Hm, I’m not sure I agree. Let’s try to synchronize our takes on the issue? With QNH set, the altimeter reads the altitude of the reference point. The altimeter assumes existing in the ISA in order to convert QFE to a fictional sea level pressure. The ISA is the only atmosphere the altimeter “knows” through it’s calibration.
The QFF is an attempt to guess a more true to life fictional sea level pressure, as it would be at sea level if the ocean was to be found nearby. It does this by using the current temperature rather than the ISA as the basis for the correction.
Thus, we fool the altimeter into reading the AD elevation by setting the actual QNH. It is not what the MSL pressure would be off a nearby drop down to the ocean, but there’s nothing phoney about it as it lives up to the definition – the reference pressure which gives the height above MSL. Nor is it QFF, as setting QFF would give a reading that is off from the actual height as soon as the temperature deviates from ISA. Yes, QFF would likely be closer to the theoretical true MSL pressure, but it would not be the MSL pressure which gives the correct altitude readout, as the altimeter is calibrated assuming the ISA.
And to address the initial question… in yet another way: At MSL, QNH and QFF both equal QFE by definition. The correction to achieve either from QFE is based on the weight of the atmosphere between the current position and MSL.
The mess when calculating it stems from the fact that the weight of that layer of atmosphere depends on the temperature in said layer. You either assume ISA temperatures and get a QNH or you base your estimate of the weight of that layer on the current temperature and get QFF.
When the current position is at MSL, there is no atmosphere between the two which means there’s no correction.
Old Smokey,
have we just invented altimeter (topic) drift to go with compass drift?
