In order for an autopilot to get an aircraft from Point A to Point B smoothly and safely, many components must work together; the main component being the autopilot computer. The autopilot computer receives input from various sensors and then outputs commands to the servo motors to position the control surfaces to fly the aircraft.
The most important input is from the vertical reference gyro that tells the computer the aircraft’s attitude in both pitch and roll. The autopilot computer uses this changing attitude signal to command smooth maneuvering keeping the aircraft level and upright.
If the autopilot were to lose this attitude signal, either because the gyro ceased to output or the connection between the computer and gyro went down, it could command the aircraft to do a complete roll over or hard pitch up or down. When a gyro is slow to respond, it can cause oscillations in roll or pitch porpoise problems. This condition is very difficult to duplicate on a ground check but will manifest itself in the air due to the interaction and response required by the computer from the gyro.
Common autopilot problems are roll or pitch oscillations commonly referred to as “wing rock” or “pitch porpoise.” These are typically caused by an Inner Loop problem. The Inner Loop consists of the servo motor electrically driven by the autopilot computer and mechanically linked to surface controls usually via cabling. It is critical that within this Inner Loop, the cable tension is up to stated specs. If the servo motor has to spool up tension on the cable before a control surface can respond, it will allow the aircraft to drift off course before correction is made and then over correct coming back. This causes it to continually be “behind” itself and cause slower and larger oscillations.
The servo motor can be another source of unwanted oscillations. Typical servos have a generated rate signal that is dependent on the speed of the servo. This tach signal is processed by the autopilot computer for speed control. Commonly the pickoffs for this signal in the servo get worn or highly resistive and put out a reduced or erratic signal. This can cause the servo to run too fast and overshoot thereby setting up smaller and more rapid oscillation. This is a relatively small amplitude signal, so resistive contacts in the signal line can cause erratic operation. Often, if you are able to see control yoke oscillation on the ground check, there is a good chance the problem is the servo motor.
Of course you can never rule out the autopilot computer as a source of oscillation problems due to processing of the signals and commanding of the servos. The symptoms can imitate those caused by both incorrect cable tensions and servo motors.
For free technical advice or troubleshooting assistance, contact your Duncan Aviation Tech Rep.
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