airspeed indicator

The airspeed indicator measures the speed at which an aircraft moves through the air mass around it, not over the ground. It does this by sensing the difference between static pressure (the ambient air pressure) and dynamic pressure (the pressure created by the aircraft pushing through air). That pressure difference is fed into the instrument via two separate plumbing lines: the pitot tube (which captures ram air) and a static port (which measures still air), giving the instrument the raw data it needs to calculate and display airspeed in knots.

Most general aviation airspeed indicators are analogue gauges with a needle sweeping across a colored arc. The face is marked with several speed ranges: a white arc (flap operating range), a green arc (normal operating range), a yellow arc (caution range, where the aircraft can fly but turbulence must be avoided), and a red line marking never-exceed speed. On some aircraft you will also see a blue line indicating best climb speed. The needle responds to changes in air pressure within milliseconds, though the instrument itself is mechanical and prone to lag in sudden maneuvers.

Blockages are the airspeed indicator's primary failure mode. If the pitot tube ices over or gets plugged with debris or insect matter, the instrument reads false and unreliable. A clogged static port causes similar problems. Pilots are trained to recognize unreliable airspeed readings by cross-checking against other instruments (vertical speed indicator, attitude indicator, engine parameters) and by feel. The needle may also stick or become sluggish due to internal friction or damaged linkages.

Variants and calibration

Glass cockpits display airspeed digitally and often show both indicated airspeed (what the gauge reads) and true airspeed (what the aircraft actually moves through the air at, corrected for altitude and temperature). Some aircraft have heated pitot tubes to prevent icing; others rely on alternate static sources that can be manually selected if the primary port fails. Airspeed indicators must be calibrated and certified; during maintenance they are often checked against a pitot-static test set to verify accuracy within acceptable tolerances.

The terms airspeed and ground speed are often confused. Airspeed is what matters for aerodynamics and flight envelope protection; ground speed (measured by GPS or inertial systems) tells you how fast you are actually moving across terrain. A headwind reduces ground speed while holding airspeed constant. This distinction is critical in takeoff and landing calculations, where airspeed determines when the wings generate enough lift to fly, regardless of wind.