ISA Overview and Application

Overview

This lecture covers the International Standard Atmosphere (ISA), explaining its role as a reference for atmospheric pressure and temperature in aviation and how deviation from ISA conditions is determined and used.

What is the ISA?

• ISA stands for International Standard Atmosphere, a model of atmospheric conditions used as a universal aviation reference.
• Developed by the International Civil Aviation Organization (ICAO) in the 1950s and published in ICAO Doc 7488.
• Provides standard pressure and temperature values at various altitudes for consistent comparison worldwide.

Purpose and Assumptions of ISA

• Local atmospheric conditions vary by weather, season, and location, so a global reference is needed.
• ISA assumes air is an ideal gas, free of moisture and particles.
• Used mainly for instrument calibration and aircraft performance determination.

Pressure and Temperature in the ISA Model

• At sea level: standard pressure is 29.92 inHg (1013 hPa), standard temperature is 15°C.
• Up to 10,000 ft, pressure decreases ~1 inHg per 1000 ft or 1 hPa per 30 ft.
• Temperature decreases by 2°C per 1000 ft within the troposphere (up to 36,089 ft).
• At 36,089 ft (tropopause), temperature remains constant at -56.5°C up to 65,617 ft.

ISA Model Calculation and Formula

• Standard temperature at altitude (in ft): ISA temp = 15 - 2 × (altitude/1000).
• Above 36,089 ft, the temperature is taken as -56.5°C.
• Actual published values are found in ICAO Doc 7488.

ISA Deviation

• ISA deviation = actual temperature minus ISA standard temperature at that altitude.
• Positive deviation ("ISA plus") means warmer than standard; negative ("ISA minus") means colder.
• Used to express current weather relative to ISA, affecting aircraft performance.
• Deviation changes throughout the day and with weather at any location or altitude.

Practical Applications

• Pilots calculate ISA deviation to use aircraft performance tables accurately.
• Some aircraft display ISA deviation automatically via onboard systems.

Key Terms & Definitions

• ISA (International Standard Atmosphere) — Reference model of atmospheric conditions (pressure and temperature) for aviation.
• ICAO — International Civil Aviation Organization, sets global aviation standards.
• Troposphere — Atmosphere’s lowest layer, where temperature decreases with altitude.
• Tropopause — Boundary between troposphere and stratosphere, where temperature becomes constant.
• ISA Deviation — The difference between actual temperature and ISA standard temperature at a given altitude.

Action Items / Next Steps

• Review ICAO Doc 7488 for detailed ISA tables.
• Practice calculating standard temperature and ISA deviation at various altitudes.
• Use ISA values and deviations when referring to aircraft performance charts.