Simultaneous use of the APU and which item is prohibited?

Simultaneous use of the APU and which item is prohibited?

• A. GPU
• B. External power
• C. HP cart
• D. Ground power unit

Answer: (C)

The key idea here is that the hydraulic system should not be pressurized from two sources at once. A hydraulic power cart provides external hydraulic pressure, and some aircraft systems can also be powered hydraulically by the APU (through an APU-driven pump). If both sources are active at the same time, pressures can conflict or backflow within the hydraulic lines, risking damage to components or unexpected actuator behavior. To prevent this, simultaneous use of the APU and the hydraulic power cart is prohibited. Electrical power from a GPU (external power) is a separate electrical supply and does not furnish hydraulic pressure, so it doesn’t create the same conflict.

The key idea here is that the hydraulic system should not be pressurized from two sources at once. A hydraulic power cart provides external hydraulic pressure, and some aircraft systems can also be powered hydraulically by the APU (through an APU-driven pump). If both sources are active at the same time, pressures can conflict or backflow within the hydraulic lines, risking damage to components or unexpected actuator behavior. To prevent this, simultaneous use of the APU and the hydraulic power cart is prohibited. Electrical power from a GPU (external power) is a separate electrical supply and does not furnish hydraulic pressure, so it doesn’t create the same conflict.