Just out to make another test flight - let's see how the cleaned up S-Tec 50 servo follows the ARR GPS 27 Approach. Motoring along, I punch the GPSS button, take the A/P from the "ready state" to "on" and immediately hit the HDG button.  Whoa!  The roll to 40 degrees of bank was unexpected.  Snapping off the A/P, I collect my thoughts (not much effort needed for this).  I say to myself, "Self, let's just try the wing stablizer."  Another exciting roll commences - the other way, even - and no sense that it would stop. With the A/P off again, I move my old collection of thoughts to the little trash can icon.

 

Hmmmm.....  Oh, that's right, Yesterday I pulled the turn coordinator (TC) breaker for some unrelated ground checks.  Yep, after restoring TC power, the red flag went away, the A/P was recycled and it held the wings rock solid.

 

Wait a second, if the TC gyro is not up to speed, the A/P ready light should never come on and, if power is lost to the TC, the A/P should automatically disconnect.  One input to the A/P computer is a voltage (about 8 VDC) representing that the TC gyro tach output is good.  Failing the voltage check, the A/P should not come ready or operate.

When I can, I will investigate and report why there is voltage on the TC Tach output line when the TC has no power at all.

 

In the meantime, I suggest that those with an S-Tec A/P perform the simple ground check:

 

1. Pull the breaker for the turn coordinator.

2. Turn on the Master/Avionics sw(s).

3. Switch the A/P to "test".

4. After the internal sequence of A/P checks, if the green "ready" light illuminates, there is a safety problem.  Diagnose the cause before serious IFR flight. Jocular IFR flights should also be avoided.