It has been said and repeated that the concept of “regional QNH” bears little sense, and I won’t argue that. Still it is a fact of life: whenever I go flying I will, upon leaving the aerodrome traffic circuit, talk to the area FIS – “Brussels Information, 126,900” and they will tell me about any paradrop or intense glider activity where I am going to fly, and sometimes they’ll tell me about possibly conflicting traffic but always immer toujours they communicate a “regional qnh”.
Today was different though: Brussels information still said a “regional qnh” of 1027, later 1028, but added a “Brussels qnh” of 1033. Obviously they did this because the difference was unusually large, but what is the use of this “brussels qnh”? In radio class I learned that I should only rely on the regional qnh?
I guess it depends on what you are using it for. If everyone in the area uses a regional QNH you will all be showing the same altitude reference which can help with separation (same as setting 1013 for FLs). But if you are trying to avoid the ground or airspace, the local QNH (to where you are) is more useful.
Even at low level I never bother with the regional QNH.
What is the concept of a regional QNH in Belgium? Sounds different from the UK, where we have
An individual airport QNH, which is for flights in the vicinity and for departure, approach and landing (as in every country)
“London QNH”, which is the QNH for in / under the London TMA, and is suitable for both terrain clearance and airspace avoidance (at least any weather anyone would still be flying in).
“Regional pressure setting” is a forecast lowest QNH for a wide area and is suitable for terrain clearance only, and hails back to a time where one would set out on long non-radio flights in uncontrolled IFR, so needed something to keep from banging into Mt. Snowdon until arriving in sunnier climes…
the RPS is definitely not useable for airspace avoidance underneath an airspace base defined in ft MSL/AGL, as by definition you will be flying higher than when using a proper QNH.
I think the idea behind the regional QNH is that it assures obstacle clearance, by being the lowest QNH forecast for the entire ASR (altimeter setting region).
The idea of using it for traffic separation assumes that everybody uses it, which is obviously not true for
It’s a useless concept, IMHO. If you are in IMC, and low enough for the QNH to matter, you need to do some serious due diligence anyway.
When i fly to CZ i always get a regional QNH … which doesn’t help me for landing at the little field at LKRK. So when i get closer i ask them for the PRague QNH, which is only 30 km away … I really never understood what the R. Q. should be good for …
Yes – anybody can just dial-up the nearest “big airport” ATIS.
You are supposed to do that (I am told) when ducking under the said airport’s CAS, anyway (in reality I think almost nobody does that).
In Germany, the concept of regional QNH doesn’t exist at all. All VFR aircraft are to obtain and set the QNH of the nearest controlled civil aerodrome. Simple, and it works well.
Using an airport QNH for enroute alimeter setting usually works but in fact it was never designed for that purpose. The only purpose is that when you hit the runway the altimeter shows the runway elevation thus all the IFR minima work out.
In flat terrain and ISA temperature that does not matter much but imagine two airports directly next to each other. For example a helipad on the ground and a helipad on a high building right next to it. As strange as it sounds they will have a different QNH if the temperature gradient differs from ISA.
Any enroute use of a QNH is always just a rough estimate but I could imagine they correct the regional QNH to MSL and ISA while the airport QNH are calibrated to the local elevation/temperature combination for IFR operations.
I could imagine they correct the regional QNH to MSL and ISA
But that would make it useless for everything.
It wouldn’t be useable for terrain avoidance (because it might be significantly off in high terrain and non-ISA temperature gradients), it wouldn’t be useable for airspace avoidance (you’d need the QNH of the airport the airspace belongs to), and for separation – well 1013 is more common for that.