This glossary is written for virtual airline and flight simulation use within Virtual Air Canada Airline. It is not intended as a real-world aviation reference.
ISA
International Standard Atmosphere
The International Standard Atmosphere is a defined model of how atmospheric temperature, pressure, and density vary with altitude, used as a universal reference baseline for aviation performance calculations. ISA defines sea level conditions as 15 degrees Celsius, 1013.25 hPa (29.92 inHg), and a density of 1.225 kg per cubic metre. Temperature decreases at a standard lapse rate of 2 degrees Celsius per 1,000 feet through the troposphere, reaching -56.5 degrees Celsius at the tropopause (approximately 36,089 feet), above which temperature is assumed constant in the lower stratosphere.
Because actual atmospheric conditions rarely match the ISA model exactly, performance data is expressed as deviations from ISA - referred to as ISA+X or ISA-X. For example, a day where the temperature at cruise altitude is 10 degrees warmer than the ISA value for that altitude is described as ISA+10. This deviation directly affects engine performance, climb gradients, cruise fuel burn, and maximum altitude capability. Aircraft flight manuals, performance tables, and computerized dispatch systems all reference ISA as the baseline, so understanding ISA deviation allows crews to interpret performance data correctly and set accurate expectations for fuel burn and thrust requirements.