Aircraft Pressure Altitude = 20000ft
QNH = 998 hPa
1013 - 998 = 15 hPa
15 hPa x 30ft = 450ft
QNH Lower than 1013 = True Altitude Lower than Flight Level
Aircraft Indicated Altitude based on QNH NOT adjusted for Temperature Error
Correction (TEC) = 19550ft AMSL.
AD Elevation = 1300ft AMSL
Aircraft Height above AD = 19550ft - 1300ft = 18250ft AGL
TEC = 4ft per 1000ft above QFE Datum per Degree of ISA Deviation
At FL200 ISA Temp = (20 x -2°) + 15° = -25°
OAT = -40°, therefore OAT = ISA -15°
TEC = 4ft x (18250 / 1000) x -15
TEC = 4ft x 18.25 x -15
TEC = -1095ft
True Altitude = Indicated Altitude
+/- TEC
True Altitude = 19550ft – 1095ft
True Altitude = 18455ft AMSL
Obstacle = 8000ft AMSL
Clearance = 18455ft - 8000ft = 10455ft