ISA Deviation

[thebusman]

Can anyone recommend a formula on how to calculate ISA Deviation?

Thanks

[john_tullamarine]

ISA deviation = OAT - ISA temperature for the height

ISA = 15 - 1.98 * Hp/1000

but we normally use 2 in lieu of 1.98.

e.g. planned cruise at pressure height (Hp) 3000 ft. Forecast OAT is 20C.

ISA dev becomes

= 20 - (15 - 2 * 3)

= 20 - (15 - 6)

= 20 - 9

= +11

e.g. 8000 ft and OAT -10

dev is

= -10 - (15 - 2 * 8)

= -10 - (15 - 16)

= -10 - (-1)

= -10 + 1

= -9

If the minuses cause a problem, this is the same as saying

ISA = -1C

dev is the number of degrees above or below ISA, so -10C is 9C colder so the deviation is -9C.

and so on. This all presumes that I can get my numbers above correct etc ...


(but, with an ATP, I'm sure that you knew all that ?)

[Old Smokey]

thebusman,

If you were looking for a "one liner" to use in a BASIC computer programme or such-like, the following should suit your purposes-

ISA Deviation = SAT-(-(15-PH/1000*1.9812)x(PH<36089.24)-56.5X-(PH=>36089.24))

Where SAT = Static Air Temperature in °C, and PH = Pressure Height in feet.

That will take care of business up to 82021 feet Pressure Height, after that a different set of parameters apply.

Regards,

Old Smokey

[oldebloke]

Crumbs guys Just double the altitude minus 15 until FL360..Thence it's -54 until about Fl450?
Mach to TAS,multiply M x 6.1-minus Ram rise..(eg:M8x6.1=488-28=460):ok:

[thebusman]

Thanks guys, appreciate your help. Oldebloke, since you mentioned Mach, how about to convert IAS to M?

Thanks

[oldebloke]

same way, TAS +RAT=488 divided by 6.1=M:ok:

[thebusman]

oldebloke, you're the man.... Thanks buddy, I appreciate your help.:ok: :cool: