Types of Airspeed

Fsxlight.bmp (153574 bytes) Variations in air pressure as altitude changes, errors in the pitot-static system, and other factors make determining airspeed more complicated than just reading a number on a dial. You should be familiar with several types of airspeed, each useful in different circumstances.

	Indicated airspeed (IAS) is the reading taken directly from airspeed indicator. It's the number that the needle points to on the dial. This value is not corrected for variations in the density of the atmosphere, installation error (caused by the position of the pitot tube and static ports), or errors caused by the instrument itself. Indicated airspeed drops as you climb, because as the density of the air decreases with altitude, fewer air molecules hit the pitot tube. This effect is most noticeable in high-performance aircraft that operate at high altitude. For example, at cruise altitude, the airspeed indicator on the Boeing 737-400 may indicate about 280 knots when the airplane's actual speed through the air is more than 400 knots. Pilots use indicated airspeed to get the proper performance from their aircraft. Takeoff, climb, approach, and landing speeds are all based on IAS.
	Calibrated airspeed (CAS) is indicated airspeed corrected for installation error and instrument error. The difference between indicated airspeed and calibrated airspeed is usually greatest at low airspeeds, when the air flow around the static ports is turbulent. The position of the pitot tube and the installation of the airspeed indicator itself also contribute small errors to the airspeed indicator. Lowering the flaps may also increase the difference between IAS and CAS. To compensate for these errors, pilots refer to tables in the aircraft handbook to determine calibrated airspeed (CAS) under various conditions of flight.
	True airspeed (TAS) is the actual speed of an aircraft through the air. The airspeed indicator displays TAS only at sea level under standard conditions, so you must calculate TAS based on IAS, the current pressure altitude, and air temperature. As a rule of thumb, you can estimate TAS by adding 2 percent to IAS for each 1,000 ft (305 m) of altitude. Pilots use TAS in navigation calculations and when filing flight plans.
	Mach number is the ratio of the aircraft's true airspeed to the speed of sound. Aircraft that operate at high altitudes use Mach number, not airspeed, as a reference for aircraft performance and navigation, because the airspeed indicator is subject to many errors at high speeds in the thin air of the upper atmosphere. Mach number is calculated by a computer and displayed on the airspeed indicator.

NOTE: The speeds used in Flight Simulator checklists, operating procedures, and in the Aircraft Handbooks section of Help are indicated airspeeds unless otherwise noted.