Hydraulics

Understanding Aircraft Hydraulics for ATPL Exams and Flight Operations

Hydraulic systems are critical to modern aircraft systems, powering high-force, high-reliability functions such as primary flight controls, landing gear, flaps, spoilers, and wheel brakes. By applying Pascal’s law, hydraulics transmit pressure uniformly, allowing small cockpit inputs to move large control surfaces. Large transport aeroplanes typically operate around 3000 psi, supplied by engine-driven and electric pumps, with a pressurised reservoir to prevent pump cavitation. In the cockpit, pilots monitor hydraulic pressure, fluid temperature, and quantity to assess system health; a hydraulic low-pressure alert generally indicates the pump cannot maintain output pressure. If primary engine-driven sources are lost, a Ram Air Turbine (RAT) may automatically deploy to provide emergency hydraulic power to essential flight controls, in line with aircraft procedures and aviation regulations.

Actuators convert hydraulic pressure into mechanical motion. A single-acting actuator is powered hydraulically in one direction with return via a spring, gravity, or aerodynamic loads, while double-acting actuators use pressure on either side of a piston for positive control in both directions. Selector valves route pressure to the correct side of an actuator, whereas check valves act like an electronic diode—permitting flow in one direction only—to prevent backflow. Shuttle valves automatically switch a consumer (e.g., brakes) to the most appropriate pressure source, such as normal system pressure or an emergency/accumulator line. Understanding hydraulic lock is essential: with fluid trapped on both sides of a piston and no internal leakage, the actuator will not move because the incompressible fluid resists displacement.

Accumulators, pre-charged with nitrogen, stabilize pressure, smooth pump pulsations, and store energy for transient demands or emergency braking. After the system pressurizes above pre-charge, the gas-side gauge will read system pressure (e.g., pre-charge 1000–1200 psi, system 1500–3000 psi → indicated equals system pressure). Correct pre-charge and isolation procedures are vital to performance. Fluid selection also matters: most large airliners use synthetic phosphate-ester-based fluids, valued for fire resistance and stable viscosity; many light aircraft use mineral-based fluids. All hydraulic fluids can irritate eyes and skin—use appropriate PPE and follow the Aircraft Maintenance Manual and operator procedures. Contamination control (filters, sampling) and correct fluid compatibility with seals and hoses are essential for reliability and regulatory compliance.

Topics Covered in This Hydraulics Question Bank

• System architecture and typical operating pressures (~3000 psi), pumps, reservoirs, and cavitation prevention.
• Actuators (single-acting vs double-acting), selector valves, hydraulic lock, and force/area relationships per Pascal’s law.
• Valves and flow control: check valves (one-way “diode” function) and shuttle valves for automatic source selection.
• Accumulators: nitrogen pre-charge, pressure indications, pulsation damping, and emergency/parking brake applications.
• Hydraulic fluids: synthetic phosphate-ester vs mineral types, properties, compatibility, and safety/handling precautions.
• Cockpit indications and procedures: monitoring pressure, temperature, and quantity; low-pressure alerts; RAT-driven emergency hydraulics.
• ATPL-relevant knowledge, procedures, and compliance with aviation regulations and aircraft systems checklists.