Indicated Altitude

Indicated altitude is the altitude read directly from the barometric altimeter with the current altimeter setting (QNH) set in the Kollsman window. It is the altitude used for all ATC altitude assignments, cruising altitude rules, and approach minimums in Canadian airspace below FL180.

Indicated vs. True Altitude

Indicated altitude equals true altitude (actual height above MSL) only when the temperature is exactly ISA standard. In cold air, the altimeter over-reads — the aircraft is lower than indicated. This is the cold temperature altitude error. At ISA −30°C, indicated altitude can be significantly higher than true altitude — relevant for IFR approaches in Canadian winters.

Cold Temperature Correction

Transport Canada requires altitude corrections for IFR approaches when temperatures are below −15°C for certain aerodromes. The correction uses the ICAO cold temperature altitude correction table. Some GPS/FMS units calculate the correction automatically; verify in the approach charts whether corrections are required.

Related Terms

See also: True Altitude, Altimeter, Altimeter Setting.