Answers courtesy of the great guys at BGS
If the temp = ISA conditions
QNH = QFF
If above MSL
If the temp > ISA then QNH > QFF
If the temp < ISA then QNH < QFF
If below MSL then this is reversed:
If the temp > ISA then QNH < QFF
If the temp < ISA then QNH > QFF
and
In ISA conditions QNH = QFF; the setting which gives airfield elevation at touchdown (QNH) = the atospheric pressure at sea level (QFF). If it is warmer than ISA the theoretical column of air between the airfield and sea level would expand and the QNH would occur below sea level which means that QNH would be greater than QFF. Similarly, if it is colder the ISA the column of air would contract and the QNH would be less than QFF.
and
QFF and QFE problems are all linked to the fact that in colder denser air pressure changes more rapidly as you go up or down.
Imagine you are on an airfield at 1000ft elevation and at 1000mb and you want to calculate the pressure at msl. In round figures for ISA conditions, at 30ft/mb you would make it 33.3mb higher at 1033.3mb. This is your QNH.
If the air is colder than ISA you get more rapid pressure changes, and we?ll use 20ft/mb for convenience. Now, using ambient temperature and this ?real? lapse rate we calculate the pressure at msl and it comes out at 50mb higher at 1050mb This is your QFF.
Now QFF is our best guess of the actual pressure at msl, but QNH is a lower pressure and zero on the altimeter with QNH set will leave you well above msl. So, in cold conditions QFF is a higher pressure setting than QNH but the QNH pressure level is at a higher altitude. As an aside, this means you can safely fly down to zero indicated altitude with QNH set and not hit the sea.
If you look at this upside down, from an airfield below msl it means that now QFF is at a lower pressure than QNH. QNH set will give you a negative height and as you climb to zero on QNH you will still be below msl
The General Rule is that in below ISA temperatures you indicated altitude on QNH will always be nearer to your station level than your true height
This works for obstacle clearance as well. If you are climbing over a mountain in cold conditions your indicated altitude will put you nearer to your station level, which is lower than your true height
This has been about cold air> If the conditions given are for temperatures above ISA then reverse the logic!
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