Honeywell KFC-225 autopilot - what could cause this failure?

Hi Peter,

As far as I know the Socata TB20 is metal but has a composite cabin,
cowl and tail. Ground Radar is one source and when you fly through a
powerful long range beam that can really upset electronics. These are
hard to discern because it can happen tens of miles from the site and
their antennas turn slowly. But the software should recover. I don't
know about aerospace electronics but in medical we must demonstrate that
our systems come back to normal within seconds after a defibrillator
hit. If they remain in la la land instead of recovering we would not get
the agency blessing.

Anyway, there is another noise source but this one could only be
correlated if you'd record the NAV or GPS data the instant the AP quits.
There could be a high powered AM station on the ground. Also, some VHF
and UHF TV transmitters use highly directional antennas so you might get
hit with the full brunt well after passing a mast. They also concentrate
the beam to a very narrow vertical range of just a few degrees, mostly
to save energy costs. Therefore, the magnitude of the EMI effect depends
on the altitude when you fly through their antenna pattern. Last but not
least there are satellite feeder stations for TV and communications
which work with a beam width of just a few degrees and point upward. Due
to the narrow beam width the field strength can be tremendous. Again,
these can often be identified as a cause if the location where the AP
fails happens to correlate.

Then there is always the chance that a certain data pattern the AP sees
upsets the software. But that would be a very bad sign.

There is a way to test for at least some of the EMI behavior but it
would have to happen in a shielded environment and that can be expensive
or hard to find. You can blast the unit with variable frequencies. It is
a test that all system have to go through after completing a design.
What I do for pre-compliance is a trick that can pinpoint vulnerable
spots: I use an EMCO near field probe kit (little loop and point
antennas on a stick with a BNC at the end) or just a 2" loop soldered to
a coax if I don't have the kit with me. Then I send a few watts into the
probe and go over the unit under test in a dousing rod fashion. It is
tedious but usually finds the culprit.

The oil pressure EMI issue is a bit scary. Does Socata know about that?
They should really fix this. Protecting an input from EMI isn't rocket
science. If it is legal you could use ferrite toroids and have these
affixed on the cable bundle right before the gauge or its electronics
box if it has a separate one. 43 material (Amidon) works pretty good at
VHF. Even Radio Shack has some but in aircraft I'd stay away from the
snap-on cores because they can come off when you hit rough air.

Regards, Joerg

http://www.analogconsultants.com