![stec 50 autopilot oscillation stec 50 autopilot oscillation](https://lahso.files.wordpress.com/2011/07/system-20.jpg)
I don't think Garmin would approve of an Avidyne retrofit for the Piper when they have their own attitude based autopilot. The addition of yaw damping (YD) minimizes yawing oscillations and helps. It is available as a retro fit to Cirrus aircraft. For aircraft that can benefit from a modern autopilot, the GFC 500 autopilot. The S-tec system was dumped for a new attitude based system, the DFC90. The debate over rate based and Attitude based systems is ongoing but I have come to the conclusion that the attitude based system is far superior. They should begin to turn with 2-3 volts on the motor.
![stec 50 autopilot oscillation stec 50 autopilot oscillation](https://i.ebayimg.com/images/g/yMsAAOSwEgZZiEQ~/s-l400.jpg)
Contamination in the servos will cause brush problems and the servo may not begin to turn until too late. Is your cables stiff? This may causes a delay from the servos and the autopilot may over correct. Installing an S-Tec autopilot in older aircraft has usually meant a lot of work on the freeing up old stiff cables, especially if that transducer is STC'd for a rear installation.
![stec 50 autopilot oscillation stec 50 autopilot oscillation](https://www.picclickimg.com/d/l400/pict/114692298005_/S-Tec-Autopilot-System-50-Controller-14V-P-N.jpg)
Without the instant attitude information to correct this, the problem sometimes gets worst. As the aircraft builds up hours, rigging and stiffness contribute to this sensor delay. Most S-Tec autopilots work well when they are installed as new systems in new aircraft. A small gain adjustment usually will solve the problem if the oscillations are not severe. Practice timeGarmin G5 GPSS to Stec 50 Cessna 195A S-tec Autopilot Installation S-Tec 40 Autopilot Roll Centering Results STEC 55 Part 1-Setting up the Altitude Alerter S-TEC 30 Autopilot Operation Garmin G5 test with Stec 55 Auto pilot N4177N-Piper Archer III - Coupled STEC 55 WAAS Approach S-Tec 55X Overview. The problem was compounded years ago when gain adjustments were no longer allowed in the field and further compounded when S-Tec no longer allowed their dealers to service them. This S-Tec problem has been the subject of many forum discussions and unfortunately there really is no solution. Information on your PFD comes from the autopilot, nothing from it goes to the autopilot. Not so with a rate based system which has no information from an attitude indicator. An attitude based autopilot relies on attitude first and static pressure second. Altitude information to the S-Tec system comes primarily from the static system and there is always a compressibility delay in the static lines. Proposing is an inherent problem with the design of the S-Tec systems. You may have water or an obstruction in the lines or a bad autopilot. This in turn causes the autopilot to over correct and an oscillation begins. The autopilot reacts too late, allowing the altitude error to increase. Proposing means the feedback loop has a problem, usually a delay in getting the information from the altitude transducer to the autopilot.