ATCO Fred said BUT...if a pressure setting is passed to a US aircraft as 999 and the pilot interprets this as 29x99 (cause it's common practice back home to drop the 2 (which I think is Gash)) then who is at fault - the one adhereing to RT protocal or the one being gash?
Quite so - unfortunately, it would be head-in-the-sand to ignore the fact that that erroneous interpretation might be made. More to the point if a concerted effort were made to bring more of the world into line by using a common reference. In the meantime, it is rather ironic that the UK insists on emphasising "millibars" for pressures lower than 1000 (for good reason) but continues to use the term millibars (for no good reason) when the rest of the world uses "hectopascals".