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Example:
Solving the previous example with this method:
-
Enter the table with 1200 ft. To avoid an interpolation, use 1500 ft.
-
Enter the table with -2°C. To avoid an interpolation, use -10°C.
-
Read that the correction on the altimeter indication must be 120 ft.
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 8 20 160 260 340 420
-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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● Formula
Low altitude correction:
.Z = [(QNHalt - Zt) * .T] / [288.15 - (QNHalt * L / 2)]
.T = difference between the airport temperature and ISA temperature of the
airport (.ISA°C).
Zt = airport altitude.
QNHalt = aircraft indicated altitude above sea level. L = temperature gradient: 0.0065°C per m or 0.00198°C per ft in function of the unit chosen for Zt and QNHalt.
The following table is illustrated in the FCOM, and is based on the above-noted formula.
Low altitude correction table:
QNH ALTITUDE MINUS TERRAIN ELEVATION (ft) . .Z CORRECTION (ft)
500 1000 1500 2000 2500 3000
.ISA -10 °C -17 -34 -51 -68 -85 -102
-20 °C -35 -70 -105 -140 -175 -210
-30 °C -52 -104 -156 -208 -260 -312
-40 °C -70 -140 -210 -280 -350 -420
TRUE ALTITUDE = QNH ALTITUDE + .Z Note: A constant .ISA from ground to airplane level has been assumed
Example:
Solving the previous example with this table:
-
Enter the table with 1200 ft. To avoid an interpolation, use 1500 ft.
-
Enter the table with .ISA = -15°C. To avoid an interpolation, use -20°C.
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