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Old 27th Jun 2011, 17:03   #37 (permalink)
Join Date: Jan 2005
Location: UK
Posts: 2
Altimeter Calculations

Use the following formulas for Altimeter calculations

TA = IA + (4 x IA/1000 x ISA Dev) + (27 x (QNH Subscale))

where TA = true altitude, IA = indicated altitude, Subscale = altimeter subscale setting, ISA Dev = ISA temperature deviation (I assume you can calculate this). This will work for the vast majority of all altimetry questions, except PPL questions use 30ft (not 27).

This will give you the exact answers to questions 1, and 3. I think you mean the indicated altitude at an MSA (true) of 12000ft in Q1.

Question 2 is slightly flawed in that radio altimeters operate only up to 2500ft, and actually read height above the surface directly below rather than altitude. If you put this aside you can determine that the ISA dev is -ve, ie colder using the equation above.

The following formula

Height change per hPa(ft)= 96 x T/P

where T= temperature in Kelvin (C + 273), P = Pressure in hPa

In Q4, T= 252K, ISA in C is -21, as 5500m is approximately 18000ft, P = 500hPa

That gives
Height change per hPa = 96x252/500 = 48.4ft
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