Originally Posted by littco
QNH can either be a regional pressure setting or a areodrome pressure setting.
Actually Regional Pressure Setting isn't a QNH. It's the lowest
forecast QNH for a time in the future in a specified area
If it is an aerodrome setting it will be at the highest elevation of the field.
It has nothing to do with what point it is on the airfield, as it is a calculation of seal level pressure, mean sea level being a constant.
QFE is QNH adjusted so that at the highest point the altimeter reads zero when on the ground.
No - QFE is a pressure derived from QFF (pressure at sensing point) and is then calculated to give a barometric pressure in the touchdown zone of a particular runway.
Now, Not all airfields use QFE and those that do, normally have more precise landing proceedures like ILS etc where you will need an accurate height above the airfield elevation.
So why do AERAD's give Decision
ALTITUDE, which is derived from QNH, for an ILS, which is a precision approach. You 'll find more Commercial / Private operators fly instrument approaches on QNH rather than QFE, as it saves a safety critical change in the event of a missed approach. I never give QFE to an aircraft on the approach unless specifically asked.
However you need to make sure that if the field is using QFE then you set your altimeter to QFE when entering the circuit otherwise you may find there could be 20mb difference between QFE and QNH and if your flying the wrong one you could be 500 ft lower or higher than you should be!
More b*ll*cks. If you fly at circuit height you set QFE, if you want to fly at Circuit
altitude you set QNH. Both vertical positions, relevant to the ground, are of course identical, because altitude is just height added to aerodrome elevation.
Littco - I think this demonstrates the farce that is the British ATPL system. You've obviously studied the exam matter by rote, without actually understanding the subject.
That, to me, is frightening!