Definition
A combined cockpit switch that controls the aircraft's main electrical power. It typically has two halves, labeled ALT (alternator) and BAT (battery), which can be operated together or separately. Turning it on connects the battery to the electrical bus and brings the alternator online to supply electrical power and recharge the battery. Turning it off disconnects the aircraft's electrical system from its power sources.
Plain English
The main on/off switch for the airplane's electrical system. One side controls the battery, the other controls the alternator (the engine-driven generator that keeps the battery charged).
Context Anchor
Seen on the cockpit electrical switch panel, in checklists, and during electrical-system malfunction procedures.
Derivation
ALT is short for alternator, from Latin alternare, 'to do by turns' — the device produces alternating current. BAT is short for battery. 'Master' indicates it is the top-level switch controlling the whole electrical system.
Why Pilots Care
Controls all electrical power to instruments, radios, lights, and avionics; improper position or failure requires immediate checklist action to restore or shed load.
Grounding Statement
BAT gives the airplane stored electrical power; ALT lets the airplane make electrical power while the engine is running.
Intuition Check
ALT does not mean altitude here; it means alternator. MASTER does not mean a person in charge; it means the main electrical control.
Example Sentence 1
After shutting down the engine, the pilot turned the MASTER ALT BAT switch off to disconnect the electrical system.
Example Sentence 2
When the alternator failed in flight the pilot confirmed the MASTER ALT BAT switch remained ON before troubleshooting further.