What is required to convert calibrated airspeed (CAS) to true airspeed (TAS)?

To convert calibrated airspeed (CAS) to true airspeed (TAS), it is essential to take into account both pressure altitude and air temperature.

Pressure altitude is critical because it influences air density, which changes with altitude. As an aircraft ascends, the density of the air decreases. This decrease in density affects the performance of the aircraft and the accuracy of airspeed measurements. Calibrated airspeed is derived from indicated airspeed, corrected for instrument and position errors, while true airspeed is the speed of the aircraft relative to the surrounding air. Therefore, knowing the pressure altitude helps to adjust for these density changes when making the conversion to true airspeed.

Air temperature also plays a significant role in the conversion process. As temperature affects air density—hotter air is less dense than cooler air—accurate temperature measurements are vital for determining true airspeed. The combination of these two factors, pressure altitude and air temperature, provides the necessary information to calculate the true airspeed accurately.

Other options lack one or both of these critical elements needed for the conversion.