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Old 21st Feb 2005, 18:48
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Capt H Peacock
 
Join Date: Jun 2001
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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!!
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