Over-reading/Underreading Altimeter - Question
Hello,
Hope you can help me with a little problem I'm having with a question in PJ Swatton's Performance Theory book.
Question:
En-route in the Northern Hemisphere an aeroplane flying with QNH set on the sub-scale of the altimeter, is experiancing 15 degrees port drift. The altimeter, if the subscale is not reset will:
(a) Read the correct height above MSL.
(b) Over-read
(c) Under-read
(d) Not be affected.
The answer given is (C) - Under-read. Why?
I've trawlled through several books/notes (Performance/Principles of Flight/Atmophere etc) and I can't find any relationship to drift/hemisphere affecting altimeter reading.
It's really bugging me.
Glad of any help you guys might be able to offer.
BF