The Airspeed indicator measures the difference between ram pressure from the pitot head and atmospheric pressure from the static source. The Airspeed indicator is subject to the following errors: POSITION ERROR (slipstream flow can cause disturbances at the static port preventing actual atmospheric pressure measurement—this can be a plus or minus value and varies with airspeed, altitude, and configuration); DENSITY ERROR (changes in altitude and temperature are not compensated for by the instrument); COMPRESSIBILITY ERROR (caused by the packing of air into the pitot tube at high airspeeds, resulting in higher than normal indications-usually occurs above 180 KIAS).

Types of Airspeed:

Indicated Airspeed (IAS) is the airspeed shown on the dial, uncorrected for instrument or system errors.

Calibrated Airspeed (CAS) is IAS corrected for position or installation and instrument errors.

Equivalent Airspeed is CAS corrected for compression of the air inside the pitot tube (same as CAS in standard atmosphere at sea level, but as airspeed and pressure altitude increase, the CAS becomes higher than it should be and a correction for compression must be subtracted from the CAS).

True Airspeed (TAS) is CAS corrected for nonstandard pressure and temperature (TAS and CAS are the same in standard atmosphere at sea level). If outside temperature increases TAS will also increase if a constant altitude and power are maintained.

Color Codes for the Airspeed Indicator:

White Arc (Flap Operating Range)—The bottom of the white arc is the Flaps-Down Stall Speed or VS0, which is the stall speed at maximum gross weight in the landing configuration (full flaps, gear down, wings level, and power off). The top of the white arc is the Maximum Flaps-Extended Speed or VFE, which is the maximum airspeed for flaps-down flight.

Green Arc (Normal Operating Range)—The bottom of the green arc is the Flaps-Up Stall Speed or VS1, which is the stall speed at maximum gross weight and wings level with the landing gear and flaps retracted). The top of the green arc is the Maximum Structural Cruise Speed or VNO, which is the normal operating limit speed or maximum speed for flying in rough air. 

Maneuvering Speed or Design Maneuvering Speed, VA, lies within the green arc. It is specified in the POH but not marked on the ASI. It is the maximum speed at which the limit load factor can be imposed (either by gusts or full control deflection) without over-stressing or causing structural damage to the aircraft. Stated differently, maneuvering speed is the speed at or below which an airplane's flying surfaces will stall before they generate aerodynamic forces sufficient to break them. For practical purposes, it is the maximum speed at which you may use abrupt control. Aviation authorities recommend reducing airspeed 10-15 knots slower than VA when flying in turbulent, gusty conditions. Pilots should remember that as the airplane gets lighter, the stall speed goes down and the maneuvering speed goes down as well.

Yellow Arc (Structural Warning Area or Caution Range)—The bottom of the yellow arc is the Maximum Airspeed For Rough Air and the top is the Never Exceed Airspeed. Fly in this range only in smooth air and only with small control inputs.

Red Radial Line (Never Exceed Airspeed)—VNEV

Exceeding this airspeed can result in structural damage to the aircraft.

If the pitot tube becomes blocked, the airspeed indicator will behave like an altimeter—it will only respond to changes in altitude. In a climb the airspeed indication will continually increase, tempting the pilot to pull up the nose to slow the aircraft down—This can lead to a dangerous stall. In a descent the airspeed indication will decrease, tempting the pilot to push the nose down—This can lead to excessive airspeed. Pilots should always verify a climb or descent by checking the attitude indicator. In aircraft that have a drain hole in the pitot-tube mast to allow water to drain from the system, indicated airspeed will immediately drop to zero as trapped ram-air pressure bleeds out of the drain hole. 

If the static port becomes blocked, the ASI will show an incorrect airspeed. As the airplane climbs, the trapped static pressure will be too high, causing the ASI to read too low. A pilot following this incorrect reading may speed up, possibly exceeding VNE. During a descent, the trapped static pressure will be too low, causing the ASI to read too high. A pilot could potentially slow the aircraft to the point of stalling.