IAS – Indicated Air Speed – The airspeed read directly on the Airspeed Indicator. It is the most accurate at design cruise airspeeds and design cruise AOA.
CAS – Calibrated Airspeed – IAS corrected for position error. Attitude, AOA changes can cause errors in the pitot-static system due to the angle of the relative airflow. Swirling air, eddies, off other parts of the aircraft can cause errors as well. Charts are available for the correction in the AFM\POH. More advanced aircraft with ADC/ARU’s may correct automatically.
EAS – Equivalent Airspeed – CAS corrected for compressibility. At high speed the impact pressure will compress inside the pitot system causing the airspeed to over read. Generally insignificant below 10,000’ and below 250 knots. It’s important for determining the dynamic pressures on the aircraft, and flight controls when designing aircraft. Again, ARU/ADC’s often factor this in to what is displayed on your ASI. I have yet to fly an aircraft where I had to apply a correction. Could also be due to the fact we have a dispatcher do our flight planning, and we fly a constant Mach number.
TAS – True Airspeed – EAS corrected for temperature and pressure (density). The only time your ASI will read correctly, is at Sea Level in a International Standard Atmosphere(ISA). 15C and altimeter setting of 29.92”. Temperature decreases with altitude, according to ISA 1.98C per 1000’, and 1” per 1000’. Density decreases (We can calculate a Density Altitude, the equivalent altitude based on non standard condition ISA for performance. The aircraft will perform as though it was at this altitude). We use a calculator or computer to calculate TAS. The ASI will under read as you increase in altitude, and/or increase temperature, roughly 2% per 1000’ as a rule of thumb. TAS a is the actual speed of the aircraft through the air.
GS – Ground Speed – TAS corrected for wind. Using trigonometry with wind direction and speed, with our direction and TAS we can calculate our GS and wind correction angle. Used for range and time calculations. Not really an airspeed, but an important part of flight planning.