Definition
A cockpit switch that connects or disconnects the airplane's battery from the electrical system. When closed, it allows battery power to feed the aircraft bus, energizing avionics, lights, fuel pumps, and other electrical equipment. When opened, it isolates the battery so no current flows from it to the rest of the aircraft.
Plain English
The switch that turns the airplane's battery on or off. With it on, the battery can power things like radios, lights, and instruments. With it off, the battery is disconnected and nothing electrical runs from it.
Context Anchor
Seen on cockpit checklists before engine start, during electrical checks, and after shutdown.
Derivation
"Master" here means the main, controlling switch — the one that governs all the others downstream. Just as a master key opens every door, the battery master switch controls whether the battery is connected to the rest of the airplane's electrical system.
Why Pilots Care
It controls all battery-supplied electrical power, so leaving it on can drain the battery and leaving it off prevents starting or operating radios and instruments.
Analogy
Like the main breaker that turns electricity on or off for an entire house.
Intuition Check
Do not assume the battery master switch starts the engine or turns every electrical device on by itself. It only makes battery power available to the airplane’s main electrical system.
Example Sentence 1
Before leaving the airplane, the pilot turned the battery master switch off to prevent draining the battery overnight.
Example Sentence 2
After engine shutdown the pilot turns off the battery master switch to prevent unnecessary battery drain.