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Let’s now assume that the Actual Outside Air Temperature (OAT) is -2°C.
The ISA deviation is then, .ISA = (13°C) - (-2°C) = -15°C.
145
It is assumed that the minimum required actual altitude to clear the obstacle is 1200 ft. In order to account for the ISA deviation, the terrain/obstacle elevation has to be increased
by: 1200 x 0.04 x 15/10 = 72 ft
In other words, the altimeter must read 1272 ft in order to have an actual height of 1200 ft.
● Tabulated corrections
ICAO publishes the following tables in the .PANS-OPS Flight Procedures. manual. They are based on an airport elevation of 2000 ft; however, they may be used operationally at any airport.
Altitude correction (m)
Aerodrome temp °C Height above the elevation of the altimeter setting source (m)
60 90 120 150 180 219 240 270 300 450 600 900 1200 1500
0°C 0 6 6 6 12 12 12 12 12 18 24 40 54 67
-10°C 6 6 12 12 12 18 24 24 24 36 48 79 103 128
-20°C 6 12 12 18 24 24 30 36 36 54 72 115 150 189
-30°C 12 12 18 24 30 36 40 40 48 72 97 150 201 250
-40°C 12 18 24 30 36 40 48 54 60 90 122 189 250 311
-50°C 12 24 30 36 40 54 60 67 72 110 146 225 299 372
Altitude correction (ft)
Aerodrome temp °C Height above the elevation of the altimeter setting source (ft)
200 300 400 500 600 700 800 900 1000 1500 2000 3000 4000 5000
0°C 0 20 20 20 20 40 40 40 40 60 80 140 180 220
-10°C 20 20 40 40 40 60 80 80 80 120 160 260 340 420
-20°C 20 40 40 60 80 80 100 120 120 180 240 380 500 620
-30°C 40 60 80 100 120 130 150 170 190 280 380 570 760 950
-40°C 40 60 80 100 120 140 160 180 200 300 400 620 820 1020
-50°C 40 80 100 120 140 180 200 220 240 360 480 740 980 1220
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