Hydraulic Fundamentals

The A330 hydraulic system is three independent loops at 3000 psi. Before the architecture and the abnormals start to make sense, the generic model of an aircraft hydraulic system has to be in place. This article builds that model — using the reference waterwheel as the conceptual anchor, mapping every aircraft power system (hydraulic, electrical, pneumatic) onto the same skeleton, walking through the two physical laws that govern hydraulic behaviour, the standard component set, common failure modes, the operating-pressure conventions of transport-category jets, the fluid chemistry, and the certification basis for the three-system architecture.

The rest of the chapter then takes the A330 implementation system by system.

1. Why hydraulics, not pure electric

Modern fly-by-wire transport aircraft are not hydraulic instead of electric — they are hydraulic because the bus could not carry the load otherwise. Flight controls, landing gear, brakes, and thrust reversers demand short bursts of very high force. An all-electric architecture would either need significantly heavier generators and feeder cables, or accept slower actuation. Neither is acceptable on a widebody.

Per standard transport-aircraft texts (ASA Turbine Pilot's Flight Manual, Ch. 4):

In larger airplanes, hydraulics provide a powerful but relatively lightweight transmission method. Instead of using high-draw electric motors and heavy mechanical drivetrains to power each flap, landing gear, and spoiler, one hydraulic pump can transfer electric or engine power through a hydraulic system to run everything.

And:

Hydraulic power is especially valuable for heavy-duty applications because it can be drawn directly from engine power, so as not to tax aircraft electrical systems. Electric motors draw tremendous current when used for heavy-duty, intermittent operations.

Hydraulics solves the high-force demand by taking mechanical power directly off the engine accessory gearbox through engine-driven pumps. The bus is not asked to move the flaps. The fly-by-wire layer carries the signal; the hydraulic layer carries the force.

That division — wires move information, fluid moves surfaces — is the design philosophy. It is why fly-by-wire and hydraulics coexist on every Airbus type and on every modern transport jet.

2. The reference waterwheel — a conceptual anchor

Standard transport-aircraft pedagogy uses the waterwheel as the reference model for understanding power systems. The waterwheel is a familiar mental image: a stream of water turns a wheel that does mechanical work; a reservoir at the source stores the working fluid (water) and provides hydrostatic pressure; pipes deliver flow from reservoir to wheel; valves direct it.

There are no waterwheels on turbine aircraft. But every aircraft power system — hydraulic, electrical, pneumatic — has the same skeleton, and learning the parts in the waterwheel makes the others easier to see.

                              ┌────────────────────┐
   inflow (river)  ───────►   │   Reservoir        │  ───► flow  ───►  Waterwheel
   (gravity = "power source") │   stores water,    │                   (output device,
                              │   regulates flow   │                    does mechanical
                              └────────┬───────────┘                    work — "gizmo")
                                       ▲                                       │
                                       │                                       │
                                       │                                       ▼
                                Pump (returns water)  ◄──────────────────── Drain
                                       ▲
                                       │
                              external power
                              (gravity / crank)

The reservoir in the waterwheel does two things at once — it stores water volume (so the wheel keeps running between inflow surges), and it stores potential energy in the form of head pressure (or, if the reservoir is sealed and pressurised above the water, in the form of gas pressure). The same reservoir, by holding water above the wheel, supplies the flow that does the work. This dual function is preserved in the aircraft architecture, where the same role is split across two components: a reservoir for volume, and an accumulator for energy.

3. The four-system mapping

Every aircraft power system — hydraulic, electrical, pneumatic — maps directly onto the waterwheel reference. The mapping makes a single learning effort serve multiple chapters of aircraft knowledge.

Two observations from this table:

• Storage is split across two components in the hydraulic and electrical systems. Volume (the bulk supply that the pump or generator draws from) sits in the reservoir or battery; energy (instantaneously available delivery during a transient) sits in the accumulator or, again, the battery. In pneumatics, the air supply is effectively unlimited (drawn from engine bleed), so the dual storage is less pronounced.
• The "protection" row is conceptually important. Every power system has a mechanism to limit the consequences of a failure: a hydraulic fuse closes when flow exceeds a threshold (indicating downstream rupture); a circuit breaker opens on overcurrent; a pressure-relief valve opens to vent overpressure. The architectures are different but the role is the same.

