ALTIMETER TEMPERATURE ERROR CORRECTION

Definition

Altimeter Temperature
Error Correction is applied to altimeters to compensate for error caused by
deviation from ISA conditions.

Description

Pressure altimeters are calibrated to ISA conditions. Any deviation from ISA will result in error proportional to ISA deviation and to the height of the aircraft above the aerodrome pressure datum.

According to ICAO PANS
OPS (Doc 8168) “The calculated minimum safe altitudes/heights must be
adjusted when the ambient temperature on the surface is much lower than that
predicted by the standard atmosphere. In such conditions, an approximate
correction is 4 per cent height increase for every 10°C below standard
temperature as measured at the altimeter setting source. This is safe for all
altimeter setting source altitudes for temperatures above –15°C. For colder
temperatures, a more accurate correction should be obtained according to the
guidance provided in section 4.3 “Temperature corrections”.

When temperature
is LESS than ISA an aircraft will be LOWER than
the altimeter reading.

For example, if the
OAT is – 40 °C then for a 2000 ft indicated altitude the true altitude is 1520
ft thus resulting in a lower than anticipated terrain separation and a
potential obstacle-clearance hazard.