Today I was able to get to the hangar and analyse my S-Tec 50 fault.  In the voltage data reported below, note that the Turn Coordinator (TC) is powered from the essential bus which sees a voltage of .7 or .8 VDC lower than the main bus.  Also, my battery appeared to be a bit weak, showing only 12.2 VDC on my EPI 800 voltage monitor.  Also remember that the AP should not reach the ready state or should disconnect if the TC gyro speed sensor (tach) see the gyro is not up to speed.  

 

Step 1. I removed the connector at the TC and powered up the AP.  The internal tests were completed and the AP reached the "ready" state.

 

Step 2.  TC Connector pin B (Ground) had a good ground (See Jeff's failure mode suggestion).

 

Step 3.  TC Connector pin A (+VDC) read 11.4 thru the essential bus.

 

Step 4. TC Connector pin E (TC gyro Tach) read .0018 VDC with the AP off and 10.2 VDC as soon as the AP was turned on to test mode and remained there when switched to ready mode.

 

Step 5.  The wing stabilizor mode (STB) was engaged and the ailerons very very slowly tried to roll the airplane.

 

-----------------------------------------

 

The TC was reconnected to the AP harness.  My breakout box was connected between the 50 pin D-sub on the harness and the AP computer/controller 50 pin socket.

 

Step 1. I verified that pin 42 was aircraft ground and all future ground references would be taken from this pin.  Also, pin 37 was carrying  + power to the AP when the test/power switch was engaged.

 

Step 2. With the TC breaker pulled and power applied to the AP: AP pin 5 = TC pin E was at 9.96 VDC.

 

Step 3. The AP pin 5 to TC pin E breakout box link switch was opened and:

a. AP pin 5 voltage was 10.1 VDC on its' side of the breakout box, and

b. TC pin E voltage was .0016 VDC on its' side of the box.

 

Step 4.  Power was applied to the TC and pin E was monitored:

a. Initial voltage was 1.36 VDC,

b. after 5 to 10 seconds, the voltage went to 9.06 VDC, and

c. shortly after TC power was removed the voltage dropped to about 2 VDC as the gyro spun down.

 

----------------------------------

 

The conclusion is that there is a fault in the AP computer itself that has caused the TC tach sensor line to see 10 volts.  This results in AP operation even though, for whatever reason, there is no tach sense or data from the TC.

 

------------------------------------

 

History

 

The S-Tec 50 was purchased in 1991-1992 in order to install the roll servo in the wing as it was being built.  The airplane first flew in 1996 and the autopilot ans servos were returned to S-Tec for sensitivity adjustments.  In September, 2001 there was an internal fuse that failed (see "The Flight of Two Madmen") and it was repaired in Casper, WY.  I can't think of any other work on the AP (other than cleaning the carbon in the servos) over the time it has operated in my airplane.

 

The Box: