Definition
A state in which an aircraft, once disturbed from its trimmed flight attitude, tends to move further away from that attitude rather than return to it. In a longitudinally unstable aircraft, a small pitch disturbance grows into a larger one unless the pilot intervenes with control inputs.
Plain English
The airplane, after being bumped out of level flight, keeps drifting further off instead of settling back on its own. The pilot has to actively correct it.
Context Anchor
Seen in aircraft stability and center-of-gravity discussions, especially when loading or design affects how the airplane responds after a disturbance.
Derivation
Unstable comes from Latin in- (not) and stabilis (able to stand firm). An unstable aircraft is one that cannot 'stand firm' on its trimmed attitude — once nudged, it keeps wandering.
Why Pilots Care
An unstable aircraft requires constant pilot corrections, increasing workload and the risk of losing control.
Analogy
It is like placing a ball on the top of a rounded hill. If the ball is nudged, it rolls farther away instead of coming back to the top.
Grounding Statement
If a disturbance makes the airplane’s motion grow instead of fade away, the airplane is in an unstable condition.
Intuition Check
Do not read “unstable” as simply meaning “dangerous right now.” Here it means the airplane does not naturally return toward its previous flight attitude after being disturbed.
Example Sentence 1
Loading the baggage too far aft shifted the center of gravity behind the limit and produced an unstable condition in pitch.
Example Sentence 2
Recognizing an unstable condition early allows the pilot to apply immediate corrective inputs.