"Must QSY. Toodle Pip!"
So did you QRF whence you came......? Perhaps via QFG followed by a QGH to QAL?
(Sorry Mods - the Q code has probably been done death here previously)
Back to QFF / QNH however, at my unit, the ATC staff are also qualified Met Observers, and on the occasions that we perform manual pressure calculations, we add to the (corrected) 'as read' pressure another correction for the QNH. This correction is taken from a table supplied by the Met Office and varies according to the ambient OAT.
As stated above, I suspect that the difference between QFF & QNH is only in the methodology of the calculation, and the actual difference in the results is probably in the order of 0.1mb or so.
Yes, I realise that with rounding down, this could mean a whole 1mb difference in the reported pressure, but in 9 out of 10 cases however, the difference will probably be neglible and only of interest to a meteorological anorak (try saying that after a few pints....).
Only a thought, but could the reason we use QNH and not QFF to pass to aircraft be that QFE & QFF look and sound very similar, which might lead to the wrong pressure being set during an approach to land, whereas QFE & QNH both look and sound different (especially phonetically), thus minimising that risk?
RC