View Full Version : QNH Validity Zone
B737NG_Pilot
15th Feb 2016, 13:58
What is geographical area in nm, upto which the QNH of a local aerodrome is valid.
Some authors mention 25NM whiles other 8KM from the ARP.
parkfell
15th Feb 2016, 21:12
The MSA for an airfield is out to 25nm, and uses the airfield QNH.
So practically the answer is 25nm.
A TMA QNH will be based on a designated airfield
Where does 8km come from?
+TSRA
15th Feb 2016, 21:51
Parkfell,
Just wondering if you have a reference for 25 NM. I was looking into a reference to answer this post earlier today and couldn't find one. I had thought 25 miles too, but then most locations I've flown into also give a 100 NM safe altitude which, presumably would also be based on the airfield QNH.
8 km is pretty darned close to 5 NM which, IIRC, is the distance out a standard TAF is published for...possibly mixing the two up?
parkfell
16th Feb 2016, 06:42
Some military charts I have seen have had the 100nm.
Now I don't know if that is based on the airfield QNH.
On a bracing day in the Scottish TMA, I have seen a 3 millibar (hPa) difference between GLA & EDI. So I would be cautious using a QNH out to 100nm.
As for the 25nm, look in the AIP etc showing approach charts. Occasionally they fail to mention it.