Which factor of in-flight stability is least preferred?

The least preferred factor of in-flight stability is to reduce airspeed. In aviation, maintaining optimal airspeed is crucial for ensuring stability and control of an aircraft. A reduction in airspeed can lead to several issues, including a potential loss of lift, which is critical for keeping the aircraft airborne. When an aircraft operates at lower speeds, it can also become more susceptible to turbulence and stall conditions, both of which can compromise the safety and stability of the flight.

Additionally, maintaining airspeed is important for maneuverability. Changes in direction or altitude require a minimum amount of airspeed to be executed safely, and reducing speed can limit the pilot's options when responding to flight dynamics. Thus, maintaining or even increasing airspeed is generally preferable for enhancing the overall stability of the aircraft during flight.