Definition
A heavy-duty, electrically operated switch (a high-current relay) that connects the aircraft battery to the starter motor when the pilot engages the start function. A small control current from the starter switch energizes the contactor's coil, which then closes a much larger set of contacts capable of carrying the high current the starter motor draws.
Plain English
A big electrical switch that lets the small starter switch in the cockpit safely turn on the very high current needed to spin the starter motor.
Context Anchor
Seen in aircraft starting system descriptions and during troubleshooting when the engine starter does not operate normally.
Derivation
Contactor' comes from 'contact' — the metal points inside the device that physically touch to complete the circuit. The name reflects its job: making and breaking electrical contact under heavy load.
Why Pilots Care
It protects the cockpit starter switch and wiring from damage by handling the high amperage required to crank the engine.
Analogy
It is like using a small control switch to operate a much stronger switch. The pilot's start control does not have to carry all the starting power; it tells the starter contactor to do that job.
Grounding Statement
When the pilot turns the key or switch to START, the starter contactor is the part that actually lets battery power reach the starter motor.
Intuition Check
Do not confuse the starter contactor with the starter motor. The contactor does not turn the engine; it controls the electrical power that lets the starter motor turn the engine.
Example Sentence 1
When the pilot turned the key to START, the starter contactor clicked closed and the propeller began to turn.
Example Sentence 2
A click from the starter contactor without engine rotation usually indicates a weak battery or a problem in the starting circuit.