Static longitudinal stability is defined as the aerodynamic pitching moments required to return the aircraft to the equilibrium angle of attack.

• A. The initial tendency an aircraft displays when disturbed from a state of equilibrium.
• B. The aerodynamic pitching moments required to return the aircraft to the equilibrium angle of attack.
• C. The pressure of air that is still or not moving.
• D. The rate of turn in a standard rate turn.

Answer: (B)

Static longitudinal stability is about the airplane’s tendency to return to its equilibrium angle of attack after a disturbance, driven by the aerodynamic pitching moment about the center of gravity. The key idea is that the restoring moment acts to bring the angle of attack back toward the trim value, and the sign of that moment’s change with angle of attack determines stability. If, after a small increase in angle of attack, the aircraft develops a nose-down moment that reduces the angle, and if a small decrease in angle produces a nose-up moment that increases the angle, the aircraft is statically stable.

The option that defines stability this way—that the aerodynamic pitching moments are what return the aircraft to the equilibrium angle of attack—directly expresses the mechanism and the result of static longitudinal stability. The other choices describe different concepts: one mentions the initial tendency without tying it to the restoring moments about the angle of attack; another refers to still air pressure, which is unrelated to stability in pitch; and another concerns turn rate, which is about maneuvers, not the static restoring tendency.