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The Difference Between TAS & IAS

The determination of speed is different in the aviation sector than with vehicles that travel on the ground. The two types of airspeed are indicated airspeed, or IAS, and true airspeed, or TAS. These types of airspeed are often close in measurement, but true airspeed offers a more accurate airspeed measurement than indicated airspeed. While vehicles that travel on the ground measure speed based on miles per hour, aviation borrows its measurement system from the nautical system, which indicates speed in nautical miles, or knots.

Airspeed Overview
- The knot is the basic increment in the measurement of the speed of watercraft and aircraft. Also known as the nautical mile, the knot equals 6,070 feet per hour, while the mile per hour, or MPH, measurement equals 5,280 feet per hour. Mach numbers also measure airspeed. The Mach number is the aircraft's speed in correlation to the speed of sound. Mach 1 is the speed of sound, and equals 661.7 knots.
Indicated Airspeed
- An instrument called a pitot tube, which usually mounts on the nose or wing of an aircraft, measures IAS. This 90-degree angled tube orients in a fashion that allows it to measure the least turbulent airflow. The tube receives heat to keep ice from forming and hindering its performance. The pitot tube measures dynamic pressure, also called ram or impact pressure, and static pressure. The difference between these measurements determines the IAS, which usually displays as KIAS, or knots indicated airspeed, on the cockpit display.
True Airspeed
- The TAS is the actual speed that the aircraft travels in respect to the air through which it travels. The TAS offers a greater level of accuracy, and additional measurements determine the true airspeed of the craft. Determination of TAS takes IAS, and the various factors that affect the reading such as outside air temperature and barometric pressure, to indicate the true air speed. The determination of TAS also uses charts. This measurement of TAS displays as KTAS, or knots true airspeed).
Considerations
- Instrumentation built into the aircraft measure the IAS to determine airspeed based on the measurement of dynamic and static pressure by the pitot tube. Three standards exist that allow for the highest level of accuracy in the IAS reading: measurement is taken at sea level; temperature and humidity fits the outlined standard; and the barometric setting of 29.92 inches of pressure. If any of these factors are not ideal, this reduces the IAS reading's accuracy, and the charts and other indicators that determine the TAS will offer a greater level of airspeed accuracy.

Aviation