In what scenario is true airspeed (TAS) primarily used?

True airspeed (TAS) is primarily used for performance calculations because it represents the actual speed of the aircraft relative to the surrounding air and is essential for accurate assessments of aircraft performance. This includes calculations for climb rates, fuel consumption, and navigation accuracy. TAS takes into account the altitude and temperature effects on air density, which is critical when evaluating how the aircraft will perform under specific conditions.

In contrast, other scenarios focus on different aspects of aviation. For instance, runway calculations often relate to ground speed and runway length, while altitude adjustments are more concerned with indicated airspeed (IAS) and vertical speed. Approach procedures rely on a mix of indicated airspeed and ground speed for managing descent and approach angles, but they don't specifically require TAS on its own as a primary metric. Thus, TAS’s importance in performance calculations stands out as fundamental for understanding and optimizing flight operations.