the airspeed.
That’s why I added it to the end of your list 
So a S-Tec 55X does most of that and is actually a good Autopilot then, except it’s expensive?
The Arrow III I’m moved forward on has an Aspen 1000 Pro and if I get it right the S-Tec 55X will a good match with it.
If I get it right you don’t need the ST-360 Altitude Pre-Selector/Alerter System as this in “included” in the Aspen.
And you only need the Auto trim module from S-Tec.
I want to have a good autopilot and if not needed to rebuild the whole panel if not necessary.
The other option if it’s the “same” amount of money would be a Garmin G3X + G5 + GFC 500 Autopilot or if there is a good solution with GFC 500 + G5 to drive the autopilot and as backup to Aspen and then get rid of the vacuum too.
I’d not be happy to get an S-Tec for free, but better than nothing. I’d never consider to install one.
There’s a reason everybody who could with reasonable effort immediately got rid of it in favor of the DFC90.
Compared to a GFC500/DFC90 it is day and night. A modern digital att based autopilot flies like on rails. In my opinion it provides the biggest possible difference and improvement to a SEP. The first time I experienced the GFC5/6/700/DFC90 I thought “Wow, feels like flying an airliner.”
I don’t see a big issue of keeping the Aspen. Manage the GFC500 via the GMC. Aspen still provides loads of information. Sure, it’s a little bit quirky as are all work arounds, but since it’s an owner flown plane you’ll quickly get used to it.
DFC90 would be nice, bo no STC for PA28 family.
How does the GMC 507 work?
It has same panel as GFC 500 ??
There’s a reason everybody who could with reasonable effort immediately got rid of it in favor of the DFC90.
A big part of that is that Avidyne, despite massive PR for years about doing their own servos, never did, and use the STEC servos. And you can’t just install STEC servos because – in one of the most flagrant examples of a completely anally retarded certification situation – no avionics shop will touch that with a 20ft bargepole, legality-wise. You have to install the whole STEC AP and then rip out the whole thing apart from the servos Then STEC issued a “poison letter” stating that their STC does not support somebody else’s autopilot for warranty or service purposes More here.
A modern digital att based autopilot flies like on rails
Actually do does any autopilot, 1980s or later, that isn’t shagged. I’d say most APs flying (or not flying) in European GA are barely working, because there is so little expertise in fixing them, and those who can fix them are “difficult to deal with” to say the least. And it is also difficult to send the system to the US. Especially as often it doesn’t fix the problem which might be due to shagged and leaky hoses. I changed my hoses (well, the parts which were accessible) and it much improved the ALT mode, for example.
I am all for people installing new stuff but not if paying tens of k extra.
Tracking precision can be achieved with roll steering.
Darkfixer wrote:
So a S-Tec 55X does most of that and is actually a good Autopilot then, except it’s expensive?
As I said before, I did exactly the kind of upgrade you are condidering, in 2014. Aspen Pro with an STEC-55x with FD and the Avionic Straubin Altitude capture module.
Am I happy with it: Yes. It massively transformed the flying I do, it’s a much more relaxed flying than before and the AP does what it should, most of the time.
It does all of Peter’s list. I love the GPSS feature and I can deal with the altitude VS mode.
What it does not too well is to follow VOR and LOC signals in crosswind. It is known to be “chasing” the track and is also to an extent doing that in GPSS. That is the main thing I do not like about it. But it is totally managable for a plane like an Arrow, which is for all practical purpose a VFR and light IFR plane.
What would I like to have in addition is the IAS Mode. Definitly. So in today’s world, I would most definitly go for the GFC500 rather than the 55×.
Seeing that to this day, the GFC500 is the only alternative and that it appears to be cheaper or similarily priced than the Stec, I would most definitly go that way, even if it means I’d have to forego some of the Aspens advantages.
However, it still is a viable solution to your particular requirement. What I would suggest is to sit down with a knowledgable avionic shop like Avionic Straubing or someone similar and let them lay out the options for you in detail. What can be connected to what, how and what will it cost. Then you can take an educated decision.
Peter wrote:
And with IAS mode you get a drawback: you get an AF447-type dependency on the airspeed. Depending on the navigator/PFD design, you can lose a large chunk of your panel if airdata is lost.
That was a main problem of the Aspen, which however got corrected with the Max version.
What an IAS mode does is that it lets you do climbs and descents on a particular airspeed rather than VS which can let the AP fly you into a stall. I’m used to IAS/FLCH mode from the airliners I was involved with and love it.
What we do with the VS mode is that we fly constant VS in climb of 500 fpm below 10’000 ft and 300 above, and we reduce the VS manually to 100 fpm within 200 ft of the altitude to be captured. That also works with a King AP.
