For operation temperature max below 25000 ft, what ISA deviation applies?

• A. ISA +31
• B. ISA 0
• C. ISA +37
• D. ISA -37

Answer: (C)

ISA deviation is how far the actual temperature is from the ISA temperature at a given altitude. A positive deviation means the air is warmer than ISA, which raises the density altitude and reduces engine and aerodynamic performance. For operation with a maximum temperature below 25,000 ft, the standard upper limit used in performance references is +37°C. This reflects the warmest permissible non-ISA condition in that altitude band, ensuring enough performance margins are accounted for. A neutral deviation (ISA 0) ignores non-ISA warmth, and a negative deviation would imply colder-than-ISA conditions, which isn’t the scenario described here. The value +37°C is the appropriate upper bound for that altitude range, so ISA +37 is the correct choice.