The previous lesson built the hydraulic system out of five words: reservoir, pump, valves, actuator, return. This lesson opens the boxes. It walks the pumps and the sources that drive them, the full component inventory an FCOM (flight crew operating manual) schematic expects a pilot to recognise, the redundancy built from them, and the quantities, pressures, temperatures and warnings by which the flight deck watches it all work.
Pumps and their power
Two pump principles divide the fleet, and the difference is what regulates the pressure:
Constant-pressure pumps (swash plate or cam plate) — a ring of pistons driven round an angled plate; the plate's angle sets the stroke, and the pump varies that angle itself, delivering exactly the flow needed to hold system pressure. At zero demand the plate flattens and the pump idles along at pressure without pumping: the pump is its own regulator.
Pumps whose output depends on rpm (gear type) — two meshing gears carry fluid round at a rate fixed by their speed. Output rises with rpm whether the system wants it or not, so a separate regulator or relief arrangement must return the surplus.
What turns the pump is a separate design choice, and the syllabus's list of pressure sources is really a list of failure independence:
Manual — a hand pump: the last-resort source, still found for emergency gear extension and ground servicing.
Engine gearbox — the engine-driven pump (EDP): the workhorse, powered whenever its engine turns, lost with that engine.
Electrical — an electric motor-driven pump: independent of any one engine, usable on the ground with the engines shut down, limited by the electrical system's capacity.
Air — pneumatically driven pumps running on bleed air (so available only while an engine or the APU can supply it), and the ram-air turbine (RAT), a small propeller that drops into the airstream and drives a pump when everything else has failed: pressure bought from airspeed alone.
Hydraulic — the power transfer unit (PTU), a hydraulic motor in one system spinning a pump in another, moving power across without moving a drop of fluid between them; reversible motor pumps do the same trick in either direction.
Accessory — a pump driven from an accessory gearbox or auxiliary power source, typical for ground operation and secondary duties, and limited to whatever its gearbox or auxiliary source is running.
One more pump completes the set: the demand pump fitted on many transport aeroplanes, a backup (usually electric or air-driven) that runs only when it is needed: switched on automatically when the primary pump's pressure falls or when a high-demand phase (gear and flaps moving together) asks for more flow than the primary alone can give. Note the naming trap: this backup does not make the system pressurised on demand in the previous lesson's sense, because the engine-driven pump holds the system at working pressure throughout; the demand pump is redundancy inside a constant-pressure system.
The component inventory
Between pump and actuator the fluid passes a standard cast of components. Storage and delivery first:
Reservoir (pressurised or unpressurised) — holds the fluid, accepts the return flow, allows for volume changes and gives the pump a positive supply. On aircraft flying high it is pressurised (by bleed air or system pressure) so that the thin air at altitude cannot starve the pump into cavitation, vapour bubbles forming and collapsing inside a starved pump until it destroys itself; an unpressurised reservoir belongs to aircraft that stay low.
Accumulator — a vessel split by a piston or diaphragm, hydraulic fluid one side and compressed gas (usually nitrogen) the other. It stores pressure, smooths the pump's pulses, absorbs shocks, and holds an emergency reserve: parking-brake pressure hours after shutdown is the accumulator's doing.
Pressure and return pipes — the arteries and veins: pressure lines built for the full working pressure, return lines for the gentle trip back to the reservoir.
Case drain and fluid cooler return lines — a pump leaks a little internally by design, and that case-drain flow carries away the pump's heat, through a cooler (often sitting in a fuel tank) and back to the reservoir. Watching its temperature watches the pump's health.
Filters — in pressure, return and case-drain lines: hydraulic tolerances are microscopic, and dirt is the system's slow poison. A clogging filter opens a bypass rather than starving the system, and shows a pop-out indicator to maintenance.
Piston actuators (single- and double-acting) — the muscle: a single-acting actuator is powered one way and returned by a spring, a double-acting one is powered in both directions, pressure switched to either side of its piston.
Hydraulic motors — actuators for rotation instead of stroke: flap and slat drives, and the motor half of a PTU.
Then the valves, each a one-job device:
Non-return (check) valves — let flow pass one way and block the other: the reason a failed pump cannot be back-driven and a charged accumulator cannot empty backwards.
Relief valves — spring-loaded overpressure guards, cracking open to return when pressure exceeds the limit: the safety valve of a gear-type pump's circuit and of thermal expansion in blocked lines.
Restrictor valves — deliberate narrowings that slow an actuator to a designed speed, often one-way: gear doors that fall gently instead of slamming.
Selector valves (linear and basic rotary, two and four ports) — the routing switches, worked either by sliding a spool along its bore (linear) or by turning a plug to line up its ports (rotary): a two-port selector opens or closes one path, a four-port selector feeds a double-acting actuator's two sides and swaps them to reverse it.
Bypass valves — open an alternative path around a component, as around a clogged filter, or connect an actuator's two sides so a surface can trail freely when its system is off.
Shuttle valves — two supply inlets, one outlet, and a floating piston that connects the outlet to whichever inlet has pressure: how an emergency source takes over a brake or gear actuator without any selection.
Fire shut-off valves — close the hydraulic supply at the engine pylon when the fire handle is pulled, so the fire is not fed fluid.
Priority valves — when pressure sags, close off the heavy non-essential users so what pressure remains serves the flight controls first.
Fuse valves — hydraulic circuit breakers: if the flow through them exceeds what any intact circuit downstream could take (a burst line), they close and trap the rest of the system's fluid. Fitted where lines are exposed, brakes especially.