Static stability : If an aircraft in flight is disturbed from its equilibrium state, for example by a gust and the aircraft has its initial tendency to return to its equilibrium state, then the aircraft is having a static stability. Static stability can be divided into statically stable system, statically unstable system and statically neutral system.

Statically stable system (positive static stability) : Aircraft has initial tendency to return to its equilibrium position.

Statically unstable system (Negative static stability) : Aircraft has its initial tendency to move away from its equilibrium position.

Statically neutral system (Neutral static stability) : Aircraft has a tendency to maintains its original disturbed position.

Dynamic stability : When an aircraft is disturbed from its equilibrium position in flight it first go through static stability and later the time history of the aircraft’s motion and attitude makes up the dynamic stability. So dynamic stability is the decay of the amplitude of response over a finite time which has been occurred due to disturbance to reach the equilibrium state.

Dynamic stability can be divided into dynamically stable system, dynamically unstable system, and dynamically neutral system.

Dynamically stable system (Positive dynamic stability ): Aircraft has tendency to return to its equilibrium position through damping of oscillations.

Dynamically unstable system (Negative dynamic stability ) : Aircraft oscillates with increased amplitudes and equilibrium position is never reached.

Dynamically neutral system (Neutral dynamic stability ) :  Aircraft maintains uniform oscillations with a constant amplitude.

Static and dynamic stability of aircraft