What are the symptoms of a failing hydraulic system?

A failing hydraulic system can significantly affect an aircraft's operations, and one of the most noticeable symptoms is difficulty in moving control surfaces. Hydraulic systems are essential for controlling various flight surfaces, such as ailerons, elevators, and rudders. When the hydraulic system is compromised, it leads to reduced fluid pressure and flow, making it harder for pilots to maneuver these essential components. This can manifest as sluggish or unresponsive control inputs, which is critical for safe flight operations.

The other options may present issues under different circumstances, but they do not specifically relate to the direct functionality of the hydraulic system. Increased fuel consumption typically pertains to engine performance or aerodynamic issues, while frequent engine stalling is usually linked to the powerplant rather than the hydraulic system. Unusual smells in the cockpit could indicate a wide range of problems, including electrical or material failures, but again, they don't pinpoint a failing hydraulic system directly. Thus, difficulty in moving control surfaces is the most clear and direct symptom of hydraulic failure.