According to the true airspeed rule of thumb, how much additional CAS should be added for every thousand feet?

The correct answer is that for every thousand feet of altitude, approximately 2% of additional calibrated airspeed (CAS) should be added to account for the increase in true airspeed. This rule of thumb is derived from the fundamental relationship between altitude and air density, which affects airspeed calculations.

As an aircraft climbs, the air density decreases, and consequently, the indicated airspeed needs to be adjusted to maintain equivalent performance. At higher altitudes, true airspeed increases for the same power setting due to the lower air density. To maintain performance characteristics such as lift and drag, it's crucial to account for this increased true airspeed.

This 2% adjustment ensures that pilots can maintain proper control and performance metrics, especially in critical phases of flight like climbs, descents, and maneuvers. Understanding this relationship helps pilots in flight planning and ensures safety by accurately calculating airspeed requirements at various altitudes.