Autopilot failure

CrashGordon wrote:

Normally, I fly air routes or VOR to VOR where necesary, so when I flew a fairly long (600NM) direct GPS flight, I wasn't expecting a drift off course of up to 30 degrees after 300 NM. I could switch from HDG to NAV and get back where I belonged, but I would have to compensate by up to 30 degrees to do it.

Any ideas as to the cause?

Gyro drift? Wind? I always let the autopilot computer figure out my GPS flights by leaving it on NAV, then it keeps the little localizer bar on the HSI centered.

Another thing is that the GPS corrects for the curvature of the earth, where-as I think the flight environment in FS9 is flat, so you're trying to follow a curving line in the GPS while the plane is flying across a straight line.

Fascinating. MS created a GPS system that ignored the actual FS environment. But that still doesn't explain why the heading is 30% off when I swithc the HDG fro NAV. I've heard of gyro drift, but 10 degrees per hundred NM?