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Old 10th April 2004 | 10:01
  #16 (permalink)  
Cuban_8
 
Joined: Oct 2000
Posts: 43
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From: Manchester, UK
Sub,

As far is I’m concerned, no (I stand to be corrected!). As long as atmosphere deviations from ISA don't cause the surface pressure to change at the airfield, then the altimeter reading will remain the same.

Remember, the creation of QNH assumes an ISA atmosphere from the airfield elevation down to MSL. The altimeter is calibrated to assume an ISA atmosphere from MSL to the airfield elevation (or any elevation, for that matter) - hence no error at the airfield. You have to appreciate that QNH is a "fictional" pressure at MSL that is created to help us, in that it eliminates temperature error to the altimeter setting point.

Hope this helps,

Cuban_8
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