In recent weeks, I have received several calls from King Air operators reporting Honeywell air data and altimeter erratic operation problems. The common symptom has been the altimeter needle oscillating anywhere from +/- 20 feet to +/- 100 feet. This causes erratic pitch and porpoising action while the autopilot is in altitude hold mode.
After troubleshooting and swapping out all components possible, we discovered that if the circuit breaker for the SP-200 autopilot computer is pulled, the altimeter needle will stop oscillating.
Our experience has taught us that the AZ-241 air data and BA-141 altimeter are very sensitive to ground loop noise. Over time, exposure will cause the signal ground connections to degrade. After cleaning up and rebonding the signal ground connections to the airframe for the air data and autopilot systems, most of the symptoms will disappear.
In another similar case, a different customer reported that changing the aircraft connector to the BA-141 altimeter helped to solve this same issue.
For more information about this and other technical support, contact your Duncan Aviation King Air Tech Reps.
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