A pilot reading an ECAM page on any of the three systems is reading a specialisation of this same skeleton. The reservoir-on-the-SD-page, the pump-status indications, the system-pressure number — all map directly onto the conceptual model above.

4. The two physical laws

Two properties of fluids govern everything that hydraulic systems do.

Pascal's law — pressure transmits equally

Per standard physical references and aviation texts:

As per Pascal's Law, the pressure of a fluid exerts a force perpendicular to any contacting surface, regardless of its orientation. Additional pressure applied to the fluid is transmitted equally throughout the system.

A 10 lbf load on a 1 in² piston produces 10 psi throughout the connected fluid; the same 10 psi acting on a 50 in² piston delivers 500 lbf at the output.

   F₁ on small piston (1 in²)          F₂ on large piston (50 in²)
        │                                       ▲
        ▼                                       │
   ┌────────┐                              ┌─────────┐
   │   ↓    │  ════════════════════════►   │   ↑     │
   │  10 lb │  fluid at 10 psi everywhere  │  500 lb │
   └────────┘                              └─────────┘
   
   F₁ / A₁ = F₂ / A₂      →     10 / 1 = F₂ / 50     →     F₂ = 500 lbf

This is the mechanical advantage of hydraulics: a small input force becomes a large output force, in proportion to the ratio of piston areas. Pedal forces in the tens of pounds become braking forces in the thousands. The trade is displacement — the output piston moves through only 1/50 of the input piston's stroke. Mechanical advantage costs travel; this is consistent across any mechanical advantage system.

A 3000-psi pump acting on a 10 cm² actuator piston delivers approximately 200 kN of force at the actuator. That figure is why hydraulics can drive flight-control surfaces, gear extension, brake clamping, and reverser deployment without enormous physical actuator size.

Incompressibility — power transmits without slack

Per the same standard reference:

Liquids (unlike gases) are noncompressible. Therefore, moving a given volume of fluid at one end of a sealed system will displace an equal volume at the other end. This hydraulic principle allows transmission of force and movement over relatively long distances with virtually no friction.

Move a volume of fluid in at one end of a sealed system, and the same volume must come out at the other end — immediately, with no compression slack and no time delay. The result is near-instant response at the actuator: a fly-by-wire computer commanding a spoiler to deflect within milliseconds gets exactly that, because the fluid does not absorb the command in the way a compressible gas would.

A non-obvious consequence: the hydraulic system is not flowing most of the time. The pumps maintain pressure against demand-driven valves. Flow occurs only when an actuator moves. The mental image is closer to a compressed spring waiting to release than to a running river — the pumps hold pressure; consumers occasionally draw from it.

This explains why a normal SD HYD page shows steady 3000 psi: there is no continuous fluid circulation in the system; the pumps are simply holding pressure against closed consumer valves. Real flow happens in brief bursts when a gear retracts, a spoiler deploys, a brake is applied — and the architecture is designed to absorb those bursts without visible pressure dip.

5. The standard component set

Every aircraft hydraulic system carries the same set of components, with implementation specifics varying by aircraft. The A330 specifics are in subsequent articles; this section establishes the generic roles.

5.1 Hydraulic pumps

Per standard texts:

Hydraulic pumps are generally rotary pumps, geared directly off the aircraft engines or driven electrically. These pumps convert the rotary motion of the power source to hydraulic pressure and flow.

The A330 has four classes of hydraulic pump:

• Engine-Driven Pumps (EDPs) — driven by the engine accessory gearbox; primary source for normal operation.
• Electric pumps — driven by AC electric motors; backup and ground source.
• Ram Air Turbine (RAT) pump — driven by an emergency-deployed turbine in the airstream; last-resort source for Green.
• Hand pump — manually operated; cargo-door support when no electrical power is available (Yellow only).

Variable-displacement pumps (the A330's choice for all engine-driven and electric pumps) automatically adjust output to match demand, holding system pressure constant. Detailed pump engineering is in Engine-Driven Pumps, Electric Pumps, Ram Air Turbine, and Manual Pump.

5.2 Hydraulic motors

Hydraulic motors are relatively small units (compared with electric motors) that convert hydraulic power back into mechanical power. They are normally rotary impeller units (basically 'pumps run backward'), which convert hydraulic pressure and flow back into rotary output to turn shafts operating, for example, flaps or landing gear.

