Definition
In aviation, instability is the tendency of an airplane, once disturbed from a steady flight condition, to move further away from that condition rather than return to it. An unstable airplane requires continuous pilot input to maintain attitude, heading, or airspeed because any disturbance grows over time instead of damping out.
Plain English
If something bumps the airplane out of straight-and-level flight, an unstable airplane will keep drifting further off on its own instead of settling back. The pilot has to keep correcting it.
Context Anchor
You encounter this term in aircraft handling and stability discussions, especially when learning how an airplane reacts after a gust, control movement, or other disturbance.
Derivation
From Latin 'in-' meaning 'not' and 'stabilis' meaning 'standing firm' or 'steady'. So instability literally means 'not steady' -- a useful anchor, because it describes an airplane that won't hold its own attitude without constant attention.
Why Pilots Care
Unstable aircraft require constant pilot corrections, raising workload and the chance of losing control.
Grounding Statement
Picture a small wing drop after a gust that keeps increasing instead of stopping or returning toward level flight.
Intuition Check
Instability does not just mean the airplane feels shaky or uncomfortable. In this context, it means a disturbance tends to grow unless it is corrected.
Example Sentence 1
Loading the airplane with the center of gravity behind the aft limit can produce longitudinal instability, making pitch control sensitive and tiring.
Example Sentence 2
Directional instability required constant rudder pressure to keep the airplane on heading.