When is the only time the hydraulic B pumps turn on?

The hydraulic B pumps are designed to operate when the respective buses are powered and at least one generator is online. This ensures that the hydraulic system has the necessary electrical power to activate the pumps for efficient operation. The hydraulic B pumps play a crucial role in providing hydraulic pressure for various systems, including flight control surfaces, landing gear, and brakes, so their activation is closely tied to the availability of power from the aircraft's electrical system.

When the buses are powered and a generator is functioning, it indicates that the aircraft's electrical system is ready to support the hydraulic pumps. This is critical for safe operation, as hydraulic systems are essential for the overall functionality of the aircraft. Other scenarios mentioned, like when the engines are at idle, during takeoff, or when flaps are deployed, do not specifically cover the condition of having power in the respective buses along with generator operation, which is why they would not accurately account for the operational parameters of the hydraulic B pumps.