DEV Question
 

Hi,

Is my calculation correct?.

38000 ft and an OAT -22

dev is

= -22 - (15 - 2 * 38)

= -22 - (15 - 76)

= -22 - (61)

= -22 + 39

= 17

Hi!

I think you have reversed the whole thing.

ISA at 38000 is 15-2*38 = -61 C

If the OAT is -22, then the ISA deviation should be +39 C.

DEV
 

Hi,

So it should be as follows.

dev is

= -22 - (15 - 2 * 38)

= -22 - (15 - 76)

= -22 - (-61)

= 39

That is almost correct, but you need to remember that the temperature in the stratosphere above 36000 feet is constant at -56(ish) degrees Celsius.

So if OAT = -22 at 38000 feet the ISA deviation = -22 - (-56) = +34

Remeber that ISA Deviation = OAT - ISA Temperature at that altitude.

Hi,

So the above calculation is correct up to 36,000 feet. After that its the OAT minus -56 degrees Celsius.

Yes, that is correct.

KW is absolutely correct regarding the tropopause. I completely forgot about that. :rolleyes:

Yes, as long as you do not go above the stratosphere.

Up to 36000 feet use ISA DEV = OAT - ISA Temperature at that altitude.

Above 36000 feet in the stratosphere use ISA DEV = OAT - (-56)

Which is of course ISA DEV = OAT + 56.

If you are studying for ATPL then that should cover all of the questions.