Indicated airspeed

This value is not corrected for installation error, instrument error, or the actual encountered air density,[2] being instead calibrated to always reflect the adiabatic compressible flow of the International Standard Atmosphere at sea level.

Since the actual density will vary considerably from this assumed value as the aircraft changes altitude, IAS varies considerably from true airspeed (TAS), the relative velocity between the aircraft and the surrounding air mass.

These speeds, in true airspeed terms, vary considerably depending upon density altitude.

[4] Furthermore, the IAS is specified in some regulations, and by air traffic control when directing pilots, since the airspeed indicator displays that speed (by definition) and it is the pilot's primary airspeed reference when operating below transonic or supersonic speeds.

where: The IAS is not the actual speed through the air even when the aircraft is at sea level under International Standard Atmosphere conditions (15 °C, 1013 hPa, 0% humidity).

TAS is the primary method to determine aircraft's cruise performance in manufacturer's specs,[2] speed comparisons and pilot reports.

Ground speed is just a pilot aid to estimate if the flight is on time, behind or ahead of schedule.

Supersonic aircraft, like the Concorde and military fighters, use the Machmeter as the main speed instrument with the exception of take-offs and landings.

Since the IAS often starts at around 74–93 km/h (40–50 kn) (on jet airliners), pilots may need extra help while taxiing the aircraft on the ground.

A basic airspeed indicator with the indicated airspeed (IAS) indicated in knots ("Kt" or "Kts" or "KIAS") -- the most common unit of measure for airspeed. Some airspeed indicators in aircraft prior to the mid-1970s indicate in miles per hour plus knots (1 knot = 1.15 mph) or kilometers per hour (1 knot = 1.85 km/h).
A primary flight display with the indicated airspeed (IAS) displayed in the form of a vertical "tape" on the left.