Its nose or tail drops or one wing dips. How the airplane reacts to such a disturbance from its flight attitude depends on its stability characteristics. Stability is the tendency of an airplane in flight to remain in straight, level, upright flight and to return to this attitude, if displaced, without corrective action by the pilot. Static stability is the initial tendency of an airplane, when disturbed, to return to the original position. Dynamic stability is the overall tendency of an airplane to return to its original position, following a series of damped out oscillations.
Negative stability is, in other words, the condition of instability.
An exceedingly stable airplane, on the other hand, may lack maneuverability. An airplane which, following a disturbance, oscillates with increasing up and down movements until it eventually stalls or enters a dangerous dive would be said to be unstable, or to have negative dynamic stability. An airplane that has positive dynamic stability does not automatically have positive static stability. The designers may have elected to build in, for example, negative static stability and positive dynamic stability in order to achieve their objective in maneuverability. In other words, negative and positive dynamic and static stability may be incorporated in any combination in any particular design of airplane.