Ok so came across this today in a Met practice:
"If the QFE, QNH and QFF have the same value,
a)The 1013.25hPa level must at MSL
b)The airport must be at MSL
c)The conditions must be as in the ISA
d)The airport must be at MSL and the conditions must be as in the ISA"
The answer given is b). However I chose d) for the following reason:
We know QFE=QNH=QFF. We also know that a measurement of QFE is used to calculate QNH using the ISA standard lapse rate and QFF is calculated using the actual lapse rate per unit of height. Then it stands to reason that in this condition, if QFE was to vary then QNH would remain equal to QFF. Say for example QFE was then taken at height 'Z' above the airfield, and the lapse rate conditions on the day = LR1 and ISA standard lapse rate = LR2.
QFF = QFE+(LR1*Z), QNH = QFE+(LR2*Z).
If QFF = QNH then QFE+(LR1*Z) = QFE+(LR2*Z)
cancelling for QFE and Z gives LR1 = LR2
hence, Lapse rate conditions on the day = ISA Standard Lapse Rate.
In answer b) I accept that it is not a wrong answer per say, however because QNH and QFF by definition are calculated, then surly d) is the more valid answer?