Q: What causes the roll and yaw angles to change radically when a body nears a pitch angle of -90 degrees?
REV 5.06

A: What you are seeing is correct and there is no fix for it! This is a problem which has plagued all for centuries: any set of Euler angles has a "singularity". For our set the singularity is at a pitch of 90 degrees.

At a pitch of 90 degrees, there is no unique way to compute roll and yaw. As a result, when you pass through this angle, "strange things appear to happen", the roll and yaw angles may change by 180 degrees. Nothing, however, is really happening - as pictures of the model will show. This is simply numbers changing to protect their integrity.

The only advise I can give here is to change your coordinate system so that you do not go through a pitch of 90; i.e. change the coordinate system so that during the process the structure rolls instead of pitches (use a &DESCRIBE PART XXX -MOVE).

This works so long as the pitch does not change by over 90 degrees during the simulation.