The C41 uses both.
Unlike competitive autopilots which are either rate- or position-based, the Century 41 uses both for more precise control and a smoother ride.
I like the smoothness how it behaves during intercept in Nav/App Mode . The Alt hold is rock solid. The C41 can be found even in King Airs so I think it’s bit better then teh average rate based stuff around…
Most attitude based autopilots use the roll rate as a 2nd corrective input.
Of course GA will take years to catch up, but the modern crop of MEMS (rate based) sensors are excellent. A fairly expensive hobby of mine is radio controlled helicopters, and the modern flybarless systems (to make RC helis even flyable at all required some additional stabilization – this comes in the form of a mechanical flybar – think like what the full-scale Hiller UH-12 has – or an electronic replacement flybarless gyro system) are impressively good despite the high levels of vibration and abuse the helicopter gives them.
Achim,
which AP do you have in your Cessna?
Now DFC90. First Cessna/ARC, then different S-TEC variants and now DFC90 which can only be installed an upgrade to S-TEC, retaining the servos and auto-trim.
The only digital attitude based APs for GA are KFC225 (no retrofit), GFC700 (no retrofit) and DFC90 (very limited STCs). A few analog attitude based exist like the Cessna/ARC 400 and Century 41.
Only S-TEC got this monster STC list. I would really like to know how they got it. Avidyne have to spend loads of money and years to certify an airframe and S-TEC got them by the hundreds, I’m sure without (complete) flight testing.
KFC225 (no retrofit)… etc
To make a slight correction, you can retrofit any AP which appears on the aircraft Type Certificate, with just a logbook entry (no modification approval is needed provided there isn’t some other resulting issue).
Only S-TEC got this monster STC list. I would really like to know how they got it. Avidyne have to spend loads of money and years to certify an airframe and S-TEC got them by the hundreds, I’m sure without (complete) flight testing.
Without much testing at all, is pretty obvious, because there is absolutely no way they could have done the full certification testing (all corners of the loading envelope and Vs to Vne, etc) with all those airframes. They must have done some sort of piggyback process, sneaking it in under the wire many years ago. Consequently, there are many reports of installations where you get porpoising (in pitch usually, sometimes in roll also) under some loading conditions. Recently I read of a Seneca doing it, with a 55X IIRC…
However I also think that “Avidyne have to spend loads of money and years to certify an airframe” is largely one of their “we are a victim of the certification regime, nobody loves us, we are spending $100M/month on R&D and will deliver a world-class avionics product tomorrow, 1800 UTC” PR statements which they seem to be generating every 5 minutes and posting on their forum 
I still plan to install 2 × IFD540 sometime but I think their constant whining is largely a result of them being massively under-resourced (read: desperately short of cash and living from hand to mouth).
In most cases where somebody is constantly running a “victim” story it’s actually partly their own doing.
Avidyne claimed they would develop King-style servos ages ago but this seems to have been shelved. It would have opened up a big market for them, comprising largely of reasonably capable IFR tourers and full of people desperate for a replacement now that Honeywell have bought and killed yet another company, and would have protected them from STEC poisoning their well with letters like this. And anybody doing a google on KFC225 will find my writeup so they would know exactly what to avoid
My KFC225 is rock solid under any turbulence up to strong moderate (roll excursions of say 30 degrees will disconnect it) but it has the old issues especially in certain airframes.
achimha wrote:
Mineral Springs, TX
Did you perhaps mean Mineral Wells, TX?
I know the main board of that very intimately as well, but logic doesn’t enter the picture there, it’s all analog electronics 
There is a resistor in the first gain stage that is supposed to be adjusted to the airframe (not even a potentiometer) – I doubt many have done that very seriously.
alioth wrote:
MEMS (rate based) sensors
Given that MEMS sensors nowadays come cheaply in 9DOF (3x angular velocity, 3x linear acceleration, 3x magnetic field) variants for mobile phones, so there’s no real justification to restrict yourself to rate based only…
The DFC90 (which i have too) is attitude based (AHRS) but is connected to a remote Turn Coordinaror for validation of the data. AFAIK the data is not actually “used” by the A/P.
Today i did an LNAV approach in VMC and although there was a tailwind (first) and crosswind later of 30 knots when doing a 90 degree intercept to the final course the A/P did a perfect job, really like on the proverbial tracks …
Peter wrote:
To make a slight correction, you can retrofit any AP which appears on the aircraft Type Certificate, with just a logbook entry
I am not sure you can. Certainly the G1000 with the GFC700 system is not retrofitable. The Avidyne DFC90 system needs an existing S-Tec 55×. I am not sure about the King systems, maybe Jesse can correct me here, but even if they are available, they are very expensive.
I don’t really think we will see real alternatives to S-Tec in the certified retrofit market, they pretty much own that market. If Avidynes system becomes available stand alone, that would be a major step forward.
Before I bought the S-Tec 55x we had massive discussions about which AP we should install, but in the end, it was clear that it was the only one halfways affordable. And when they had a promotion selling me the 55x for the price of the 30 in Friedrichshafen 2 years ago, I grabbed it. Ok, I had to buy the autotrim additionally but I still thought it a pretty neat deal considering what it can do in comparison to the System 30.
The basic deal is: Either you buy an airplane with an AP fitted or what you will end up installing will be an S-Tec.
