calculating pressure altitude

[bookworm]

Using the relationship there, I think you're on the right track.

I make it:

Pressure_altitude = a*(1-(QNH/P0)^b) + Indicated_altitude*(QNH/P0)^b

where a = 145000 ft, b = 0.190 and P0 is 1013.25 hPa. The correction factor to the second term makes little difference.

So for, say, a QNH of 1000 hPa at an airport at 400 ft, the first term is 355 ft, the second 399 ft, for an overall pressure_altitude of 754 ft.

For small QNH - P0,

Pressure_altitude = Indicated_altitude + a*b*(QNH - P0)/P0

which is an additive correction of 27.2 ft/hPa.