A Learjet 35 pilot reported intermittent audio clacker in his headset, even though the stab actuator was not trimming. The audio clacker is designed to alert the pilot when the horizontal stabilizer actuator is trimming.
At Duncan Aviation, we connected the stab actuator to a Learjet test stand for function testing. The results indicated that everything was working properly. Knowing we couldn't give the unit back with a No Fault Found, we investigated and tested further.
With the trim-in-motion potentiometer connected to an "analog" Simpson meter, we ran the stab actuator the full length of its travel. We discovered if we stopped the stab actuator in a specific spot and lightly tapped on the potentiometer, the analog meter would read an erratic resistance (the needle jumped around). A change in resistance triggers the audio clacker in the pilot's head set.
After replacing the trim-in-motion potentiometer and retesting the whole unit, no other faults appeared. On the next trip, the pilot reported the audio clacker only went off when the horizontal stabilizer actuator was really trimming.
Written by Chris Gress, Component Solutions Manager