It looks like you are a US pilot. Don't take this wrong but US ATP's don't have any theory knowledge. The ATP written is 100 mutilple choice questions.
The JAA ATPL is about 14 exams that must be completed in an 18 month period. I'm working on it for a JAA license conversion. I am learning things I never knew and a lot of old material now makes more sense. I personnaly think the course is harder than a Masters of Business Administration which I recently completed.
A possible answer to your question is remote compasses that use flux valves, such as your HSI can have an error introduced when flying at high speed and making a slow turn while heading in a northerly direction.
