Why hydraulics, and how systems are laid out
Hydraulics transmits large forces through small pipes: by Pascal's law pressure in a confined fluid acts equally in all directions, and the force on a piston is F = P × A. Fluid is almost incompressible, so actuators respond immediately and hold position firmly. Most transport aircraft operate at 3 000 psi (207 bar); the A380 and B787 use 5 000 psi (345 bar), which allows smaller, lighter pipes and actuators.
Because flight controls, landing gear, brakes and steering depend on it, hydraulic power is divided between two to four independent systems (for example Green, Blue and Yellow on the A320; A, B and standby on the B737; Left, Centre and Right on the B777). Each has its own reservoir, pumps, filters and accumulators; important services are powered by more than one system.
Hydraulic fluids
| Fluid | Type and colour | Seals | Notes |
|---|---|---|---|
| MIL-H-5606 / MIL-PRF-5606 | Mineral (petroleum) based, red | Synthetic rubber (nitrile / Buna-N, neoprene) | Flammable; light aircraft, shock struts, older systems |
| MIL-H-83282 / MIL-PRF-87257 | Synthetic hydrocarbon (polyalphaolefin), red | Same as 5606 (compatible) | Fire-resistant; military and some civil use |
| Phosphate ester (Skydrol, Hyjet) | Synthetic, purple (Type IV/V) | Butyl rubber, ethylene-propylene (EPR), Teflon | Fire-resistant — the transport-aircraft standard. Attacks paint, some plastics and wiring insulation; irritates skin and eyes |
| Vegetable-based (obsolete) | Blue | Natural rubber | Found only on very old aircraft |
Fluids of different types must never be mixed, and seals must match the fluid: phosphate-ester fluid destroys nitrile seals, and mineral oil swells butyl seals. A system contaminated with the wrong fluid must be flushed and its seals replaced. When handling Skydrol, wear gloves and eye protection, and clean spills promptly — it softens paint.
Contamination — particles, water, air and wrong fluid — is the main enemy. Fluid samples are taken for particle counts and analysis, filters are inspected for debris (which can reveal pump wear), and all connections are capped when disconnected.
Reservoirs, accumulators and filters
Reservoirs
- Store fluid, allow for thermal expansion and for the volume difference as actuators extend and retract, separate air from the fluid and provide a head of fluid for the pumps.
- Pressurisation prevents pump cavitation at altitude: by regulated bleed air (around 50 psi) or as a bootstrap reservoir, in which system pressure acting on a small piston pressurises the fluid through a larger piston.
- A standpipe feeds the engine-driven pump from above a reserve: if a leak in that pump's circuit drains the reservoir down to the standpipe, fluid below it remains for the electric or emergency pump.
- Fittings: sight gauge and quantity transmitter, temperature sensor, relief and vent valves, drain, and the servicing connection.
Accumulators
- Functions: damp pressure surges and pump ripple, supplement pump flow at peak demand, maintain pressure for small leaks, and provide an emergency supply — notably for the brakes and parking brake.
- Pre-charge pressure is typically a third to a half of system pressure (for example 1 000 psi in a 3 000 psi system). It is checked and adjusted only with the hydraulic side fully depressurised; with fluid pressure present, the gauge shows system pressure instead.
- Charge with dry nitrogen only: oxygen with hydraulic fluid can explode.
Filters
- Pressure filters downstream of the pumps, return filters before the reservoir, and case-drain filters on the pump drain lines (debris here is an early sign of pump failure).
- Elements of fine paper, glass fibre or sintered metal remove particles down to a few microns.
- A differential-pressure (clogging) indicator pops out when the element is nearly blocked; a thermal lockout stops false indication when the fluid is cold and thick. A bypass valve opens at high differential pressure so flow continues — but now unfiltered, so a popped indicator must be investigated.
