Oh my gosh!.  I had to actually hand fly my airplane when I got down low in the Cup Race and had to deal with turbulence - S-turns don't win races.  Recently I have been unhappy with my S-Tec 50 performnace in turbulence and a flight test indicated up to 15 degrees of wandering from GPSS controlled courses (1 dot off after 15 miles), DG control had the bug set 5 degrees to the left of the desired course and the ground wing leveler test required the manual turn knob set to 10 o'clock to keep the stick from wandering right.  Control hasn't been crisp for a while and this was just ridiculous.

 

Analysis:

 

1.  The S-Tec turn coordinator was suspect because a CFII thought the little airplane didn't return to level quickly enough (well, he flies a Falcon Jet).  Maybe it was worn out.  Even the S-Tec Tech contact thought it could be the gyro.

 

Answer:  The local avionics shop tech (JA) saidthat the S-Tec turn coordinator is very well constructed and typically lasts 2000 hours before an overhaul would be considered.  Mine is 10 years old but only has 500 hours on it - 499 of those hours managing flights.  Because of its function, it is well damped and would appear slow compared to an ordinary turn coordinator.  OK, I test-tried a fresh one off the shelf and found little, if any, improvement in performance.

 

2.  JA's AP tech said that servo motors can become "sluggish" because of carbon build up where the brushes come in contact with the commutator.  This is especially true of motors that don't move much.  In this case, the servo does not respond to small corrections.

 

Note:  Ahah!.  My 2 seat airplane is almost always flown solo.  I have a fuel system that empties both tanks simultaneously and equally.  I almost never have to adjust lateral trim and the usual method of operating the auto pilot is to trim out the airplane, activate the "direct to" GPS function and utilize GPSS.  This means there is very little course adjustment (servo movement) as the centerline is followed.  Hmmmm........

 

Answer:  A test is to disconnect the D-sub connector from the computer and check the motor pins by applying voltage and watching the current draw.  If the current drops at about 2-3 volts (motor starting voltage), the brushes are probably OK.  My roll servo was marginal.  Here, the avionics tech runs the voltage up to about 40 VDC and spins the motor, without engaging the clutch, for about 5 minutes.  He then reversed the voltage to run the motor in the opposite direction for 5 minutes.  This cleans the brush to commutator contact area.  If the motor is removed and actually cleaned, the result is long lasting.  If the motor is cleaned as mentioned above, it should be done every year (mine had never been cleaned before).

 

In the following ground tests, in wing leveler mode, the stick remained centered.  If a test VOR box was used, the a/p immediatly followed the needle.  In the flight test, the a/p followed the bug in heading mode and turbulence did not upset the course very much (1-2 degrees) in GPSS tracking mode.

 

The pitch servo was in better shape because altitude hold in a 320/360 is a significant task requiring a continous servo two-step dance.

 

The servo motor cleanup is a simple task for a well equipped avionics shop or, if servo removal is easy, by anyone who knows what the inside of a motor looks like and can not only clean it, but reassemble it correctly.

 

If you are not pleased with the performance of your S-Tec A/P, consider these procedures before pulling your hair and the system out.