# Altimeter Temperature Error Correction

• Pressure altimeters are calibrated to indicate true altitude under ISA conditions. Any deviation from ISA will result in erroneous readings, except that the altimeter will read the correct elevation of the airfield regardless of temperature when the aircraft is on the ground with QNH set.
• When temperatures are lower than ISA an aircraft’s true altitude will be lower than the altimeter reading.
• The error is proportional to the difference between actual and ISA temperature, and the vertical distance of the aircraft above the altimeter setting datum.
• The height correction is 4 feet per degree Celsius deviation from ISA per 1000 feet.  Note: the calculation must be made over the indicated height difference from the datum for the pressure setting.

With temperatures colder than standard consideration must be given to the effect of temperature on terrain clearance.

For example:

A flight is planned at FL180 over Mont Blanc (elevation 15 782 ft). The mean sea level pressure is 983 hPa, from an aerodrome at mean sea level, and the temperature of the air up to the summit is 25°C colder than ISA. Determine the true altitude of the aircraft at Mont Blanc and hence the terrain clearance. See Figure below:

The indicated altitude is 18 000 ft above the 1013 hPa datum; the height correction for the 30 hPa pressure difference is:

30 × 27 = 810 feet so the corrected altitude is 17 190 ft. See Figure Below:

The height correction for the temperature deviation from ISA is:

4 × (- 25) × 18 = -1800 ft

Hence the true altitude of the aircraft is 15 390 ft. See Figure Below:

But Mont Blanc is 15 872 ft high so if we do not do something about it we will hit the mountain 392 feet below the summit. To simplify the calculation use the formula:

true altitude = indicated altitude + (indicated altitude/1000 × ISA deviation × 4) + 27(actual pressure – pressure setting)

For all of the following quiz assume that 1 hPa = 27 ft.

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