airspeed
The speed of an aircraft relative to the air through which it is flying.
airspeed: what the air around you tells you about motion
Airspeed is the velocity of an aircraft measured relative to the surrounding air mass, not relative to the ground. This distinction matters because wind carries the air itself. An aircraft flying 100 knots through calm air moves 100 knots over the ground; the same aircraft flying 100 knots through a 20-knot headwind moves only 80 knots over the ground. For flight control, the air speed is what matters.
The airspeed indicator uses a pitot-static system to measure this difference. A pitot tube points forward into the airstream, capturing dynamic pressure (ram air pressure). A static port samples undisturbed ambient pressure. The gauge subtracts static pressure from pitot pressure to display airspeed. This works because dynamic pressure increases with velocity: the faster you move through air, the higher the ram pressure becomes.
Indicated airspeed (IAS) is what the gauge shows. It assumes sea-level density and can lag at altitude. True airspeed (TAS) corrects for actual air density; at 10,000 feet, TAS may be 20 percent higher than IAS even at the same throttle setting. Equivalent airspeed (EAS) accounts for both density and instrument error. Pilots use IAS for flying the aircraft, but dispatchers use TAS for navigation and fuel planning.
In maintenance, a pitot tube blockage or static port icing causes airspeed failures. Dirt, insect nests, or ice over the opening reduces or prevents airflow, so the gauge either reads too low or fluctuates wildly. A blocked static port without a blocked pitot produces a climbing airspeed indication during descent. Both require visual inspection and cleaning or replacement. Pitot heaters must function in icing conditions.
Airspeed also defines aircraft performance limits. Stall speed varies with weight and configuration; a loaded twin-engine aircraft stalls at a higher airspeed than the same aircraft empty. Never-exceed speed (Vne) is the structural limit, often 320 to 360 knots for light aircraft and much higher for transports. Maintenance crews track airspeed-related incidents because a faulty reading can lead a pilot to operate outside safe margins.