How is air generally removed from an aircraft hydraulic system?

The process of removing air from an aircraft hydraulic system is effectively achieved by operating the various hydraulic components through several cycles. This practice allows the fluid to flow through the system, and as the hydraulic components extend and retract, any trapped air bubbles are displaced and forced to migrate toward high points in the system, where they can then escape. This method capitalizes on the dynamic movement of the system, which enhances the separation of air from the hydraulic fluid.

Operating the hydraulic components multiple times helps achieve a more thorough purge of air as the system is designed to handle fluid flow effectively. Additionally, air bubbles naturally rise to the surface of the fluid; thus, cycling the system encourages that process, leading to better ventilation of the hydraulic lines.

Other methods, such as using an external vacuum pump to extract air or bleeding the system through relief valves, may be applicable in certain systems but are not the primary means used in typical aircraft maintenance practices. Replacing the hydraulic fluid with a gas is fundamentally incorrect, as it would compromise the system's ability to function properly since hydraulic systems are specifically designed to operate with incompressible fluids.