What are the three main DC distribution buses in many aircraft electrical systems?

• A. AC bus, DC bus, and avionics bus.
• B. Main bus, essential (or emergency) bus, and avionics/essential avionics bus.
• C. Main bus, standby bus, and power distribution bus.
• D. Main bus, essential bus, and spare bus.

Answer: (B)

In aircraft electrical systems, power is arranged in a prioritized three-bus setup so essential functions stay powered during failures. The main bus carries the majority of noncritical loads that can be shed if power is limited, keeping the overall system operational under normal conditions. The essential (or emergency) bus is designed to stay energized when the main bus or a generator fails, powering the most critical flight-and-control systems and safety-related equipment. The avionics bus (often paired with an essential avionics bus) specifically powers cockpit electronics and avionics equipment, with the essential avionics bus providing a dedicated path to keep the most vital avionics active even if other buses are down.

This arrangement—main, essential, and avionics/essential avionics—fits how DC distribution is typically organized to maximize reliability and maintain capability for flight-critical tasks. Other options either mix AC with DC, reference standby or spare concepts that aren’t part of the standard three-bus DC distribution, or use terminology that doesn’t align with common practice.