ISA and ISA± deviations describe temperature deviations and their effect on engine thrust and aerodynamics. Which statement is correct?

• A. ISA is the Standard Atmosphere temperature; deviations indicate ambient temps hotter or colder than ISA, with higher temps reducing air density, lowering engine thrust and lift; and colder temps increasing density.
• B. ISA is the standard altitude reference.
• C. ISA deviations only affect cabin humidity.
• D. ISA stands for International Standard Air.

Answer: (B)

ISA defines a standard atmosphere with temperature, pressure, and density that change with altitude, and ISA± deviations describe how the actual air temperature differs from that standard. When ambient air is hotter than ISA, it becomes less dense, which reduces engine air mass flow, lowers thrust, and decreases lift. Cooler-than-ISA air is denser, increasing thrust potential and aerodynamic lift. This is why the correct understanding is that ISA is the standard atmospheric model used as a reference for temperature with deviations indicating hotter or colder conditions and the resulting density and performance effects. The other statements misstate what ISA is or what deviations affect (for example, treating ISA as only a altitude reference, limiting deviations to cabin humidity, or defining ISA acronym incorrectly).