Getting a little hung up on ISA DEV when you have a minus C

 

So lets say at FL100 and the OAT is 2c

 

My calculation is FL100 x 2 = 20 then take 15 minus 20 = -5c thats ISA TEMP at 10000ft

 

I would then take OAT 2 minus -5 = -3 ISA DEV but the correct answer is ISA DEV 6c

 

Why is that? If you have a minus you are going down the temp?

ISA at FL100 is -5C (if based on 2 degrees drop pr 1000 ft).

 

If the actual temp at FL100 is 2C, the deviation is actually ISA +7C. 

 

The atmosphere at FL100 is 7C warmer than the standard atmosphere. Makes sense?

So ISA DEV is infact how far its deviating from the standard atmosphere -5 c

Yes, you got it!

At FL180 the ISA temp is -21 and if you have an OAT of -20 your ISA dev will be 1c

 

What i done get is if you have a number thats minus (-20) for this example and you further deduct -21 wouldn't that take you to a much colder temp to -40 ? If you get what i mean but maths tells us its 1

DEV stands for 'deviation'. Deviation means the difference between. The difference between -21 and -20 is only 1. -20-(-21)=1. When you subtract a negative, you have to add it, as you should have learned in 5th grade math.

as you should have learned in 5th grade math.

Uncalled for.

That was not meant as an insult. If someone is going to be going for a pilot's license, then there are certain academic skills, such as grammar school math that they are going to need a good grasp of. If they don't then there will be points in their training that is going to be extremely difficult and end up befuddled by simple problems. This will cost him extra, money and stress. What this student should do is go back and review some basic skills such as math before wasting any more time on the ATP test, because there is enough new stuff to grasp that he doesn't need to be handicapped by a lack of basic math understanding.

Well this OP isn't doing any sort of training to become a pilot PPL or ATPL. I'm just very interested in all this sort of stuff. Well the job i'm in i can guarantee i'm on alot more than the basic FO makes at most airlines. And maths ain't required which is a positive for me