Altimeter Correction - Temperature
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Altimeter Correction - Temperature
The Chief Standards chappy hit me with this one today...and not as part of my yearly beasting session, but mainly because no one could agree how to apply the correction or interpret the tables.
Now assume the airfield in question lies at 2000 ft amsl, and the aerodrome OAT is -10 deg C, the ILS MDH is 200ft. Now according to the tables in the American equivalent of the ERS the pilot should add 20 ft to the approach minima, making the minima now 220 ft indicated. Now assume that we now have a platform 5000 ft above the airfield, at 7000 ft amsl. The temp up there (assuming ISA lapse rate 2 deg/1000 ft) is measured at -20 deg C. According to the tables the correction for a 200 ft ILS approach to the platform is also 20 ft, as the temp was measured on the platform. Is that correct..or have I misinterpreted it? In free air the the altimeter correction at 5000 ft above the airfield with an OAT of -10 deg C, assuming ISA lapse rates temp would be -20 deg C up there, would be 710 ft. How do you guys apply this correction flying into cold and high airfields (Kabul etc)?
The ICAO Air Ops Procedures doc Vol 1, states that corrections must be added to minima when the surface temp is lower than ISA. Approx 4% height increase per 10 deg variation from below standard temperature measured at the altimeter setting source. Now reading that makes me think that it's not the OAT that should be important, but the difference between OAT and ISA temp. Or have I just tied my self in knots?
Now assume the airfield in question lies at 2000 ft amsl, and the aerodrome OAT is -10 deg C, the ILS MDH is 200ft. Now according to the tables in the American equivalent of the ERS the pilot should add 20 ft to the approach minima, making the minima now 220 ft indicated. Now assume that we now have a platform 5000 ft above the airfield, at 7000 ft amsl. The temp up there (assuming ISA lapse rate 2 deg/1000 ft) is measured at -20 deg C. According to the tables the correction for a 200 ft ILS approach to the platform is also 20 ft, as the temp was measured on the platform. Is that correct..or have I misinterpreted it? In free air the the altimeter correction at 5000 ft above the airfield with an OAT of -10 deg C, assuming ISA lapse rates temp would be -20 deg C up there, would be 710 ft. How do you guys apply this correction flying into cold and high airfields (Kabul etc)?
The ICAO Air Ops Procedures doc Vol 1, states that corrections must be added to minima when the surface temp is lower than ISA. Approx 4% height increase per 10 deg variation from below standard temperature measured at the altimeter setting source. Now reading that makes me think that it's not the OAT that should be important, but the difference between OAT and ISA temp. Or have I just tied my self in knots?
Last edited by Hueymeister; 21st Feb 2005 at 14:05.
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Huey - please use 'search' in 'Tech Log' for 'altimeter' or 'cold weather'. There are numerous threads on this topic which should keep you and your 'CSC' happy for days 
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Ok, not the most scintilating of topics me'old, but the bloke got me 1/2 an hr before the end of a 7 day 110 duty hr SAR stint, I was tired and vulnerable!!!!! and I didn't have a yellow book with me......and now to bed!
Last edited by Hueymeister; 22nd Feb 2005 at 10:02.
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The temperature correction applies to the layer of air between the aircraft and the altimeter setting reference. In your case this means the Kabul aerodrome QNH and the altitude of the aircraft. The practical application for this correction is that you use the average temperature of the layer of air beneath you as a function of ISA, (ISA-10 in your example) and calculate a correction based of 4% per 10 degree difference from ISA applied to the QFE value so to speak.
Gobbledigook? So platform altitude 7000’, ISA-10, touchdown zone elevation 2000’, platform height 5000’. 4% of 5000’ is 200’ so your altimeter will read 7000 plus 200’ at platform altitude when set to aerodrome QNH.
If you’re doing a VOR approach then these corrections are CRITICAL to ensure obstacle clearance, especially in cold high altitude airfields. If you’re doing the ILS, expect to arrive at the platform altitude/distance LOW on the glideslope in LOW temperatures. You might want to consider flying above that platform altitude until intercepting the glideslope if terrain is a factor.
You can verify the correction by using the radio altimeter after having taken account of the terrain under the approach path.
Better still, fly south, it’s warmer!!
Gobbledigook? So platform altitude 7000’, ISA-10, touchdown zone elevation 2000’, platform height 5000’. 4% of 5000’ is 200’ so your altimeter will read 7000 plus 200’ at platform altitude when set to aerodrome QNH.
If you’re doing a VOR approach then these corrections are CRITICAL to ensure obstacle clearance, especially in cold high altitude airfields. If you’re doing the ILS, expect to arrive at the platform altitude/distance LOW on the glideslope in LOW temperatures. You might want to consider flying above that platform altitude until intercepting the glideslope if terrain is a factor.
You can verify the correction by using the radio altimeter after having taken account of the terrain under the approach path.
Better still, fly south, it’s warmer!!
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In the aircrafts I fly, the FMC gives you a possibility to put it values for temperature correction. Then it is automatically transfered to ADC and your instruments. I believe...
Also, as said before, in some countries, when it is very cold, ATC gives you corrected value for initial approach altitudes with a phrase ("xxx123 decent to 1500 ft on QNH 1023 corrected for temperature error by ATC"), or something.
Kabul airfield´s elevation is almost as high as 5800 ft and there you have to be extra carefull in any way and sense. Temperature correction being just one of them.
Great answer by the way Capt H. P.
Cheers!
Also, as said before, in some countries, when it is very cold, ATC gives you corrected value for initial approach altitudes with a phrase ("xxx123 decent to 1500 ft on QNH 1023 corrected for temperature error by ATC"), or something.
Kabul airfield´s elevation is almost as high as 5800 ft and there you have to be extra carefull in any way and sense. Temperature correction being just one of them.
Great answer by the way Capt H. P.
Cheers!
