12000ft on 1030 QNH It is stated that temp deviation is +10 deg cel from ISA
To calculate take 3 key figures
4 :Allow 4ft per 1000ft above press datum
10 :Temp deviation from ISA (+10deg cel)
12 :Min safe alt 12000ft (or height in question)
4x10x12=480 ft
ISA deviation warmer; alt underreads
ISA deviation colder; alt overreads
Temp is warmer; alt underreading; 12000ft - 480ft = 11520ft
No rules of thumb and I hope easier to understand.
Sorted
