DFC,
I disagree. The QNH is
not the sea level pressure! It is normally assumed to be so, but it is fact:
The pressure 'reduced' to mean sea level, assuming ISA temperature profile from the station/airfield to MSL
(
Reference, but I've aded the emphasis.)
Normally this difference does not have much practical impact. But when we're specifically talking about a case when the temperature does not fit the ISA profile, the difference
is important. That's why you can't perform a temperature correction calculation like this without knowing the elevation of the airfield or station whose QNH you are using.
Also, although I don't have a whiz wheel with me to check this, I think that the only time the whiz wheel allows you to set a temperature/altitude combination is for TAS/IAS conversions, not for temperature corrections to altitudes? Hence why we had to learn the rule of thumb that I quoted above (and also, coincidentally, on the QFF thread which is also running in this forum) for our ATPL exams.
FFF
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