Thank you everyone. Nice to know I was on vaguely the right track. I "resdicovered" this during a rapid descent recently and the rest of the crew cried
when I tried to work out why it occured. Of course, I was probably taught the whys and the wherefores at groundschool but no-one's perfect.
Oldbeefer - Don't suppose you have a title for the section of Prouty's that it's in? I have the combined works but I can't seem to find the right bit. (Excellent book btw - Helibooks.com).