Re: [Flightgear-devel] incorrect altimetry

[Alex P. <ale...@ie...>]

From: John Denker <sf...@av...>
> Please do not write to me saying that SLP must equal QNH
> when you are flying at sea level.  That's true in that
> narrow special case, but not representative of the
> general case.

Supporting John's point:
In real world operations, even when flying at sea level, QNH is often not equal to SLP.  In areas with a dense array of airports, the QNH values are intentionally correlated to increase vertical separation in the airspace between aircraft that have taken off from different airports.  As a result, even an accurately calibrated altimeter may not indicate field elevation when sitting on the runway.

More generally:
It is always very important to distinguish between the facts that arise from the simulation of the planet (such as SLP and variation), and the facts that arise from simulation of the airspace (such as QNH and VOR alignment).  There tends to be fairly good correlation between the two, because that makes engineering sense, but the differences are routinely enough to kill people.



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