What is true airspeed?

True airspeed (TAS) is defined as the actual speed of the aircraft relative to the surrounding air. It is calculated by adjusting calibrated airspeed (CAS) for both altitude and atmospheric conditions, specifically temperature and pressure deviations from standard.

When a pilot flies at higher altitudes, the density of the air decreases, which affects the indicated airspeed and calibrated airspeed readings. Therefore, to determine the true performance of the aircraft and engine efficiency, it's critical to account for changes in air density, which are influenced by non-standard atmospheric conditions. The corrected CAS accounts for these factors accurately, allowing pilots to determine how fast their aircraft is actually moving through the air.

The other options do not correctly describe true airspeed. For instance, the speed of the aircraft relative to the ground refers to ground speed, while CAS corrected just for altitude does not incorporate temperature adjustments. Similarly, indicated airspeed reflects the reading from the aircraft's instruments without corrections for altitude or temperature, and thus is not correlated with true airspeed. Understanding these distinctions is crucial for accurate flight planning and performance calculation.