Types of Airspeed

Measuring speed on ground is much easier for vehicles like cars, buses, bikes, etc. by the use of speedometer but in the case of aircraft. Airspeed is not simply what you see inside the cockpit reading through the indicator. Depending upon the instrumentation terms and physical properties of air, Airspeed can be classified as :

1. Indicated Airspeed 

Airspeed as indicated by the airspeed indicator inside cockpit that the pilot reference for speed changes. Indicated airspeed is the measured speed of an aircraft as it moves through the air, it is based on the pressure readings collected by the pitot-static system. It changes with the altitude as at higher altitudes the density of air is low and there are fewer molecules to enter the pitot tube and create pressure hence, Indicated airspeed decreases during climb.

2. Calibrated Airspeed

Calibrated airspeed is the indicated airspeed corrected for instrument and positional errors. At certain airspeed and flap settings instrumental errors can add up to several knots. The calibrated airspeed takes the aircraft specific known value of calibrated airspeed offset as defined by the manufacturer and printed in Pilot Operating Handbook. At lower altitudes and slower speeds calibrated airspeed show significant variations in contrast to Sea Level International Standard Atmosphere where Calibrated airspeed is equal to indicated airspeed (CAS = IAS). 

3. True Airspeed 

True airspeed is a measure of the speed of your aircraft in relation to the airmass it is flying through. If the air is till and the aircraft is flying steady and level then, true airspeed is equal to the groundspeed of the aircraft. True airspeed is calculated by indicated airspeed adjusting for pressure and temperature variables. As you climb your true airspeed will be higher than indicated airspeed because at higher altitude fewer and fewer molecules enter the pitot tube which imparts lower pressure and hence, lower value of indicated airspeed than true airspeed. In fact for every thousand feet above sea level true airspeed is 2 % higher than indicated airspeed.

4. Equivalent Airspeed 

Calibrated airspeed corrected for compressibility of air at a non-trivial Mach number. It is also the airspeed at sea level in the International Standard Atmosphere where dynamic pressure is same as the dynamic pressure at true airspeed (TAS) and the altitude of flying aircraft. It is helpful for predicting aircraft handling loads, stalling characteristics, etc. In low-speed flights it is the value shown by airspeed indicator of zero error. Equivalent airspeed is related to true airspeed as follows:-   

where,  is actual air density 
and,  is Mean Sea Level Density (1.225 kg/m³ or 0.00237 slug/ft³).