View Full Version : DEV Question
Michael_B767_ATP
17th Jun 2011, 18:41
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
GearDownThreeGreen
17th Jun 2011, 19:26
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.
Michael_B767_ATP
17th Jun 2011, 19:39
Hi,
So it should be as follows.
dev is
= -22 - (15 - 2 * 38)
= -22 - (15 - 76)
= -22 - (-61)
= 39
keith williams
17th Jun 2011, 19:45
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.
Michael_B767_ATP
17th Jun 2011, 19:58
Hi,
So the above calculation is correct up to 36,000 feet. After that its the OAT minus -56 degrees Celsius.
GearDownThreeGreen
17th Jun 2011, 20:01
Yes, that is correct.
KW is absolutely correct regarding the tropopause. I completely forgot about that. :rolleyes:
keith williams
17th Jun 2011, 20:03
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.