What triggers the auxiliary hydraulic system to power the lower rudder PCU?

The auxiliary hydraulic system is designed to provide backup hydraulic power in the event of a primary hydraulic system failure. In this scenario, the trigger for the auxiliary hydraulic system to power the lower rudder Power Control Unit (PCU) is a failure in the right hydraulic system.

When the right hydraulic system experiences a failure, the aircraft relies on the auxiliary system to maintain control over critical flight surfaces, such as the rudder. This redundancy is essential for safe flight operations, ensuring that pilots have the ability to control the aircraft even if one of the primary systems becomes inoperative. The auxiliary systems are engineered to engage automatically in response to specific failures, enhancing the overall safety and reliability of the aircraft's flight control mechanisms.

In contrast, failures in the left hydraulic system, the fuel system, or the electrical system would not specifically engage the auxiliary system to power the lower rudder PCU, as those systems operate independently or have different fail-safes and responses. Thus, the focus on the right hydraulic system failure underscores the critical design aspect of maintaining full control of the aircraft under all circumstances.