Final answer:
To improve the Longitudinal Stability of an aircraft, a design feature that is commonly used is a horizontal stabilizer or tailplane.
Step-by-step explanation:
The utilization of a horizontal stabilizer, also known as a tailplane, is a design element of an aircraft that aids in improving longitudinal stability. The main purpose of the horizontal stabilizer, which is attached to the aircraft's tail, is to stabilize the aircraft's pitch, or its up and down motion. It does this by producing what is known as negative lift, or downforce, which is a downward force. This downforce helps the aircraft maintain a constant pitch attitude while in flight by counteracting the nose's tendency to pitch up or down.