Indicated airspeed

Indicated airspeed is the speed displayed by an aircraft's airspeed indicator. The instrument derives it from the difference between total pressure from the pitot system and static pressure from the aircraft's static ports. It is normally read in knots and is abbreviated KIAS when expressed as knots indicated airspeed.

How it works

The pressure difference responds to dynamic pressure, which depends on air density and the square of speed through the air. The instrument is calibrated to the standard atmosphere at sea level. Because the calibration assumes standard density, indicated airspeed is not the same as the aircraft's actual speed through the air when altitude or temperature changes.

Many aircraft operating speeds are published as indicated airspeeds. Stall speeds, flap limits, maneuvering speed, and never-exceed speed are useful on the indicator because the associated aerodynamic loads and pressures relate closely to indicated speed. The applicable POH or AFM takes priority over general explanations.

Example

An aircraft may show 100 KIAS at sea level and at a higher altitude. In the higher, colder or warmer air, its true airspeed will differ even though the cockpit indication remains 100 knots. The airspeed indicator continues to provide the reference needed for the aircraft's published operating limits.

Why it matters

Pilots use indicated airspeed for control, configuration changes, and aircraft limitations. If the pitot or static system is blocked, the indication may become unreliable or behave differently as altitude changes. Preflight checks and the aircraft's approved procedures address those failure modes.

True airspeed corrects indicated airspeed for air density. Density altitude also describes the air density affecting aircraft performance, but the two terms answer different questions. One describes a speed reference, while the other describes an atmospheric condition.

Related terms and tools

• True airspeed
• Density altitude
• FAA Pilot's Handbook of Aeronautical Knowledge