Indicated Airspeed Calculator

Calculate Indicated Airspeed (IAS) from True Airspeed (TAS)

Simply enter the true airspeed, pressure altitude, and outside air temperature, and the calculator will instantly compute the indicated airspeed using the ISA model, accounting for altitude and temperature effects.

To compute Indicated Airspeed (IAS) from True Airspeed (TAS), the process involves correcting for air density, which is affected by Pressure Altitude, and for temperature using the ISA 1976 atmospheric model. First, the TAS is adjusted based on the air density at the given altitude by calculating the ratio between the current air density and standard sea-level density. Then, a correction is applied for temperature by considering the difference between the Outside Air Temperature (OAT) and the standard ISA temperature at that altitude. This method provides an accurate conversion from TAS to IAS, accounting for altitude and temperature effects.

What is Indicated Airspeed (IAS)?

Indicated Airspeed (IAS) is the airspeed read directly from an aircraft's pitot-static system, without any correction for altitude or temperature. It's the speed displayed on the airspeed indicator in the cockpit. IAS is crucial because it reflects the dynamic pressure experienced by the aircraft, which is directly related to the forces acting on the airframe, such as lift and drag.

Key Characteristics

• Depends on Air Density: IAS is influenced by the density of the air, which changes with altitude and temperature. As altitude increases, air density decreases, so IAS will read lower at high altitudes compared to the actual speed through the air (True Airspeed or TAS).
• Primary for Piloting: Pilots rely on IAS for maintaining safe flight operations, as it indicates aerodynamic performance. Critical speeds like stall speed, takeoff speed, and approach speeds are often given in IAS, since it reflects how the aircraft "feels" in the air.
• Not Corrected for Environmental Factors: IAS does not account for changes in pressure altitude, temperature, or compressibility effects at high speeds. To get an accurate measure of speed through the air, corrections need to be made, which result in True Airspeed (TAS).

How to Calculate Indicated Airspeed (IAS)?

To calculate Indicated Airspeed (IAS) from True Airspeed (TAS), Pressure Altitude, and Outside Air Temperature (OAT), you must adjust for the differences in air density and temperature at various altitudes using the International Standard Atmosphere (ISA) model. First, determine the atmospheric pressure and temperature at the given altitude, which decreases with height according to the ISA model. Next, compute the air density based on the calculated pressure and temperature. Finally, adjust the TAS using the ratio of the standard sea-level air density to the air density at the current altitude. This correction accounts for the thinner air at higher altitudes, which affects how the aircraft’s speed is measured by onboard instruments. Indicated Airspeed will always be lower than TAS in thinner air, reflecting the reduced aerodynamic force on the aircraft.

Step 1: Calculate Pressure (P)

• For altitudes below 11,000 meters:
  • P = 101325 × (1 - 0.0000225577 × h)^5.25588
• For altitudes between 11,000 and 20,000 meters:
  • P = 22632.1 × e^(-0.0001577 × (h - 11000))
• Where:
  • h = Altitude in meters
  • 101325 Pa is the standard atmospheric pressure at sea level
  • 22632.1 Pa is the standard atmospheric pressure at 11,000 meters

Step 2: Calculate Temperature (T)

• For altitudes below 11,000 meters:
  • T = 288.15 - 0.00649 × h
• For altitudes above 11,000 meters:
  • T = -56.5°C (constant)
• Where:
  • 288.15 K is the standard temperature at sea level
  • 0.00649 is the temperature lapse rate in the troposphere

Step 3: Calculate Air Density (ρ)

ρ = (P / (R × T_K)) × 1000

• Where:
  • ρ = Air density (kg/m³)
  • R = Specific gas constant for air (287.05 J/kg·K)
  • T_K = T + 273.15 (temperature in Kelvin)

Step 4: Calculate IAS from TAS

IAS = TAS × √(ρ / ρ₀)

• Where:
  • IAS = Indicated Airspeed
  • TAS = True Airspeed
  • ρ₀ = Sea level air density (1.225 kg/m³)
  • ρ = Air density at altitude (calculated in Step 3)

Limitations: The calculator is designed to work for altitudes up to 20,000 meters and is restricted to subsonic speeds, as it does not account for compressibility effects at supersonic speeds.