Indicated airspeed (IAS) is the airspeed value shown directly on the airspeed indicator in the cockpit, based on the pressure difference between the air hitting the aircraft and the surrounding air.
How It Works#
The airspeed indicator measures two types of air pressure. Pitot pressure (also called ram air pressure) is collected by a forward-facing tube on the aircraft. Static pressure is the ambient air pressure around the aircraft, collected through separate ports on the fuselage or pitot tube housing.
The instrument compares these two pressures. A greater difference means faster airflow, which means higher speed. The dial then displays this as a speed in knots or miles per hour.
IAS does not account for air density, temperature, or altitude. It reads the pressure difference only. This is why IAS differs from your actual speed through the air at higher altitudes, where the air is thinner.
Example in Aviation#
A student pilot climbs through 8,000 feet and notices the airspeed indicator reads 100 knots. That is their indicated airspeed. Their actual speed through the air (called true airspeed, or TAS) is higher, perhaps around 115 knots, because the thinner air at altitude produces less pressure for the same physical speed.
Despite this difference, the pilot still uses IAS to fly. The aircraft's stall speed, flap limits, and approach speeds are all published as IAS values. The aircraft "feels" the same pressure regardless of altitude, so IAS remains the correct reference for aircraft performance.
Why It Matters#
Pilots use IAS as their primary speed reference for nearly every flight maneuver. Stall speeds, best climb speeds, and landing approach speeds are all defined in IAS. Flying by IAS keeps aircraft handling consistent and predictable across different phases of flight.
Understanding IAS also helps pilots recognize its limits. Navigation and wind correction require true airspeed, not IAS. Knowing the difference prevents errors in flight planning and cruise performance calculations.
Key Takeaways#
- IAS is the speed reading shown directly on the cockpit airspeed indicator.
- It measures the pressure difference between pitot (ram) air and static air.
- IAS does not correct for altitude, temperature, or air density.
- Aircraft performance limits (stall speed, flap speeds) are always published in IAS.
- True airspeed (TAS) is IAS corrected for altitude and temperature.