ILS 119.5. WB's SATCO is alluding to other airspace users who use a regional pressure setting to maneouvre up to say 45 000'. In this instance, when coordinating with said users, finding that the 30' per millibar assumption is wrong has come as quite a shock and no-one is quite sure exactly what to do about it.
Can I take issue with this one? From an altimetry point of view, the 30 ft per millibar assumption holds nicely, because the difference between altimeter settings is millibars at or close to sea level.
We presumably agree that if you have an aircraft at 1,000 ft on an altimeter setting of 1013, it will be about 300 ft below an aircraft 1,000 ft on an altimeter setting of 1003.
If you have an aircraft at 45,000 ft on an altimeter setting of 1013, it will still be about 300 ft below an aircraft 45,000 ft on an altimeter setting of 1003.
The first aircraft is 45,000 ft above the 1013 mbar surface, the second is 45,000 ft above the 1003 mbar surface. The difference between the 1013 and 1003 mbar surfaces is about 300 ft.