Which system failure requires the use of the emergency hydraulic system?

The emergency hydraulic system is specifically designed to take over when the primary hydraulic system fails. The primary hydraulic system is responsible for various critical functions in an aircraft, including operating flight controls and landing gear. When this system experiences a failure, the emergency hydraulic system provides a backup mechanism to maintain control, ensuring the safety of the aircraft and its occupants.

In contrast, other systems, such as the fuel system, electrical system, and navigation system, do not have a direct dependency on hydraulic operations. While failures in these systems can significantly impact aircraft performance and safety, they do not necessitate the use of the emergency hydraulic system. Thus, recognizing the primary hydraulic system's crucial role highlights the importance of having an emergency hydraulic system available for situations involving its failure.