Definition
A control system that continuously measures its own output and uses that measurement to automatically adjust its input, keeping the result at a desired target. The measurement signal is fed back to the controller, which compares it to the target and corrects any difference.
Plain English
A system that watches what it is doing, compares it to what it should be doing, and corrects itself automatically. The loop is 'closed' because the result is sent back to the start to keep it on target.
Context Anchor
Seen in engine-control and aircraft system discussions, especially where sensors let a control unit adjust fuel flow, air flow, temperature, pressure, or another controlled condition.
Derivation
Closed-loop' describes the path of the signal: it leaves the controller, acts on the system, is measured, and returns to the controller — completing a closed circle. 'Open-loop' would mean no return path; the controller would simply send a command and hope for the result.
Why Pilots Care
It keeps engine speed, mixture, and other parameters stable even when conditions change, reducing pilot workload and preventing damage.
Analogy
Like a home thermostat: it senses the room temperature, compares it to the setting, and turns the heat on or off to match. The sensor reading is constantly fed back to keep the room at the target.
Intuition Check
Do not read 'closed' as 'off' or 'unavailable.' In closed-loop control, 'closed' means the system has a return path that reports the actual result back to the controller.
Example Sentence 1
The propeller governor uses closed-loop control to hold a constant RPM as engine load changes.
Example Sentence 2
During troubleshooting, the technician verified that the fuel control unit was operating in closed-loop mode by checking that sensor data adjusted the metering valve.