Definition
A cockpit switch that connects or disconnects the aircraft battery from the electrical system. When turned on, it allows battery power to flow to the bus, energizing electrical equipment; when turned off, it isolates the battery from the rest of the aircraft.
Plain English
The main on/off switch for the aircraft's battery. Turn it on and the battery can power things in the airplane. Turn it off and the battery is disconnected from everything else.
Context Anchor
Seen on the cockpit electrical panel during preflight, engine start, shutdown, and electrical-system troubleshooting.
Derivation
“Master” comes from an older meaning of “chief” or “main.” In this term, it means the main controlling switch for battery power, not a person or instructor.
Why Pilots Care
It must be turned on before engine start and is critical for managing electrical power to prevent battery drain or system failures.
Intuition Check
Do not assume “master” means this switch controls every electrical item by itself. It mainly connects or disconnects the battery; individual equipment still has its own controls, and some aircraft have separate controls for charging power.
Example Sentence 1
Before leaving the aircraft, she turned the master battery switch off to prevent draining the battery overnight.
Example Sentence 2
In the event of an electrical fire, the procedure calls for turning off the master battery switch immediately.