How can true airspeed be roughly computed from indicated airspeed at higher altitudes?

To understand why the computation of true airspeed from indicated airspeed at higher altitudes involves adding a percentage based on altitude, it's important to recognize the relationship between pressure, air density, and airspeed.

At higher altitudes, the atmospheric pressure decreases, which results in a decrease in air density. Indicated airspeed (IAS) is based on the pressure of the air entering the pitot tube and does not account for these variations in air density. As altitude increases, indicated airspeed can underestimate the actual performance of the aircraft, as it does not reflect the true dynamics of the airflow around the aircraft.

To convert indicated airspeed to true airspeed (TAS), pilots add a correction factor due to this density loss. Specifically, a common rule of thumb is to add approximately 2% of the indicated airspeed for every 1,000 feet of altitude increase. This adjustment accounts for the change in air density and provides a more accurate representation of the aircraft's actual speed through the air, which is essential for safe flying, particularly when flight planning and in performance calculations.

Thus, adding 2% to the indicated airspeed for each 1,000 feet of altitude allows pilots to obtain a rough estimate of the true airspeed