Definition
The action by which a fire-detection sensor closes an electrical circuit when it detects fire or excessive heat, allowing current to flow and trigger the cockpit fire warning indication. In a typical thermal switch or thermocouple-based system, the sensing element is open under normal conditions; when temperature exceeds a set threshold, the element closes the circuit, energizing the warning light and aural alert.
Plain English
The fire sensor finishes the electrical loop when it senses a fire, which lets electricity flow and turn on the fire warning in the cockpit.
Context Anchor
Seen in aircraft systems and maintenance descriptions of engine, heater, cargo, or baggage compartment fire warning systems.
Derivation
A circuit is a closed loop that electricity flows around. 'Complete' here means to finish or close the loop. Until the loop is closed, no current flows and no warning appears. When the sensor detects fire, it closes (completes) the loop and the warning activates.
Why Pilots Care
Immediate activation of the warning allows the crew to identify the fire location and initiate the correct extinguishing sequence before the situation escalates.
Analogy
It is like flipping a wall switch to turn on a light. The switch closes the path, electricity can flow, and the light comes on.
Intuition Check
“Completes” does not mean the system is finished or successfully inspected. Here it means the electrical path has been closed so current can flow and activate the warning.
Example Sentence 1
When the thermal switch in the engine nacelle reaches its trigger temperature, it completes the fire warning circuit and the red fire light illuminates on the instrument panel.
Example Sentence 2
A chafed wire can complete the fire warning circuit on its own and trigger a false alarm during flight.