(a) Don't lose sleep over any of this;. .. .(b) You'll never need to know how it works when you fly aeroplanes which have them;. .. .(c) Really, it's only in the syllabus because it used to be, (and it helps keeps some dunderheads out, so best it's left there);. .. .(d) Don't buy from Transair - save money and buy direct from my chum Bill at <a href="http://www.fmcguide.com" target="_blank">Bill Bulfer's page</a> (type in <a href="http://www.fmcguide.com" target="_blank">www.fmcguide.com</a> if the hyperlink doesn't work). Shopkeepers who own jets mark up far too much.