Definition
The tendency of an aircraft, once disturbed from a given attitude or flight condition, to depart further from that condition rather than return to it on its own. In an unusual attitude, instability characteristics include increasing bank, accelerating pitch change, rising airspeed in a nose-low attitude, or decaying airspeed in a nose-high attitude — all of which worsen if the pilot does not intervene promptly.
Plain English
The ways an aircraft tends to get worse, not better, on its own once it has been knocked out of normal flight. Left alone, the situation keeps building in the wrong direction.
Context Anchor
Seen in unusual attitude training, especially when discussing why some aircraft require prompt, correct control inputs after a nose-high, nose-low, or banked upset.
Derivation
‘Instability’ comes from the Latin in- (‘not’) + stabilis (‘able to stand firm’). In aviation it means the aircraft will not return to steady flight by itself — disturbances grow rather than fade.
Why Pilots Care
The pilot must anticipate whether the aircraft will assist or resist recovery inputs to avoid over-controlling or entering a secondary upset.
Grounding Statement
Picture an airplane that starts to roll after a disturbance and keeps rolling farther unless the pilot stops it.
Intuition Check
Instability characteristics does not mean the aircraft is broken or unsafe. It means the aircraft has certain tendencies that may make a disturbance grow unless the pilot controls it.
Example Sentence 1
The instructor pointed out the instability characteristics of a nose-low unusual attitude — increasing airspeed and steepening bank — and stressed the need for immediate recovery action.
Example Sentence 2
Recognizing the plane's instability characteristics helped the pilot stop the nose from rising further during the unusual attitude recovery.