Difference
between pressure alt and true alt = 4ft per 1,000ft per ºC dev
SO
4 x 13 x ºC
dev = 1,000ft ºC dev = 1,000/(4
x 13) = 19.23º colder than ISA because true altitude is lower than pressure
altitude
OR
On CRP5 put
true altitude (12,000ft) on outer main scale next to pressure altitude
(13,000ft) on inner main scale. Go to the ALTITUDE window and read
air temperature next to pressure altitude (FL130) = -32ºC
FL130 ISA = +15º - (2 x 13) = -11ºC
Outside Air
Temperature = -32ºC
Deviation from
ISA
= -20ºC