Thread: QNH or QFE ?
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Old 9th Aug 2018, 19:51
  #52 (permalink)  
LookingForAJob
 
Join Date: Jun 2001
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Originally Posted by BackPacker
Mixing two altimeter datums in the same airspace is dangerous. For safety, all aircraft in the same airspace should use the same datum. Whatever that datum is, I don't care, as long as everybody knows and uses it.
As an ex-controller who used to work terminal areas, having more than one level datum was just day-to-day business. Aircraft coming into my area of responsibility were often at or descending to a FL, below TA QNH was the common pressure setting used and around the aerodromes QFE might well have been used. Include in the mix aircraft transiting the control area which might have been on the Regional Pressure setting, with the added bonus that being close to the boundary between two Altimeter Setting Regions, the RPS might have been different depending on whether the aircraft was coming from the North or South.

The rules of the game were to separate aircraft that needed separating and give traffic info on those which didn't. For separation there was a simpler system that displayed what FLs were separated from aircraft below TA and for traffic info, levels rounded to the nearest hundred feet were easy to to work out (and if they were VFR and not going to be in my airspace for long I saw no reason to get all aircraft to change to one of the aerodrome QNHs). I don't dispute that more than one pressure datum means that there is more to think about, but it concerns me to think that there may be pilots or controllers who have trouble with simple altimetry calculations when values rounded to the nearest hundred feet will suffice for most purposes.
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