Need help understanding KFC-200 operation

In article .com,
"Michael" wrote:

Correct. The only time there is a datalink between the GPS and AP is
when you have GPSS, and even then XTE is not what drives it.

We've got GPSS in a couple of our planes (not this one). It's an add-on
box that sits between the HSI the GPS, and the A/P (Century-2000 in that
case). You engage the GPSS and put the A/P in HEADING mode. The GPSS
widget then takes XTE (or maybe angular error?) from the GPS and
synthesizes a heading error signal that makes the A/P do the right thing.

Or, at least that's the theory. It's basically just a huge ball of duct
tape interconnecting several systems that were never meant to talk to each
other in this fashion. The one I've flown is hideously under-damped and
just flies big S-turns if you're dumb enough to turn it on. I'm told
they're working on getting it adjusted. A big ball of duct tape with rows
of trim pots and an STC does not a useful tool make.

Can't comment on all this... but the 225 has an option
that does GPSS in the NAV mode. You can tell if it's
in that mode by moving your d-bar or heading bug... if
it's tracking the GPSS, these heading references will have
no effect. I think GPSS in the 225-- if you have it-- overides
the apr coupling functions.

The GPSS that is s-ing all over the place is busted. All
the ones I've seen are flawless. Their output to the
autopilot is only a turn rate command--up to standard
rate. All the other roll components in the AP are shut off.

OK- I meant FD+ALT+AP. Key concept remains... you must
aim the airplane by whatever means to get an intercept or
it will never go to clpd. If you have GPSS, this is done
automatically.

Peter sure has had worse than average luck with his 225.

Bill Hale


Peter wrote:
The tip of the day is to always use APR mode when coupling to
your GPS, even enroute. NAV mode is dumbed down
considerably to make crossing over VOR stations less
traumatic. APR doesn't suffer that, and since the GPS
course needle is rock solid APR works great with sharp,
precise coupling. You can fly a T shaped GPS approach
without ever decoupling it-- just feed the new headings in as
cued by the cnx-80.

I have missed most of this thread, but it's very interesting because I
fly a KFC-225 (Socata TB20).

I've had to look into this in more depth than some because, in 4
years, I have had the computer replaced 4 times (various faults,
notably the internal pitch sensing accelerometer), the roll servo
replaced 6 times (keeps burning out), and the pitch and pitch trim
servos replaced once

Interesting about the APR mode. I thought it worked only with a
localiser input, i.e. somehow knowing if the KX-165 radio/nav was set
to a LOC frequency (I believe the radios have an output which
indicates if the nav freq is within the LOC/ILS range).

The NAV mode is seriously defective in that if you engage it when the
HSI deflection is less than 3 divisions, the AP doesn't continue to
intercept as it should but immediately turns onto the HSI course
pointer track, and then (over a very long time) reduces the error.

If the GPS (KLN94 in my case) is set to the default enroute
sensitivity of 5nm full-scale then you have to be at least 3nm off
track to engage the NAV mode.

It works better if the GPS is set to 1nm full-scale.

It might work even better if the GPS was set to 0.3nm full-scale but
the loop gain is then so high it doesn't track the GPS; it oscillates
too much. I wonder what this is telling me about the system's ability
to fly a proper GPS approach, past the FAF...

However, the AP does fly an ILS perfectly, and the final phase of that
has to be at least as sensitive as 0.3nm FS.

heading is selected. But you can turn
FD and ALT modes back on and it will wing-level at a constant
altitude which can be very useful if you are trying to figure
out what to do after a gyro failure. Explore this by pulling
out the HSI breaker.

Can one turn on AP+ALT? If you turn on FD+ALT you don't get a
wing-levelling autopilot; you just get a wing-levelling flight
director.

I will have to do that too.

Have you completely figured out the pitch modes?

You mean the one that suddenly puts the airplane into a 3000 FPM dive?
Yes, I've got that one figured out :-)

What is this? One of my failed computer units would randomly select
+2000fpm or -2000fpm, and fly that vertical speed!! Quite exciting.
Honeywell blamed a static leak for this

I have had extensive comms with Honeywell (Olathe) about this stuff
but nobody there seems to know anything about it.

After a lot of work I managed to get my hands on the
maintenance/installation manuals for the AP and its three different
types of servos, and (I am an electronics hw/sw designer) found some
highly suspect stuff in the servos, but (apart from not having a
watchdog) the main unit seems well enough designed.

The performance, when it's working fine, is excellent - probably the
best general aviation AP I have ever used. Excellent ride in
turbulence too.