Definition
A system in which the output is not directly proportional to the input. A given change in the input does not produce a predictable, equal-sized change in the output, so the system's behavior cannot be described by a straight-line relationship.
Plain English
A system where doubling the input does not simply double the output. The response curves, jumps, or behaves unevenly instead of following a steady, straight-line pattern.
Context Anchor
Seen in discussions of aircraft control, aircraft stability, automatic control systems, and how an aircraft responds under different flight conditions.
Derivation
Non- means 'not.' Linear comes from the Latin linea, meaning 'line.' A linear system plots as a straight line on a graph of input vs. output; a nonlinear system does not.
Why Pilots Care
Many real aircraft systems behave nonlinearly near their limits — control response, engine performance, and aerodynamic effects often change disproportionately as inputs grow large. Knowing a system is nonlinear reminds the pilot that small inputs may behave one way while large ones behave very differently.
Grounding Statement
Picture moving a control slightly: in one condition the aircraft responds gently, but in another condition the same movement may produce a stronger response.
Intuition Check
Nonlinear does not just mean “confusing” or “unpredictable.” It means the input and output are not tied together by a simple direct proportion.
Example Sentence 1
Control surface effectiveness becomes a nonlinear system at high angles of attack, where a small additional input can produce a much larger change in response.
Example Sentence 2
The flight control computer accounts for nonlinear behavior when scheduling gains at different airspeeds.