Definition
The aircraft's main electrical battery, normally connected to the bus through the master switch, which provides power for engine starting and serves as a backup electrical source if the alternator or generator fails in flight.
Plain English
The aircraft's main battery — the one used to start the engine and to keep electrical equipment running if the engine-driven power source quits.
Context Anchor
Seen in electrical system discussions, especially when comparing the aircraft’s normal power source with a standby battery.
Derivation
Primary here means main or first in importance — not first in time. It is called primary to distinguish it from any secondary or standby battery fitted as a backup.
Why Pilots Care
Pilots must know which battery handles routine power needs so they can respond correctly if it fails or if the standby system activates.
Intuition Check
Primary does not mean “disposable battery” here. In this context, it means the aircraft’s main battery, as opposed to a standby or backup battery.
Example Sentence 1
After the alternator failed, the pilot shed non-essential loads to extend the life of the primary battery.
Example Sentence 2
Once running, the alternator charges the primary battery while also powering the electrical system.