Duncan Aviation Avionics Tech Reps have fielded a number of calls about erratic yaw damper operations in the Honeywell SPZ-500 Integrated Flight Control (IFC) System. This system is installed in Citations and King Airs.
During flight, the heading signal from the C-14() directional gyro is fed into the SP-200 Auto Pilot (A/P) computer where it calculates the yaw rate and drives the SM-200 yaw servo motor to correct for yaw conditions. Any one of these can be the cause of the erratic yaw damper operation, but which one?
Because it is very difficult to duplicate this squawk in a ground check, the following troubleshooting actions will help to narrow the field and identify the failing unit.
The fastest and easiest way to determine which unit is causing the squawk is to swap them out one by one for a loaner or exchange unit until the problem is eliminated. If your aircraft has dual C-14() gyros, swap their positions. If this does not correct the problem, the gyros are not the cause.
If the servo motor has high time, it is very likely the cause of the erratic operation. If a known-to-be-good servo is not available to swap in, the most economical solution is to send in the high-time servo for a bench check and repair as needed.
If you have eliminated the C-14() and the SM-200 servo as discussed previously, you can assume with some certainty the problem is the A/P computer. Most likely, a very sensitive circuit in the A/P that senses the heading signal and calculates the yaw rate. Your options are to send the unit in for a bench check and repair or purchase an exchange A/P computer.
While the cause of erratic yaw damper operation is most likely to be one of the three components named above, the possibility still exists for that resistive connections or switching relay contacts in the aircraft may be the cause. This requires advanced troubleshooting. Call your Duncan Aviation Avionics Tech Reps for assistance.