Cold-weather altimeter correction

Cold air effect (TC AIM AIR 1.5.4):

• Denser than standard atmosphere: pressure levels sit lower.
• Altimeter over-reads; true altitude below indication.
• Equal surface pressure: cold column shorter, pressure falls faster with height. Pressure at a given altitude LOWER than in warm column (FAA AWH 8.2.4.2).

Rule (TC AIM AIR 1.5.4): indicated altitude off about 4% per 11°C the column's average temperature differs from ISA average.

Winter (TC AIM AIR 1.5.4):

• Strong low-level inversions likely; altimeters habitually over-read.
• Ground separation marginal: add 50% to error found from flight-level temperature.

Finding temperature deviation (use pressure altitude, not indicated):

1. Momentarily set 29.92" on subscale.
2. Read pressure altitude.
3. Compare ISA temperature at that level to actual OAT.
4. Apply 4%-per-11°C rule to indicated altitude; pressure altitude only finds deviation.

Worked example (TC AIM AIR 1.5.4):

• Indicating 6 000 ft on 29.80".
• Momentarily set 29.92": pressure altitude 6 120 ft.
• OAT -20°C: temperature error large enough to shrink mountain-crossing margin to just over 200 ft.

Corrected altitude (FAA AWH 8.4.1.3):

• Correcting indicated altitude gives corrected (approximately true) altitude.
• Flight computer derives it from OAT.
• Close enough to true altitude for terrain clearance once altimeter set from nearby reporting station.
• Reasonably accurate when actual lapse rate near Standard Atmosphere rate of 2°C per 1000 ft (TC AIM AIR 1.5.4).

Mountain wave compounds error (TC AIM AIR 1.5.8): with non-standard temperature, overread can reach 3 000 ft, far beyond temperature error alone.

Trap: 4%-per-11°C rule multiplies indicated altitude, not the pressure altitude used to find deviation.

Practice