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CPL Performance Book pg 38 ISA Temp Calculation

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SHEEKEY created the topic: CPL Performance Book pg 38 ISA Temp Calculation

Hi Bob,

I am working through the CPL performance book pg 38. P charts. If given density height "enter chart on the left hand scale and move horizontally to the right to the ISA temperature"

ISA Temp = 15-(2x the number of thousands of feet) using pressure height.

Given Elevation of 1200 ft
ISA Temp = 15-(2x1)= 13 deg c

The book is using ISA temp calculated using 2000 ft (density altitude). ISA Temp = 15-(2x2) = 11 deg c

Is this correct? Should we be using pressure height or density height to work out the ISA Temp?

Also does CASA have guidance material on how to use these charts.

Regards,

Steve
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SHEEKEY replied the topic: CPL Performance Book pg 38 ISA Temp Calculation

I should of posted this in the performance category, sorry. Feel free to transfer it over to the correct category. I'm new here.

Regards,

Steve
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bobtait replied the topic: CPL Performance Book pg 38 ISA Temp Calculation

If the chart doesn’t have a density height entry scale and you are given a density height in the question data, enter that density height value on the pressure height scale and go to the right to the ISA temperature for that pressure height. ISA temperature for any given pressure height is density height.
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bobtait replied the topic: CPL Performance Book pg 38 ISA Temp Calculation

If the chart doesn't have a density height entry argument and the question gives you a density height as part of the data do the following. If necessary, calculate the pressure height if the the elevation and QNH is given or, if the pressure height is given, enter at that pressure height on the left-hand pressure height scale.

If the examiner gave you a DENSITY HEIGHT, he/she would not have given you a temperature

So, go across to the ISA TEMPERATURE AT THAT PRESSURE HEIGHT. Pressure height at ISA temperature IS density height. Proceed around the chart as usual. Note that a density height already has accounted for temperature, so if a density height is given, there will be no mention of temperature.
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  • John.Heddles
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  • ATPL/consulting aero engineer
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John.Heddles replied the topic: CPL Performance Book pg 38 ISA Temp Calculation

An additional comment to Bob's observations, if I may. If the charts have been drawn with a bit of care, then the bottom line of the density altitude carpet will be scaled to density height, regardless of whether the scale is explicit or implicit. That is to say, if you follow Bob's technique suggestion, the point you arrive at will be the density height as scaled on the other axis of the carpet.

Engineering specialist in aircraft performance and weight control.
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