what is ISA

sixpack

sixpack

New Member
I've never used ISA, but have run across it a couple of times.
ISA = International Standard Atmosphere.
Looks to me like it's a temperature lapse rate, that works for high altitudes.

Can one of you jet guys give me a quick explanation? Do you use it reguarily, or just when studying for the ATP written?
 
15*C and 29.92"hg at sea level, is that what you're asking
confused.gif
 
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I've never used ISA, but have run across it a couple of times.
ISA = International Standard Atmosphere.
Looks to me like it's a temperature lapse rate, that works for high altitudes.

Can one of you jet guys give me a quick explanation? Do you use it reguarily, or just when studying for the ATP written?

[/ QUOTE ]

International Standard Atmosphere,

29.92" Pb 1013mb (hPa)
15deg C
 
I found the abbreviation. What I didn't find was how to use it.
What's the difference between the terms "Standard Temperature (15deg@0msl)" and "International Standard Atmosphere"?
Are they interchangeable at altitudes above 36000?

There was a question on the ATP test that made me think was non-linear.
It appears you use 2 degC/1000 upto 36000, then something different.

i.e. What is ISA temperature at FL420.
 
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i.e. What is ISA temperature at FL420.

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FL420 is an unusable alt. but ISA at FL430 is -57C.

Departure from ISA (warner) is used in regards to engine performance. ISA+10 or +15 at FL410 degrades power tremendously.
 
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i.e. What is ISA temperature at FL420.

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FL420 is an unusable alt. but ISA at FL430 is -57C.

Departure from ISA (warner) is used in regards to engine performance. ISA+10 or +15 at FL410 degrades power tremendously.

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Clearly, unless my arithmetic is bad, the lapse rate is non-uniform (not 2degC/1000ft for the entire range of altitudes).

<ul type="square"> T = +15 - 2*(43) = +15 - 86 = -71C [/list]
Can you elaborate on the lapse rate used for ISA.
Where does it deviate from the 2degC/1000.
Is it a formula, or do people just use some ISA chart?

Like I said, on the ATP written, there's a question the uses ISA, and the explanation says that 36000ft has the same temperature as 37000. I couldn't tell if they was saying "close enough for government work" or if the lapse rate for ISA flattens out (which if I remember right, happens in the atmosphere around this altitude).
 
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Clearly, unless my arithmetic is bad, the lapse rate is non-uniform (not 2degC/1000ft for the entire range of altitudes).

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You are correct. Above the tropopause (&gt;~36,000', aka stratoshpere) the ambient temperature remains relatively constant, meaning the rate of change of the temperature changes to zero.
 
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