Ray - I have a 3-yr old Simrad autopilot on my boat that is tied to a Garmin chartplotter. It does the same thing. Not a full 90 degrees but it definitely starts to turn off course. I learned about the problem the first time I used it in the NAV mode following a course from the chartplotter. It goes off course going under most bridges. I always have thought it was caused by loss of satellite signal but the flux-gate compass and nearby metal could be the real cause. I now go to Standby mode when going under any bridge. Marv Storey Osprey 43 Viking Aft Cabin Washington, DC In a message dated 5/30/2015 8:28:26 A.M. Eastern Daylight Time, raymacke@egyptian.net writes: I am assuming there must have been enough metal in the bridge piling to cause the variation but never would have thought it could produce a 90 plus degree compass swing. Is this the norm with a flux gate compass?