Aviation Calcs

Cold temperature correction

Cold Temperature Altitude Correction Calculator

In cold air a barometric altimeter reads high — you are lower than indicated. Enter the reported airport temperature and a published altitude (or a height above the field) to get the correction from the ICAO Doc 8168 formula, the AIM Table 7-3-1 value when the height and temperature fall in the table, and the 4% rule of thumb. Fly a higher indicated altitude by the correction; do not change the Kollsman-window setting.

Correct
MSL, same units as elevation

The correction depends on height above the altimeter-setting source, not on altitude MSL. An MSA high above the field needs a much larger correction than the MDA.

Altitude to fly

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Why the altimeter reads high in the cold

A sensitive altimeter is calibrated to the standard atmosphere. When the column of air is colder than ISA, the true height of a given pressure surface is lower than the instrument indicates — “look out below.” To fly the charted obstacle-clearance height, you must fly a higher indicated altitude by the correction. The true altitude calculator shows the related E6B identity; this page is the operational correction you add to a published approach altitude.

The ICAO formula and the AIM table

ICAO Doc 8168 (PANS-OPS) gives the approximate correction, with heights in feet and temperatures in °C:

t0 = taerodrome + 0.00198 × Hss
Δh = H × (15 − t0) / (273 + t0 − 0.5 × 0.00198 × (H + Hss))

H is the height of the published altitude above the altimeter-setting source; Hss is the elevation of that source (normally the airport). The 4% rule of thumb — add 4% of the height for every 10 °C below ISA at the source — is a coarser check, described by ICAO as acceptable above about −15 °C.

AIM Table 7-3-1 is the ICAO cold-temperature error table. It assumes a sea-level airport, so it is slightly conservative at higher airports. This page interpolates between tabulated heights and temperatures, and always reports the formula as well. A common operator practice is to round the correction up to the next 10 or 100 ft; both rounded indicated altitudes are shown.

Cold Temperature Airports

In the United States, designated Cold Temperature Airports carry a snowflake symbol and a temperature on the approach chart. When the reported airport temperature is at or below that value, correct the segments the CTA list requires (intermediate, final, missed approach, or all). Use the reported airport temperature and the height above the airport. Never change the altimeter setting to accomplish the correction, and do not correct assigned radar-vector altitudes.

Sources

ICAO Doc 8168, Aircraft Operations (PANS-OPS), temperature correction of minimum altitudes; FAA Aeronautical Information Manual Table 7-3-1 (ICAO Cold Temperature Error Table); FAA Cold Temperature Airports procedure. For a generic true-altitude check, see the true altitude calculator.

Frequently asked questions

At a 1,000 ft airport, −25 °C, and a 3,000 ft FAF, how much do I add?

The ICAO formula gives about 308 ft, so you would fly about 3,308 ft indicated, or 3,310 ft if you round the correction up to the next 10 ft. Uncorrected, your true altitude at a 3,000 ft indication would be a few hundred feet below the charted FAF.

Should I change the altimeter setting to make the correction?

No. Leave the official QNH in the Kollsman window and fly a higher indicated altitude by the correction. Changing the setting would move every subsequent assignment and is not the approved method.

How is this different from the true altitude calculator?

The true altitude calculator answers “if I am indicating this, what is my true height?” This page answers the operational inverse: “what indicated altitude should I fly so that my true height matches the published value?” It also reports AIM Table 7-3-1 and the 4% rule used in approach procedures.