Which buses are powered in flight with a single generator online?

The main and essential buses are designed to receive power from a single generator while the aircraft is in flight. In typical aircraft systems, the main bus distributes power to the majority of the electrical systems needed for normal operation, while the essential bus provides power to critical systems that are necessary for safe flight, particularly in the event of an electrical failure or in emergency scenarios.

When only one generator is online, both the main and essential buses can still function, as the essential bus typically draws power from the main bus's supply. This configuration ensures that vital systems remain operational even when electrical load management needs to be prioritized.

The other options refer to combinations of buses that do not align with the standard operational practice of relying on a single generator. For instance, auxiliary buses are generally powered through additional means or multiple generators and are intended for non-essential loads. Similarly, standby and emergency buses serve specific functions that typically involve different power sources or configurations that exceed the single generator setup. Hence, the main and essential buses offer the most critical support while maintaining functionality with just one generator in operation.