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I just came across this question in Exercise 1.5 in perfomance book.
The pressure height is -270.
The ISA temperature what I thought was 15+(2*0.25).
But the ISA temp was calculated as 15+2*0.5
I am a bit confused here. Can you please help me get around this. Screenshot attached.

bobtait replied the topic: BT book exercise question

Also, referring to your original question. There has to be a limit to the degree of interpolation in these situations. I'm sure even CASA would agree. 1 hPa for every 30ft is itself an approximation so the calculated pressure height of -270ft is not likely to be exact. We do encourage students to use the calculation (flow chart) method for an exam because it eliminates the possibility of misreading a scale on the computer and it's just as time efficient. CASA accept an answer based on a flight computer which would be nowhere near that level of accuracy.

Surely, we are not going to start working with fractions of degrees of temperature. Based on the assumption of 2° per 1000ft temperature lapse rate, -270ft could be rounded up to the nearest 500ft to give an answer of 15 - 1 = 16°. Even CASA accept that.