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TECHNICIAN'S BENCH
Troubleshooting the System Thirty & System 50 on the Flightline

Mike LaCaze, Tech Rep


Troubleshooting the System Thirty and System 50 on the flightline is not a complicated process if you know what symptoms to look for. The following is a guide based on questions we are frequently asked to help you quickly troubleshoot the autopilot system to the correct LRU (Line Replaceable Unit). Keep in mind, this list is not inclusive of all possible failures and corrective actions but should make the process much easier. Look for a guide on the System 60 & 55 in the next issue of the Approach.

SQUAWK: The Ready light does not come on after the self-test sequence.
CHECK:
1. Was the test procedure initiated properly?
2. Does the turn coordinator power flag pull out of view?
3. Is there power to the turn coordinator?
4. Does the turn coordinator gyro spin?
5. For a valid TACH signal from the turn coordinator. The valid signal is 8 to 9 VDC.
6. Check the autopilot fuse.
7. For a System Thirty, unhook the PFGC and retest. If the test passes, the PFGC or wiring could be faulty.

SQUAWK: The Autopilot Ready light illuminates but a mode cannot be engaged.
CHECK:
1. For a faulty autopilot disconnect switch.
2. Is the mode switch cushion or membrane bad? (System 50)

SQUAWK: The heading mode will not function.
CHECK:
1. Is the autopilot configured for the correct DG/HSI?
2. Is the HDG system harness configured for the correct DG/HSI?
3. Is the HDG signal reaching the autopilot inputs?
4. Is the HDG jumper disabled in the main harness?
5. Is the roll servo functioning correctly?

SQUAWK: There is no trim light on the autopilot controller during self-test.
CHECK:
1. For a blown fuse in the computer.

SQUAWK: The Autopilot does not go through the self-test properly.
CHECK:
1. For an offset in the accelerometer in the PFGC.
2. Is the autopilot mounted in the correct orientation?
3. Disconnect the PFGC and retest (System Thirty). If the sequence is correct, the PFGC or the wiring could be faulty.

SQUAWK: The Trim Light on the autopilot is on all the time.
CHECK:
1. For a sticking trim switch on the pitch servo.

SQUAWK: Control Wheel Movement cannot be nulled with the Roll Centering Adjustment.
CHECK:
1. For the Turn Coordinator Gyro signal exceeding 150 mV.
2. For a broken roll centering potentiometer.

SQUAWK: No Display on System 50 but the autopilot functions properly.
CHECK:
1. The aircraft dimmer control may be set for night flight or the dimmer switch may be bad.

SQUAWK: Aircraft continues to climb or descend in Altitude hold.
CHECK:
1. For the correct direction of rotation in the pitch servo, especially in new installations.

SQUAWK: Aircraft rolls opposite of the Heading Bug or Stabilizer command input.
CHECK:
1. For direction of rotation in the roll servo, especially in new installations.

SQUAWK: Heading and stabilizer modes function properly but navigation mode "S" turns or wanders off.
CHECK:
1. Check for high start voltage in roll servo,
2. Loose bridle cable


Additional Comments: Wire Harness problems can cause a multitude of failures that may have similar symptoms. Always check out the harness and associated wires before sending a component in for repair. If you’re ever in doubt as to which component may need repair, just give us a call in S-TEC technical support.