ISA stands for which term?

• A. International Standard Atmosphere
• B. International System of Aeronautical Standards
• C. Integrated Standard Atmosphere
• D. International Safety and Atmosphere

Answer: (A)

The main concept here is the standard name for a reference atmosphere used in aviation. International Standard Atmosphere is a defined model of how temperature, pressure, density, and viscosity change with altitude. It gives pilots and engineers a common baseline so performance calculations, instrument readings, and engine and airframe testing are consistent regardless of actual weather. It sets sea‑level values (temperature around 15°C and pressure about 1013.25 hPa) and a standard decrease in temperature with altitude (roughly 6.5°C per kilometer up to about 11 km). By using this model, altimeters, airspeed indicators, and performance charts can be calibrated and compared reliably.

Other phrases listed aren’t the recognized name for this aviation reference atmosphere, so they don’t serve the same standard role in calculations and instrument calibration.