Yes but...

"QNH is a calculated number that makes your altimeter read
correctly (true altitude = pressure altitude) "

True alt is above sea or land in true feet or meter.
Pressure alt. is alt above the Std 29.92/1013.25 level, and that surface can
be above or below SL depending on atmosphere pressure.

Jan Carlsson
www.jcpropellerdesign.com


"T o d d P a t t i s t" skrev i meddelandet
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Frank van der Hulst wrote:

QNH is the pressure that the air would be at sea level, if it wasn't for
the land all being above sea level. If you were to bore a hole down to
sea level, and lower a barometer down the hole to the bottom, then QNH
is what pressure the barometer would read. Theoretically.

I think "Theoretically" is the important part of this. When
you set QNH at the local airport, the altimeter reads
correctly only at the altitude of that local field (unless
the atmospheric temp and pressure changes with altitude in
the standard way, which it almost never does.) So, just
like there is a difference between true altitude and
pressure altitude when flying above the airport, the
imaginary barometer-in-the-hole won't really read QNH when
at the true altitude of the sea.

QNH is a calculated number that makes your altimeter read
correctly (true altitude = pressure altitude) at the
altitude/elevation of the reporting station. Everywhere
else (including at the true altitude of the sea) it's wrong,
but as long as we're all wrong by the same amount, we don't
run into each other.

T o d d P a t t i s t
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