Operational Temperature Max is defined by two altitude categories. Which pairing is correct?

• A. Under 25000 ft uses ISA +37; Over 25000 ft uses ISA +31
• B. Under 25000 ft uses ISA +31; Over 25000 ft uses ISA +37
• C. Both categories use ISA +31
• D. Both categories use ISA +37

Answer: (A)

The key idea is that the maximum ambient temperature an aircraft can operate at isn’t a single fixed value; it’s defined relative to the International Standard Atmosphere (ISA) and split into two altitude bands because thermal and performance margins change with altitude.

Below 25,000 feet, the aircraft can tolerate a larger positive deviation from ISA, so the Operational Temperature Max is ISA plus 37°C. Above 25,000 feet, cooling and performance margins are tighter due to thinner air, so the allowable deviation is smaller—ISA plus 31°C. This two-tier approach accounts for how systems behave differently in denser, lower air versus thinner, high-altitude air.

So the pairing that matches this design is: under 25,000 ft uses ISA +37; over 25,000 ft uses ISA +31.