Definition
A control system in which the corrective output is proportional to the size of the error between the desired value and the actual value. The larger the deviation, the larger the correction applied; the smaller the deviation, the smaller the correction.
Plain English
A control method where how hard the system pushes back depends on how far off things are. A small error gets a small correction; a big error gets a big correction.
Context Anchor
Seen in discussions of automatic flight control, autopilots, and other aircraft systems that correct toward a selected setting.
Derivation
From Latin proportio, meaning 'in relation to' or 'in ratio with.' The correction is in ratio to the error — that is the whole idea behind the name.
Why Pilots Care
Proportional control gives smooth, stable handling rather than abrupt on/off corrections. It is why a properly tuned autopilot eases the aircraft back onto altitude instead of jerking it back.
Analogy
Like steering a car: if you drift slightly out of your lane, you nudge the wheel a little; if you drift a lot, you turn it more. The size of your input matches the size of the problem.
Grounding Statement
A proportional control system makes a bigger correction for a bigger miss and a smaller correction as the miss gets smaller.
Intuition Check
Do not read “proportional” as just meaning “balanced” or “reasonable.” Here it means the correction changes in size according to how large the error is.
Example Sentence 1
The autopilot uses proportional control to apply gentle aileron input when the aircraft drifts a few degrees off heading, and stronger input when the deviation is larger.
Example Sentence 2
The hydraulic actuator used proportional control to match rudder deflection directly to the amount of pedal pressure applied by the pilot.