1033 -1013 =
20hPa x 30 = 600ft. Altitude in ISA conditions = 20,600ft.
FL200 ISA = +15ºC – (2º x 20)
= -25ºC
OAT = -40ºC
Dev. = -15ºC
Difference between true altitude and pressure altitude is calculated as:
4ft per1,000ft per ºC ISA deviation = 4 x 20.6 x 15 = 1,236ft
True altitude = 20,600 – 1,236 =
19,364. Or use CRP5