Definition
A cockpit switch that controls the main electrical power circuit of the airplane, connecting or disconnecting the battery (and usually the alternator or generator) from the aircraft's electrical system. When turned off, it removes electrical power from most systems, including the fuel pump, avionics, lights, and electrical ignition components.
Plain English
The main on/off switch for the airplane's electrical power. Turning it off shuts down nearly everything that runs on electricity in the aircraft.
Context Anchor
Seen on cockpit checklists and in emergency procedures, including engine-fire procedures where electrical power may need to be shut off.
Derivation
"Master" here means the controlling switch in a circuit -- the one that overrides everything else. Flip the master off, and the smaller switches downstream lose power regardless of their position.
Why Pilots Care
Turning it off removes power that could create sparks or keep electrical equipment running during a fire.
Intuition Check
Do not assume “master” means it shuts down every system or stops the engine. In many piston airplanes, the engine ignition can keep working with the electrical master switch off; the switch mainly controls aircraft electrical power.
Example Sentence 1
After shutting down the engine, the pilot turned the electrical master switch off before leaving the cockpit.
Example Sentence 2
Before beginning the engine start sequence, the pilot verified the electrical master switch was in the off position.