To summarize
To summarize all what has been said (just a few years later):
True Pressure Laps Rate (feet/hPa) = 27 * (1 + 4*ISADEV/1000)
ISADEV = OAT(at airfield) - ISA T°(at airfield elevation/2)
It is the difference between the actual OAT at airfield and ISA temperature at mid height between SL and airfield.
QNH (hPa) = QFE + AirfieldElevation/27
QFF (hPa) = QFE + (AirfieldElevation / True Pressure Laps Rate)
Units :
Temperatures in C°
Elevations in Feet
Pressures in mb or hPa (it's the same)
Emmanuel Cordier.
Last edited by manucordier; 30th Aug 2010 at 14:19.