Cold temperature correction

Altimetry — cold correction and ISA reference

The barometric altimeter is calibrated to the ICAO Standard Atmosphere (ISA). When the actual atmosphere differs from ISA (colder or warmer than standard), the altimeter shows incorrect values relative to true altitude.

Source: ICAO Doc 7488 Manual of the ICAO Standard Atmosphere; ICAO Annex 6 Part II; EASA AMC SERA; FAA-H-8083-25B PHAK Ch. 8.

Altimeter calibrated to ISA

The altimeter is calibrated to standard atmosphere. Deviations of the actual atmosphere from the standard atmosphere cause incorrect altitude values regarding the true altitude.

A barometric altimeter in aircraft operates by correlating measured air pressure to corresponding altitudes based on the International Standard Atmosphere model, providing an indicated altitude that assumes standard conditions.

True altitude — actual altitude

A true altitude is an altitude above mean sea level, corrected for non-standard temperature.

Q-codes — pressure references

Standard setting — pressure altitude

When the altimeter is set to standard, the altimeter indicates aircraft pressure altitude.

Pressure altitude is the vertical distance above the standard datum plane where atmospheric pressure is 1013.2 hPa, representing the height in the International Standard Atmosphere at that pressure level.

Higher QNH = higher indication

A higher QNH setting will cause a higher altitude indication:

• Example: QNH from 1013 to 1023 → +280 ft indication (1 hPa ≈ 28 ft at MSL).

Subscale — update before every flight and on cross-country

A current QNH needs to be set regularly, before every flight and during cross country flights.

The altimeter subscale needs to be reset before every flight and on cross-country flights.

Subscale setting calibrates the system

Setting of the pressure value in the subscale of the altimeter calibrates the entire measurement system to the chosen reference pressure.

Example: subscale change in descent

When resetting the altimeter from standard pressure to a lower QNH of 1003 hPa during approach from FL065, the indicated altitude decreases by approximately 270 ft due to the 10 hPa pressure difference:

• 1013 - 1003 = 10 hPa difference.
• 10 hPa × 28 ft/hPa = 280 ft (rounded 270 ft).
• FL065 after QNH setting becomes ~6230 ft AMSL.

Subscale error — what about a 5 hPa error?

An altimeter subscale setting error of 5 hPa causes a deviation in altitude indication of approximately 135 ft or 40 m, as each hPa roughly equates to 27 ft near sea level:

• 5 hPa × 27 ft/hPa = 135 ft error.
• 10 hPa error: 270 ft.
• Consequence: an inaccurate QNH = inaccurate altitude = terrain risk.

Pressure-error mnemonics

"From high to low, the mountains grow" — when flying into lower pressure without updating QNH, the altimeter indicates a higher altitude than actual, terrain appears lower and collision risk rises ("the mnemonic 'From high to low the mountains grow' warns pilots that when flying into lower pressure areas without adjusting QNH, the altimeter will indicate a higher altitude than actual, making terrain appear lower and increasing collision risk (assuming temperature effects are neglected)").

"HIGH to LOW, LOOK OUT BELOW" — from high to low pressure without QNH update → altimeter reads more than true altitude → aircraft is lower than indicated.

Flight toward low pressure — true altitude drops

During a flight with constant altimeter indication towards a low pressure area, the altitude above MSL reduces.

Temperature error — cold day

Flying in an air mass which is colder than the standard atmosphere, the altimeter will indicate a value too high compared to true altitude:

• Cold air = denser → corresponding MSL pressure → the altimeter thinks "pressure is low → aircraft high" but true altitude is lower.
• At -20 °C instead of ISA standard 0 °C at a given altitude: up to ~10 % error possible.

Low temperatures may lead to altitude indications that are too high.

Mountain winter vs summer

It is possible for an aircraft to fly below a mountain top in winter and above it in summer at the same atmospheric pressure and indicated altitude because when true air temperature is colder than the ICAO standard atmosphere:

• Winter (cold): actual altitude < indicated → could be below the summit.
• Summer (warm): actual ≈ indicated → stays above.

Cold correction — rule of thumb

For critical operations (mountain approach, IFR minimum altitudes) a correction must be added in cold weather:

(Exactly in ICAO Doc 8168 PANS-OPS Vol I, Table III-1-1.)

Altimeter check

The altimeter can be checked on the ground by setting the subscale so that the indication matches the airfield elevation and comparing the subscale to the actual QNH.

Hysteresis error

A hysteresis error is a potential fault in altimeters — mechanical lag of the aneroid capsule after rapid altitude changes.

Term recap

Transition altitude / level

• Transition altitude (TA): altitude at and below which the pilot must set QNH.
• Transition level (TL): first usable FL at least 1000 ft above TA.

Height becomes zero at touchdown

Height becomes zero when the aircraft touches the ground.