What is the correct symbol for the minimum steady flight speed at which an airplane is controllable?

The symbol representing the minimum steady flight speed at which an airplane is controllable is denoted as "Vs." This speed is critical for pilots because it is associated with the stall speed of the aircraft in a clean configuration—typically without flaps or other lift-enhancing devices deployed.

At this speed, the aircraft can maintain controlled flight, and flying below this speed can lead to a stall, where it loses lift and therefore control. Understanding and recognizing "Vs" is essential for safe flight operations, especially during takeoff and landing phases.

The other options represent different but related terms. "Vso" refers to the stall speed in landing configuration (with flaps, for example), which is lower than "Vs" and specific to landing scenarios. "Vs1" is used in the context of specific aircraft performance limitations, and it also relates to stall speeds but under different configurations or conditions. "VNE" stands for the velocity never exceed, which is the maximum speed limit for flight; it does not describe controllability in terms of minimum flight speed. Thus, "Vs" is the appropriate term for the minimum speed necessary to maintain control of the aircraft in flight.