Delta Alt true = Delta Alt ind +/- (4 x ( T - T std) x Delta Alt ind) / 1000
Delta Alt is the air layer from wich you have to correct the temperature
( ie if the aircraft is at 2000 ft sea level but at 1000ft over the ground, you wil have to correct 1000ft)
+ if temp over std
- if temp below std
Ex: Aircraft 2000ft above the groun on std - 25 ( indicated temp at 2000ft= -14 deg cel) => true alt = 2000 - (4 x 25 x 2000) / 1000 = 1800 ft