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Temperature-Pressure Error (Static Weather)


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When static weather is used and the pressure is 760mmHg and temperature 15°C (standard atmosphere) then QNH is not correct for high altitude airports, e.g. Beslan.

 

Expected behavior is that when sea level temperature and pressure are standard (15°C, 760mmHg) then standard altimeter setting is equal to QNH for all surface locations.

 

The altimeter reads the correct elevation when set to QNH only when the sea level temperature is set excessively low according to a predictable pattern.

 

E.g. if SLT is +15.00°C then Beslan local temperature is +11.47°C. When altimeter is set to 760mmHg the field elevation of 1772' is not correct displayed.

 

However if mission temperature is set -7.94°C such that local Beslan temperature is -11.47°C (negative of +11.47°C) then the altimeter displays the correct elevation when altimeter is set to QNH.

 

This effect is observed in multiple aircraft (A-10C, Mirage 2000, suspect more if not all) so the conclusion is that the error is with DCS reported atmospheric pressure or with code for barometric altimeters shared by multiple modules due to a logic error in the code such that a mathematical operation was incorrectly applied twice.

 

This error is not demonstrated in dynamic weather where a single cell is given a pressure deviation of zero Pascals. This suggests the code for translating local pressure into altimeter value is correct and that it is the local pressure generated by static weather which is in error.

 

Additionally it should be noted that "QNH" is not a global value in cases where the temperature is non-standard. Suggest the term be changed to "QFF" if informing the user of the sea level altimeter setting that results in zero instead of equaling field elevation at starting field. QNH is only valid for a defined position such as an airfield and as such would be a different value for every airfield present in the mission (unless sea level temperature is standard). Ideally the user is informed of the QNH setting for his starting location but this is only possible if the code is given the user's starting location or it is otherwise defined.

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