A hydraulic motor is essentially a pump operated in reverse — fluid flowing in produces rotation out. Its size, for a given power output, is dramatically smaller than an equivalent electric motor: this is the architectural reason flap drive motors, landing-gear retraction motors, and similar consumers are hydraulic rather than electric on transport aircraft.

A330 hydraulic motors include the flap drive motors (one Green channel, one Yellow channel), the slat drive motors (one Blue channel, one Yellow channel), and the landing-gear retraction motors.

5.3 Hydraulic cylinders (actuators)

Hydraulic cylinders use pistons to translate hydraulic pressure into linear mechanical movement. These are used for many purposes, brakes being among the most obvious. Hydraulic cylinders are also used to power control surfaces, gear doors, air stair doors, and other devices with relatively short travel.

Linear actuators on the A330 include the flight-control surface servocontrols, the brake actuators, the gear-door actuators, the cargo-door actuators, and many smaller secondary actuators. The cylinder is the simplest form of hydraulic output device; the geometry is direct (force × stroke ÷ pressure = piston area).

5.4 Hydraulic lines

Hydraulic lines (flexible and rigid versions) deliver hydraulic power from the pump to the hydraulic motor.

The A330 uses three classes of line:

• High-pressure (HP) lines — 3000 psi nominal, rigid metal tubing for most runs.
• Return lines — typically below 50 psi, conducting fluid back to the reservoir.
• Case-drain lines — small flow from pump internal leakage, separately routed to the reservoir through its own filter.

The lines themselves are uneventful in normal operation; their architectural significance is in their separation between systems. No line carries fluid between Green, Blue, and Yellow systems — by design and by certification.

5.5 Valves

Valves direct the flow of hydraulic fluid, and therefore power, to where it's needed. For example, in the case of hydraulic landing gear, valves can direct hydraulic flow/pressure to one side of a hydraulic landing gear motor for retraction or to the other side to reverse the motor's direction for extension.

The A330 uses several valve types:

• Check valves — one-way flow direction.
• Selector valves — direct flow to one of several consumers.
• Pressure regulators — maintain output pressure within a band.
• Priority valves — automatic shed of secondary consumers when pressure drops (covered in Priority Function and Fire Shut-Off Valves).
• Fire shut-off valves — close on ENG FIRE pushbutton or automatic command.
• Leak measurement valves — isolate flight controls on the ground for leak-rate testing.

5.6 Accumulator

The accumulator is the energy-storage component. A small fluid volume is held behind a compressed-gas pre-charge; when system pressure drops briefly (a demand transient), the accumulator delivers fluid to smooth the dip; when system pressure recovers, fluid refills the accumulator and re-compresses the gas. Detailed engineering in System Accumulators.

5.7 Reservoir

The reservoir is the volume-storage component. Bulk fluid sits at low pressure, available for the pump to draw from. Each system has its own reservoir with no fluid path between them. Detailed engineering in Hydraulic Reservoirs.

5.8 Hydraulic fuse / leak measurement valve

The hydraulic fuse is a conceptually familiar but often overlooked component. From the same reference:

A hydraulic fuse is a safety component designed to prevent a catastrophic loss of hydraulic pressure. (The term 'fuse' is an analogy to the more familiar electrical fuses that serve similar functions in electrical circuits.) Hydraulic fuses are installed at strategic locations throughout an aircraft hydraulic system and are designed to detect leak-producing failures such as a failed fitting or hydraulic line break. If a leak develops, a hydraulic fuse prevents excessive fluid loss while still permitting operation of remaining hydraulic system components.

The A330 does not document a part labelled "hydraulic fuse" in its maintenance documentation. The protective philosophy is implemented through two components instead:

• The leak measurement valves (upstream of the primary flight controls), which can be closed on the ground for leak-rate testing, but more importantly serve the architectural role of allowing the system to identify and locate fluid loss.
• The priority function, which automatically isolates heavy consumers when system pressure indicates a loss condition — effectively serving the leak-containment role for the architecture as a whole.

Generic understanding of "hydraulic fuse" helps in seeing why these A330-specific components exist. They are the architectural answer to the same problem.

6. Common failure modes

Hydraulic systems share a small set of failure modes that recur across architectures. Recognising the modes makes specific aircraft failures easier to interpret.

Each of these modes has a specific A330 response architecture, covered in the relevant articles. The point at this conceptual level is that the modes are predictable — they recur across all transport-aircraft hydraulics, and the architectural responses follow the same patterns.

