Glasair Aviation issued Service Bulletin 73 for all Sportsman and GlaStar Aircraft equipped with Garmin GSA 28 autopilot servos.
From the Service Bulletin:
Compliance Time: Before next flight
Discussion and Background Information: There has been a report of an autopilot cable unwinding from the capstan on a Garmin GSA 28 Autopilot Aileron Roll Servo. The swaged ball may come loose from the capstan and bind in the cable guide causing the flight controls to bind.
Required Action: Before your next flight inspect the servo capstan cable routing and cable ball location. If you have the inspection ports in the seat pans you will be able to see the routing of the autopilot cables without removing the seat pans. If you do not have the inspection ports you must remove the seat pans to determine the routing of the cables and the position of the cable ball.
Phil Martineau described his experience in a forum post:
I’ve got a full Garmin panel including the GSA28 autopilot servos which were part of my TWTT build.
While on the ground away from home at a fuel stop, my control stick jammed solid laterally (roll) even though front/back (pitch) it moved ok; I was able to determine there was an issue with my autopilot, as I was able to free the stick by cycling the autopilot at the panel. I pulled the autopilot breaker and flew home.
Next day in my hangar I replicated the jam. This time, not possible to free the control stick laterally with or without power.
Seat cushion and inspection panel was removed. Using a flashlight and mirror, I was able to see that the centering pin (sometimes referred to as swaged ball) of the bridle wire had pulled out of the capstan when it was at “6 o’clock” position, and then as a result the pin was jamming on the capstan cable guard. I question whether I would have been able to free the control stick if I’d been in the air.
As I’ve got approximately 80 hours on my plane, a logical question is “why now?” My own conclusion is that the cable on my capstan had not been installed correctly and only when I let the autopilot move the stick to it’s extreme roll position in the hangar without intervening, it unwrapped the cable from the capstan far enough to pull the pin (“ball“) out of the slot, never to go back in until I physically uninstalled the cable.
Read more about the issue in the forum.
Download the Service Bulletin.