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Altimeter Temperature Error Correction
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Altimeter Temperature Error Correction is applied to altimeters to compensate for error caused by deviation from ISA conditions.
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.
When To Apply Corrections
When the aerodrome temperature is 0°C273.15 K
491.67 °R or colder, the temperature error correction must be added to:
- DH/DA or MDH/MDA and step-down fixes inside the final approach fix (FAF).
- All low altitude approach procedure altitudes in mountainous regions (terrain of 3000 ft914.4 m AMSL or higher)
According to ICAO PANS OPS Chapter 4 "Altimeter Corrections" pilot-in-command is responsible for the safety of the operation and the safety of the aeroplane and of all persons on board during flight time (Annex 6, 4.5.1). This includes responsibility for obstacle clearance, except when an IFR flight is being vectored by radar.
When pilots intend to apply corrections to the FAF crossing altitude, procedure turn or missed approach altitude, they must advise ATC of their intention and the correction to be applied.
Pilots may refuse IFR assigned altitudes if altitmeter temperature error will reduce obstacle clearance below acceptable minima. However, once an assigned altitude has been accepted, it must not subsequently be adjusted to compensate for temperature error.
Minimum Sector Altitude
Currently, there is not a European-wide common procedure to deal with adjustments to Minimum Sector Altitudes (MSAs). Some regulators do not specify adjustments to MSAs and consequently ATC providers do not apply a temperature correction to published MSAs for cold temperatures. It is the flight crew reponsibility according to the provisions of ICAO PANS OPS referred above.
Some operators advise flight crews to add 1000 ft to the MSA when the temperature is - 30 °C or colder. (RAF FIH)
Minimum Vectoring Altitude
MVAs are established for use by the Air Traffic Controller (ATCO) when Air Traffic Control (ATC) provide a surveillance service (usually radar). Each MVA chart contains sectors large enough to accommodate the vectoring of aircraft within the sector at the MVA. The minimum vectoring altitude in each sector provides 1000 ft above the highest obstruction in non-mountainous areas and 2000 ft above the highest obstacle in designated mountainous areas.
According to ICAO PANS OPS minimum vectoring altitudes shall be corrected for temperature. The temperature correction shall be based on seasonal or annual minimum temperature records. In turn, ATC authorities are required, as per ICAO PANS ATM, 126.96.36.199, Note 2, “to provide the controller with minimum altitudes corrected for temperature effect”.
- Flight Safety Foundation ALAR Briefing Note 3.1 — Barometric Altimeter and Radio Altimeter
- Airbus ALAR Briefing Note 3.1 — Barometric Altimeter and Radio Altimeter
- Royal Air Force Flight Information Handbook
- see also ICAO Doc 8168 - PANS-OPS, Volume 1, Chapter 4 and associated tables.