I was told that the correction you have to apply to the altimeter equals:
4% of the height (so AGL) per 10°C ISA deviation.
When flying in colder than ISA the altimeter will overread (indicate a higher altitude than your actual altitude) so you would have less terrain clearance than you would expect...
OK, let 's imagine you would like to fly over a mountain of 16000 ft with a vertical separation of 2000 ft. When flying at FL180 (let us assume the QNH is 1013 hPa) what would be the actual terrain clearance knowing that the temperature is ISA-10°C?
It is obvious that, when flying at FL180 I will not have the 2000 ft vertical separation, but what will be my actual vertical separation with that mountain?
Do you take the 4% of the 18000 ft or take the 4 % of the 2000ft (height)???? I always thought you had to take the 4% of the 18000ft but then somebody argued that the first 16000ft isn't air but mountain and can therefore not be compressed due to the lower temperature...
Does anyone know which I should take and why?
Thanks!
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