What is ISA deviation and how should it be treated when applying performance charts?

• A. ISA deviation is the difference between ambient pressure and standard; charts assume ISA
• B. ISA deviation is the speed difference; charts require adjustments
• C. ISA deviation is the difference between actual temperature and standard ISA; charts assuming ISA may over/under-predict; apply temperature corrections
• D. ISA deviation is altitude difference; ignore

Answer: (C)

ISA deviation is the difference between the actual ambient temperature and the ISA temperature for that altitude. Charts that assume ISA don’t account for temperature differences, so when the air is warmer or cooler than ISA, air density changes and performance changes accordingly. If the air is warmer than ISA, the air is less dense, which tends to reduce engine/airframe performance and can cause takeoff and climb performance to be worse than the ISA-based charts predict. Because of that, you apply temperature corrections or use density altitude in the calculations.

To apply it, determine the ISA temperature for the altitude, compute the deviation (OAT minus ISA temperature), and then adjust the performance using density altitude. Use charts that are provided for density altitude or apply the POH’s temperature correction factors. In short, when there’s ISA deviation, you must correct for temperature because ISA-based charts can over- or under-predict performance.