What is True Airspeed (TAS) and how does it relate to IAS?

• A. TAS is the actual speed of the aircraft through the air; it equals IAS corrected for air density and instrument error.
• B. TAS is the airspeed read relative to the ground.
• C. TAS is the indicated airspeed.
• D. TAS is the speed over the ground corrected for wind.

Answer: (A)

True Airspeed is the actual speed of the airplane through the air mass. It reflects how fast the aircraft is moving relative to the air around it, not relative to the ground. To get TAS from what you read on the airspeed indicator, you start with the indicated airspeed and correct for factors that affect the reading: instrument and installation errors (to get calibrated airspeed) and the air density (which changes with altitude and temperature). These corrections account for how dense the air is and how the instruments might be biased, yielding the true speed through the air.

At low, standard conditions TAS is close to what the cockpit shows, but as you climb and air gets less dense, the TAS must be higher than the indicated value to produce the same dynamic pressure. At high speeds, compressibility effects also matter and are included in the corrections. This relationship helps explain why TAS is not the same as the speed over the ground, which is affected by wind.