AUTOPILOT PROS & CONS

There are secondary effects but these are second order. If you apply
roll, the main result will be roll. The pitch change should be small.

Think about this carefully. Imagine you have the plane trimmed for
straight and level flight. Now let go of the controls and allow the
plane to enter a spiral. Will the pitch change be small? Somehow I
doubt it. When an airplane rolls, the vertical component of lift is
reduced. This is a first order effect on pitch. The second order
effects modify it.

I think that either there is more to this, or the manufacturers are
not doing the most basic analysis of the stability margin.

Your latter conjecture is most likely correct. I once interviewed an
electrical engineer who worked for Meggitt. If he is at all typical,
they are simply not capable of doing the most basic analysis of the
stability margin (of course I did not hire him).

The thing which suprised me, on the KFC225, was that the software does
not detect an increasing error. So, if say a roll servo goes dead, it
never realises it.

S-TEC's are the same. A friend of mine had a servo go intermittent,
and finally die. Never got any diagnostics.

I even have a couple of videos of this. The unit
even passes its power-up tests, including servo tests, with a totally
dead servo.

I believe the only failure mode the KFC is designed to detect is a dead
open circuit. Some A/P's won't even do that.

I realise this isn't on the same topic as control
stability but if they don't get this right then .... ?

Your implication is that their design is not up to the standards you've
come to expect from a professional operation. And it's not.

I often wonder just what "certification" really means in this
business.

It means the paper is worth five times as much as the actual product.
It's obvious to you how poorly autopilots are designed because you have
a good understanding of the technology. If you believe that the
situation is different for other aircraft components, I have a bridge I
think you might be interested in purchasing.

Michael