My observations are similar to this. However, I don't think that it's a 
problem with the accuracy of the calculations as such. The route manager code 
is relatively old and tries to set a straight course to it's next waypoint. 
It doesn't consider the wind blowing the aircraft off its lateral track. The 
autopilot tries to compensate by keeping the aircraft on a heading straight 
to the next waypoint. Now, as we get closer, this happens at an increasingly 
greater rate, until the rate of required course adjustment exceeds maximum 
turn-rate. I've seen a few occasions where the 747 couldn't reach the next 
waypoint because of this phenomenon and started flying around it in endless 
circles, until I manually popped it. 


On Saturday 05 June 2004 03:28, Ampere K. Hardraade wrote:
> The violent maneuvers I was describing occur when the plane is a few
> kilometers away from the waypoint.  Therefore, it should have little to do
> with the way that pid controller reacts to the jump in waypoints.
> One explanation for the violent maneuvers that I thought of is this: as the
> distance between the plane and the waypoint decreases, the accuracy
> required in the course calculations increases.  Since it takes time for the
> autopilot to respond, and takes even more time for the plane itself to
> respond to the commands of the autopilot, the plane will never align itself
> perfectly with the waypoint.  Hence, the autopilot will keep trying to
> "catch" the waypoint until the very last moment, thus causing the violent
> maneuvers.
> One solutions to the above problem is to pop the waypoint when the plane is
> still some distance away, thereby preventing the autopilot from making all
> those course adjustments.

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