In a typical aircraft DC electrical system, what is the nominal system voltage and the main sources of power?

• A. Nominally 24 V DC; powered by a battery and a solar panel.
• B. Nominally 28 V DC; powered by a generator only.
• C. Nominally 28 V DC; powered by a battery only.
• D. Nominally 28 V DC; powered by a battery and an alternator/generator.

Answer: (D)

The main idea is that typical aircraft DC systems operate around a 28 V DC bus, which provides enough power for avionics, lighting, and other equipment while keeping wiring practical. The two primary power sources are the battery and an engine-driven generator (alternator). The generator supplies the bulk of power when the engine is running, keeping the system charged and active in flight, while the battery provides starting power and serves as a backup or emergency source if the generator isn’t producing power or is overloaded. A strictly generator-only or battery-only setup would either lack startup capability or fail to sustain power in flight, and solar panels aren’t used as the main power source in standard aircraft DC systems. So, the normal arrangement is 28 V DC powered by both a battery and an engine-driven generator.