7. Operating pressure — 3000 psi (2500 with RAT)

Transport-category jets operate hydraulic systems at high pressure, conventionally around 3000 psi:

Aircraft hydraulic systems operate at very high pressures, usually around 3,000 psi.

The A330 follows this standard. Per FCOM DSC-29-10:

• Normal system operating pressure: 3000 psi.
• When Green is supplied by the RAT pump (dual-engine-out case): 2500 psi.

The reduced figure under RAT is not a fault — it reflects the limited extraction power of an air-driven turbine against the same downstream consumers. When the RAT pressurises Green, the aileron, elevator, and spoiler servo control operating speeds are reduced; the surfaces still travel their full range, more slowly.

Newer airframes use 5000 psi to reduce pipe diameter and weight at the cost of tighter sealing tolerances. Neither is "better"; they are different engineering trade-offs. 3000 psi was the industry standard for transport aircraft for decades and remains the standard for the A330's generation.

The pressure shows up in two places for the pilot:

• The SD HYD page shows pressure in green when normal, with LO annunciations against pump-specific thresholds.
• After RAT extension, 2500 psi is the new normal for Green. Treat it as expected for the configuration, not as a fault.

8. Why three systems, not two

The A330 carries three independent hydraulic systems with no fluid transfer between them. This is regulatory, not a redundancy preference.

Transport-category certification (CS-25.1309 / FAR 25.1309) requires that no single failure produces a catastrophic condition, and that catastrophic failure probability remain "extremely improbable" — conventionally below 10⁻⁹ per flight hour.

Two interconnected systems cannot meet that figure. They share common-mode failure paths:

• A single contaminated batch of fluid (one shared reservoir).
• A ruptured cross-connection (one fluid path).
• A shared bleed source for reservoir pressurisation (one bleed path).

Each shared path is a single point of failure for both systems simultaneously. The architecture cannot meet 10⁻⁹ per flight hour against all such paths combined.

Three independent systems break the common mode:

• Three separate reservoirs of fluid.
• No cross-connections at the system level.
• Even where bleed is shared (Engine 1 HP feeds all three reservoir cushions), the crossbleed provides backup, and the bleed function is cavitation prevention, not fluid delivery.

The Ram Air Turbine adds a fourth, fully independent power source for the Green system in the loss-of-all-engines case. By extending Green via an entirely separate power source (ram air), the RAT closes the architectural margin against multi-engine failure.

Understanding this re-frames every system question that follows: redundancy is not a designer's preference; it is a regulatory constant. The pilot's job in any hydraulic anomaly is to identify which system is degraded and what is lost — never to expect fluid cross-feed to recover the loss.

9. The fluid — phosphate-ester (Skydrol family)

The fluid in Airbus hydraulic systems is a phosphate-ester (commonly called Skydrol-type, after the dominant brand). Per ASA Turbine Pilot's Flight Manual:

Hydraulic fluids are specially formulated to withstand these conditions without vaporizing... Incidentally, hydraulic fluids are often highly caustic; avoid getting them on your skin or in your eyes.

The fluid is chosen for a single reason: fire resistance. Mineral hydraulic fluids ignite when sprayed onto a hot brake or engine surface; phosphate ester does not propagate flame.

Three operational consequences follow:

• Not interchangeable with mineral fluid. The seal compounds (EPDM, butyl) used in Skydrol-compatible systems swell or dissolve in mineral oil. A wrong-fluid event means fleet-wide reseal — measured in days, not hours.
• Corrosive to skin, eyes, and many paints. A visible leak on the airframe is reported and traced; it is never "wiped off and dispatched". The stain is the symptom; the source must be identified.
• Hygroscopic and degradation-sensitive. Moisture, contamination, and high temperatures shorten fluid life. Maintenance monitors fluid condition; the crew's role in flight is to flag reservoir overheat indications when they appear.

Detailed fluid chemistry and thermal management is in Hydraulic Fluid.

10. Pilot's contact points with the hydraulic system, across the flight

The hydraulic system is mostly invisible in normal operation, but the pilot encounters it at specific points across the flight. Recognising these contact points helps anchor the abstract architecture to operational reality.

