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Thread: ICAO Altimeter
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Old 4th Aug 2012, 07:31
  #6 (permalink)  
Oktas8
 
Join Date: Jun 2001
Location: Australia
Posts: 890
If I understand correctly:

My guess is it should read Altitude(in feet)=ICAO(P)+27.31*(s-1031.25)
It should read altitude in the standard atmosphere. If it doesn't, it isn't a "sensitive altimeter"! Hence I think that the answer to your question is actually the definition of the standard atmosphere - which I know you have already studied.

QNH is the pressure at the measuring station, reduced to sea level according to ISA. That definition is found in any meteorological textbook or dictionary, and is internationally accepted just as the values for ISA are internationally accepted.

(As an aside, QFH is the pressure at the measuring station, reduced to sea level according to the actual temperature of the air. Therefore QNH does not account for density fluctuations, but QFH does. Aviation is not interested in QFH.)

Since the rate of pressure change varies with height, the value of 27ft/hPa is valid only at ISA sea level. Altimeter movement is not constant at different pressures; 30ft/hPa is a better average value throughout the lowest 5000 foot layer. 100ft/hPa is appropriate to higher altitudes.

Each ICAO State will publish maintenance standards which altimeters must meet when undergoing bench testing every two years or so. If neither your regulator nor your Ops Manual states a pre-flight accuracy (checked before take-off), you might consult your national maintenance regulations. 50' or 60' maximum error on the ground is reasonable around the world for IFR I believe.

Hope that helps,
O8

Last edited by Oktas8; 4th Aug 2012 at 07:32.
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