Pressure generation
| Source | Notes |
|---|---|
| Engine-driven pumps (EDPs) | Variable-displacement axial piston pumps on the accessory gearbox; a depressurising (blocking) valve offloads the pump when switched off; its case-drain flow cools and lubricates it |
| Electric motor-driven pumps (EMPs / ACMPs) | AC motor-driven piston pumps; ground operation, back-up and peak demand |
| Air-driven pumps (ADPs) | Driven by a bleed-air turbine — the pneumatic pressure source of the syllabus (e.g. demand pumps on some wide-bodies) |
| Ram air turbine (RAT) pump | Emergency pressure for flight controls when engine pumps are lost |
| Power transfer unit (PTU) | A hydraulic motor in one system driving a pump in another — transfers power, never fluid; often operates automatically when the difference in pressure between the systems exceeds a set value |
| Hand pumps | Light aircraft, ground servicing, cargo-door operation and some emergency functions |
| Fixed-displacement (gear) pumps | Simpler systems; need an unloading valve or relief valve to limit pressure, since they always deliver the same flow per revolution |
Pressure control and distribution
| Component | Function |
|---|---|
| Pressure compensator (in the pump) | Holds system pressure by varying pump displacement |
| System relief valve | Opens to return if pressure exceeds a set value (a back-up to the compensator) |
| Thermal relief valve | Relieves pressure rise from heating in a trapped (isolated) line |
| Unloading valve / pressure regulator | With a fixed-displacement pump, sends flow to return once the accumulator is charged |
| Pressure-reducing valve | Supplies a lower pressure to a sub-system (e.g. some brake or steering circuits) |
| Priority valve | When system pressure falls, shuts off less important users so that essential services such as the flight controls keep pressure |
| Check valve | Allows flow in one direction only |
| Shuttle valve | Automatically connects a user to an alternate supply when the normal one fails (e.g. normal/alternate brakes) |
| Hydraulic fuse | Shuts off flow if a downstream line ruptures (excessive flow or volume), saving the rest of the fluid |
| Selector and sequence valves, restrictors | Direct flow to actuators, make actions happen in order (e.g. gear doors then gear), slow movements |
Distribution: rigid pipes (stainless steel or titanium for pressure lines, aluminium alloy for some return lines), flexible hoses where there is movement (with fire sleeves in fire zones), quick-disconnect couplings at component interfaces, and identification bands on every line. Ground service connections allow a hydraulic ground cart to pressurise each system.
Indication and warnings
For each system the crew sees pressure (transmitters), reservoir quantity, and cautions for low pressure, low quantity, high fluid temperature or pump overheat (case-drain temperature), and pump faults; maintenance also sees filter-clogging indicators. Glass-cockpit aircraft show a hydraulic synoptic page with the state of every pump, valve and PTU.
Servicing
- Put the aircraft in the configuration the AMM specifies for reading the reservoir level — typically all systems depressurised, accumulators pre-charged, landing gear down, flaps and spoilers retracted, thrust reversers stowed — because fluid moves between actuators and reservoir as they operate.
- Read the sight gauge or quantity indication and compare with the full mark for that configuration.
- Replenish with the correct, clean fluid through the servicing connection and its filter (hand pump or service cart), never by pouring into an open reservoir.
- After component changes, bleed air from the system as the AMM directs and check for leaks with the system pressurised.
- Always depressurise a system (and its accumulators, where relevant) before disconnecting any line — a 3 000 psi jet of fluid can inject fluid through the skin.
Emergency pressure and interfaces
- Emergency sources: the RAT (automatic deployment on loss of engines or of the relevant systems), the PTU, electric pumps on essential power, accumulators for brakes, and hand pumps on some aircraft.
- Interfaces: hydraulic power operates the flight-control PCUs, spoilers, flaps and slats, landing gear extension and retraction, nose-wheel steering, wheel brakes, thrust reversers, cargo doors and sometimes airstairs; on some aircraft a hydraulic-motor-driven generator provides emergency electrical power, and the electrical system powers the electric pumps — so the two systems back each other up.