If all four engines fail, which hydraulic system is powered by the RAT-driven hydraulic pump?

In the C-17, if all four engines fail, the Ram Air Turbine (RAT) automatically deploys to provide emergency hydraulic power to one of the hydraulic systems. Specifically, the RAT drives hydraulic pump number 4. This system is critical for maintaining control of essential aircraft functions, particularly during an emergency situation when engine-driven hydraulic systems are no longer operational. The RAT and the corresponding hydraulic pump ensure that there is still a means to operate essential flight controls and landing gear, which are crucial for a safe emergency landing. The design prioritizes reliability and safety, allowing for some level of control even in the event of catastrophic engine failure.