Is it possible the aircraft was tracking a navigational ground station (VOR) while on autopilot? It might fly a sinusoidal course if the right circumstances exist.
I can think of four reasons this might occur. There could be more.
1. The ground station is transmitting a poor or weak signal.
2. The signal from the ground station is being distorted. (Terrain, weather, mechanical)
3. The aircraft is not receiving the signal well.
4. The autopilot is weak or malfunctioning.
For example:
I fly out of Anchorage, Alaska. On some days when we are to the East or South of the VOR tracking a radial to or from the Anchorage VOR we get distorted signals. There is a large range of mountains 15 miles (25 kilometers) east of Anchorage that run NNE to SSW. I don’t believe it is a problem with the aircraft or autopilot because it has happened on different airplanes. Also, when we use a different VOR like Kodiak or Johnstone on the same day, the aircraft tracks straight.
When we use the VOR mode of the Autopilot it tracks the radial (course) selected by the pilot. When flying back to Anchorage from Cordova, we are westbound flying on the 084 degree radial of the VOR (HSI needle is set to 264 degrees To the station). The autopilot will keep the aircraft centered on the course. One moment we will be centered, and the next it will show a deflection to the right of about ½ to 1 dot. The autopilot will correct the aircrafts heading to get itself back on course. The needle will center and then not much later after that it will deflect to the left the same amount. The autopilot will correct again and turn to the left to intercept the course. The result is we end up flying a zigzag path unless we do something to correct it. Besides being a little uncomfortable to the passengers and annoying to us pilots, it looks unprofessional.
This doesn’t occur all the time, but most of us chose not to use the VOR mode. We instead will use the Heading mode of the autopilot. Instead of tracking the radial the pilot has selected it flies a heading the pilot has selected. We will select a heading and allow the needle to swing back and forth. If the needle stays to one side or the other for an extended period of time, we will then adjust our heading to get back on course. This method results in a lot less banking and or zigzagging and I think is more professional. It is a lazy pilot that allows it to zigzag back and forth on VOR mode in this case.
This may or may not have been the cause of the sinusoidal contrail.
Anyway, hope this helps.
