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Thread: How to calculate true altitude
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Old 18th Oct 2023, 00:32
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eckhard
 
Join Date: May 2005
Location: France
Age: 69
Posts: 1,143
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ISA dev is -15

Alt error is approx 4ft per degree dev per 1000ft

So error is 4x15 = 60ft per 1000

60 x 20 = 1200

So true Flight Level is 188

QNH = 1033, so approx 20 x 30 = 600 ft correction

18800 + 600 = 19400
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