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Old 24th Jun 2011, 19:07
  #36 (permalink)  
Conan the Vulgarian
Join Date: Mar 2010
Location: Not from round these parts
Posts: 20
Q1. True altitude differs from indicated by 4% for every 10 degrees difference in temperature from ISA. True altitude is higher than indicated in warm air, lower in cold air. At 12,000 ft ISA temperature is -9 degrees, if the actual temperature is +9 then the temperature is ISA+18, so 1.8 x 4 = 7.2 percent difference, the actual altitude is 107.2% of the indicated.

Q2. If the QNH was 1013mb, you'd be at 13,500 feet when at FL135. Since if you increased the subscale setting to 1019 the indication would increase, then FL135 is actually a bit lower than 13,500 ft. (You're flying into higher pressure air than ISA, so you're higher than you would be if it were ISA). Since the radio altimeter says you are really at 13,500 feet, you must be in warmer than ISA air.

Q3. FL160 is 15,730 feet above sea level if the QNH is 1003. (FL measures from 1013 and since the pressure at sea level is 1003 the datum you are measuring from is below sea level). OAT is -27 wheras ISA temperature at your level would be -17, so it's ISA-10, so take off 4% to get 15100ft - 15090 is close enough.

Q4. 50ft and you just have to memorise these, although you could make a decent guess if you knew it was 27ft at sea level and about 100ft at airliner cruising levels, then 5500m=16000 odd feet will be somewhere in the middle.
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