Definition
A characteristic of an aircraft in which, after being disturbed from a trimmed condition, the resulting oscillations grow larger over time rather than fading away. The aircraft initially tries to return toward its original attitude, but each successive swing past that point is bigger than the one before, so the motion diverges instead of settling.
Plain English
If something nudges the aircraft off its trim, it wobbles back and forth, and each wobble gets bigger instead of smaller. Left alone, the motion keeps growing rather than dying out.
Context Anchor
Seen in aircraft stability discussions, especially when comparing how an airplane responds over time after a gust, control input, or other disturbance.
Derivation
Dynamic comes from the Greek dynamis, meaning power or motion. Dynamic stability is about how motion behaves over time, not just the initial reaction. Negative here means the motion works against stability — the swings grow rather than shrink.
Why Pilots Care
An aircraft with negative dynamic stability will become increasingly difficult to control and may require constant pilot input or automatic systems to prevent loss of control.
Analogy
It is like a playground swing that should slow down after a push, but instead swings higher each time. The motion is not calming down; it is building up.
Grounding Statement
Picture an airplane hit by a gust: instead of gradually returning toward steady flight, each movement carries it farther from where it started.
Intuition Check
Negative does not mean the airplane is pointing downward. Here, negative means the aircraft’s motion gets less stable over time after it is disturbed.
Example Sentence 1
Because the aircraft showed negative dynamic stability in pitch, the pilot had to actively dampen each oscillation rather than let it settle on its own.
Example Sentence 2
Designers added a yaw damper to eliminate the negative dynamic stability that appeared in the aircraft's lateral axis during flight tests.