What is the criteria for the minimum cruising speed formula in FAR part 23 to be accurate?

The criterion for the minimum cruising speed formula in FAR part 23 to be accurate is that the speed must be less than or equal to 20 knots calibrated airspeed. This specification is based on the operational characteristics and safety margins established for aircraft within the regulations.

Having a maximum speed limitation of 20 knots allows for the accounting of performance variances at lower speeds, which can ensure safety and efficiency during flight operations. Above this threshold, the accuracy of the cruising speed calculations may not hold due to various aerodynamic factors and potential changes in aircraft control responses. Therefore, keeping the speed at or below 20 knots ensures that the aircraft operates within a range where the performance metrics are reliable and consistent.