I was at the UK Phraseology Working Group (UKPWG) meeting where this was discussed along with the change from millibars to hectopascals, the UK always having required the units to be transmitted after the pressure setting when less than 1000 in order to differentiate between mb/hPa and the 'abbreviated 'inches (of mercury), the thinking behind it being that 'QNH 992' transmitted to a crew from a country where inches was used might be misinterpreted as '9.92 inches (or actually 29.92 inches) I presume this must have arisen as a result of an incident sometrime in the past when someone actually did this..
There happened to guest from the IAA (Irish Aviation Authority) and he was asked what they did in Ireland, replying that the use of mb/hpa was not required in Ireland.
Last edited by chevvron; 22nd Oct 2019 at 00:59.