What auxiliary system powers the lower rudder PCU during a right system failure?

The auxiliary hydraulic system is specifically designed to ensure continued operational capability of critical flight controls, such as the lower rudder Power Control Unit (PCU), in the event of a primary system failure. In aviation systems, redundancy is essential for safety, and the auxiliary hydraulic system acts as a backup to provide necessary hydraulic pressure when the primary system becomes inoperative.

In circumstances where the right hydraulic system fails, the auxiliary hydraulic system takes over the function of supplying pressure to the lower rudder PCU. This transfer of power is crucial for maintaining control during flight, preventing situations that could lead to loss of control of the aircraft.

The other options do not serve the same function in providing hydraulic pressure to the rudder during system failures. The primary hydraulic system is naturally the main source of pressure but is unavailable during a failure. The fuel hydraulic system is not typically involved in power transfer for control surfaces, and the backup hydraulic system, while a reasonable thought, is not the standard terminology used in this context. Hence, the correct answer highlights the auxiliary hydraulic system's role as a key redundant system in aircraft operations, ensuring safety and controls are maintained.