Hi Luke
I must confess, I don;t know how long ATC has used the Q codes for - a long time though I would imagine. As far as I know, the UK used Q codes and millibars during the war - but I'm perfectly willing to stand corrected on this. Technically - the UK uses hectopascals - one millibar equals one hectopascal. But in practice, most people refer to millibars. Yes the USA use Inches of Mercury, and they issue clearances in feet on the local altimeter setting, based on inches Hg below the transition altitude of 18000 ft. ABove that it's FL's but still based on inches, but they call it 29.92 in HG instead of 1013 millibars/hectopascals.
Hope this helps
SLT
