FAA Private Pilot Airplane (PAR)Aerodynamics and Aircraft SystemsMedium
In a light airplane with a functioning alternator supplying the electrical bus during cruise flight, what is the primary role of the battery?
- AIt serves mainly to start the engine and to provide a reserve/buffer for electrical loads, since the alternator is the primary in-flight power source
- BIt is the main continuous source of electrical power for all avionics during flight
- CIt converts AC power from the alternator into DC power for the instruments
- DIt regulates alternator output voltage to prevent overcharging
Show answer & explanationAnswer & explanation
Correct answer: A. It serves mainly to start the engine and to provide a reserve/buffer for electrical loads, since the alternator is the primary in-flight power source
During normal flight with the alternator operating, the alternator is the primary source of electrical power and also recharges the battery. The battery's main roles are providing power for engine starting and acting as a buffer/reserve, especially during transient loads or if the alternator fails.
Why the other options are wrong
- B. The alternator, not the battery, is the primary continuous power source when it is functioning.
- C. Voltage regulators, not the battery, handle rectification/regulation in most GA systems.
- D. That function belongs to the voltage regulator, not the battery.
Battery Role with Alternator Operating
With the alternator functioning normally, it supplies the electrical system and recharges the battery; the battery's role is primarily starting and serving as a reserve/buffer.
- Alternator = primary power source in flight
- Battery = starting power + buffer/reserve
- Battery becomes primary source only if alternator fails
Memory trick: Alternator drives, battery backs up.