hello all,
correction rule for cold wx is : 4 FEET per °C isa deviation per 1000ft OR 1% per 2.5°C isa deviation, both rules give same result.
take FL180 & isa - 10°C gives 4ft x 10 x 18 = 720ft or (1% x 10)/2.5=4%. 4%from 18000ft= 720ft. so if QNH=std then at fl180 your true altitude = 720ft lower. so from high to low(pressure) & from hot to cold(temperatures) : look below.
i agree, its more user friendly to use correction tables than above formulae.
bm.
