Motion of an aircraft, relative to the parcel of air it is currently flying in, is termed as airspeed. Aircraft speed is, however, further categorized in terms of its relation with the airspeed indicator and its dependence on values such as atmospheric pressure, density and temperature.
This concept of categories or a different criterion to measure a value which is apparently the same in both cases is often confusing. To understand the difference between indicated airspeed and true airspeed, one must first understand how these terms are generally defined.
Defining Indicated Airspeed and True Airspeed
The indicated airspeed (IAS) of an aircraft is simply the value that an airspeed indicator denotes on its scale. The value is obtained through a pitot-static system which includes a pitot tube and two static vents. Indicated airspeed is used aerodynamically, and is important to aircraft performance. Speeds related to take-off, stall, lift, turns, etc. are all in terms of IAS.
On the other hand, the true airspeed (TAS) of an aircraft is a relative measurement. The actual flight speed of an airplane relative to an air mass is termed as true airspeed, and it is primarily used for navigational purposes. In short:
- Flight speed shown on the instrument (not corrected for instrument error, altitude, density and temperature) is called indicated airspeed.
- The actual speed of an aircraft through the air is termed as true airspeed.