Definition
A main electrical distribution bar in the aircraft that receives power from the alternator or battery and feeds power to most of the aircraft's electrical equipment through individual circuit breakers or fuses. It is the principal point from which electrical loads are distributed throughout the aircraft.
Plain English
The main power strip in the aircraft. Power comes in from the battery or alternator, and from this bar it is split out to run most of the equipment on board.
Context Anchor
Seen in aircraft electrical system diagrams, especially when tracing how power moves from the battery or alternator to cockpit equipment and other electrical loads.
Derivation
The word 'bus' here is short for 'busbar,' from the Latin 'omnibus' meaning 'for all.' A busbar is a single conductor that supplies power to many circuits at once, so 'primary bus' simply means the main shared power bar.
Why Pilots Care
Knowing which systems connect to the primary bus helps a pilot understand what may stop working during an electrical failure or bus fault.
Analogy
Think of it like a main power strip. Power comes into one place, and several devices receive power from that shared point.
Intuition Check
Primary does not mean “more important than everything else in every situation,” and bus does not mean a vehicle. Here, primary means the main distribution point, and bus means a shared electrical path that feeds multiple circuits.
Example Sentence 1
After starting the engine, the alternator brought the primary bus online and powered the radios and instruments.
Example Sentence 2
Power from the alternator reaches the primary bus before it is sent to the lights and instruments.