>> maggot17
Every IFR pilot knows how to deal with the type of QNH available at his/her destination.
Ah, maggot, if only that were true.
And why is it that one has to look for a 'good' IFR school? Sign of the times! Go easy on him mate, there are some pilots of large kero burners that could not tell you the difference!
>> novicef
This is not a Jepp v AsA issue. I think you have not had this bit of altimetry properly explained to you. Changing to the Jepp presentation has possibly revealed the misunderstanding.
Refer to Bendo's post above. He is on the money. There are 3 types of QNH for your altimeter, not 4.
You asked:
what is the correction if any when using Actual Area QNH (given by ATC)for an approach?
If ATC ever gives you an "actual" area QNH, reply by demanding an "actual" case of beer on arrival! There is
no such thing as an "actual" area QNH.
Jepp MET AU-1 Definitions
AREA QNH - A forecast altimeter setting which is representative of the QNH of any location within a particular area.
Area QNH are forecast for Area QNH Zones (AQZ) which normally match ARFOR boundaries (Jepp MET AU-11).
"representative of the QNH of any location" means
+ or - 5HPa. If the QNH of any location is likely to vary more than 5HPa from the forecast area QNH during the forecast period, the MET office will subdivide the AQZ/ARFOR and provide one area QNH for each of the smaller areas.
e.g subdivide ARFOR area 30 to make two areas, one east of a line MIA to ML, one west of the line.
Why 5HPa? Well, 1 HPa = roughly 30' so 5 HPA = roughly 150'
which is... the maximum possible difference between the Actual
Aerodrome QNH (AsA chart -100') to Area QNH (AsA chart +50').
Make sense?