What piece of information is critical for calculating true airspeed?

To determine true airspeed, one needs to know indicated airspeed and altitude because these factors allow for the necessary adjustments to account for variations in air density.

Indicated airspeed, which is the speed shown on the airplane's airspeed indicator, is not adjusted for altitude or temperature. At higher altitudes, air density decreases, which affects the aircraft's performance. True airspeed is corrected from indicated airspeed using the density altitude, which is a function of both the aircraft's altitude and the local air temperature.

By knowing both the indicated airspeed and the altitude, pilots can use standard methods or formulae to calculate true airspeed accurately. This is critical for navigation and performance calculations, as true airspeed represents the actual speed of the aircraft relative to the surrounding air mass, which is essential for safe flight operations.