The activity pattern across the day shows that the heavy hydraulic events are at takeoff (gear, flap), descent (speed brakes), and landing (spoilers, brakes, reversers). Cruise is hydraulically uneventful. The architecture is designed to handle the heavy events without visible system perturbation — which is why a healthy cruise produces a near-static SD HYD page.

11. Five questions for any hydraulic anomaly

Every hydraulic ECAM event reduces to five questions, asked in order:

1. Is the indication real? Pressure transients during heavy demand (gear cycling, flap retraction, simultaneous brake and spoiler activity) can briefly drop SD readings. A genuine fault persists.
2. Pump failure or system failure? PUMP LO PR means the pump has dropped offline; the system may still be pressurised by another pump on the same loop (Green has two EDPs; Blue and Yellow each have an electric pump). SYS LO PR means the entire loop has lost pressure.
3. What is lost? What remains? Each hydraulic system drives a specific list of consumers, and the flight control law degrades according to which systems remain.
4. What does ECAM ask for? Procedures may call for isolating a pump (to protect against reservoir depletion or pump damage) or transferring a function to an alternate path. Trust the procedure design and execute as written.
5. What is the degraded profile? Approach speed, available flap detents, brake channel, and reverser availability all change after a hydraulic loss. The amendment to the landing performance appears in the STATUS page after the failure is processed.

These five questions reappear, with type-specific numbers, in every abnormal article in this chapter.

12. Reading order for the chapter

The remaining articles take the A330 implementation system by system:

• General Description — three independent systems, 3000 psi, HSMU monitoring, no cross-feed.
• Hydraulic Generation Overview — pump configurations across the three systems.
• Hydraulic Reservoirs, Reservoir Pressurisation, Hydraulic Fluid — storage, pressurisation, and chemistry.
• Engine-Driven Pumps, Electric Pumps, Ram Air Turbine, Manual Pump — pump engineering, system by system.
• System Accumulators, Priority Function and Fire Shut-Off Valves, Filters and Leak Measurement Valves, HSMU — distribution and protection.
• Power Distribution Map, Gravity Gear Extension, ECAM HYD Page — interface and consumer mapping.
• Q&A articles on warnings reference, single- and dual-system loss, pump-vs-system distinction, MEL, typical day, maintenance view.

Self-test

[!note]- Q1. The reference waterwheel's reservoir does two things at once. What are they, and how does the aircraft hydraulic architecture split them?

The waterwheel reservoir stores water volume (the bulk supply that lets the wheel run between inflow surges) and stores potential energy (the head pressure that drives the flow, or the gas pressure above a sealed reservoir). In the aircraft hydraulic architecture, these two functions are split across two components: the reservoir stores fluid volume at low pressure (the pump's bulk supply), and the accumulator stores energy as a small high-pressure fluid volume behind a compressed-gas pre-charge. Both exist on every A330 system because they solve different problems: continuous supply versus instantaneous delivery.

[!note]- Q2. A 10 lbf force is applied to a 1 in² piston in a closed hydraulic system. The output piston is 50 in². What is the output force, and what is the trade?

Output force is 500 lbf — pressure of 10 psi acts on 50 in² to give 500 lbf. The trade is displacement: the output piston moves through only 1/50 of the input piston's stroke. Mechanical advantage in any system costs travel. This is the principle behind brake-pedal force amplification on a transport jet — moderate pedal force produces the braking force needed to stop a heavily loaded aircraft.

[!note]- Q3. The hydraulic system is described as "not flowing most of the time". What does this mean, and why is it counterintuitive?

It means the pumps maintain system pressure against closed demand-driven valves; fluid does not continuously circulate. Flow occurs only when an actuator moves — a spoiler deploys, a gear retracts, a brake is applied. The image of a "river of fluid" is wrong; the image of a "compressed spring" is closer. This is counterintuitive because the term hydraulic suggests continuous flow (as in plumbing), but the operating state of a transport hydraulic system is mostly steady pressure with intermittent demand.

[!note]- Q4. Why does the A330 have three independent hydraulic systems instead of two interconnected ones?

Certification. CS-25.1309 / FAR 25.1309 require the probability of catastrophic failure to be extremely improbable, conventionally below 10⁻⁹ per flight hour. Two interconnected systems share common-mode failure paths (contaminated fluid, shared cross-connections, common bleed source) and cannot meet that target. Three fully independent systems break the common mode. The choice is regulatory, not a redundancy preference, and it explains why no fluid cross-feed exists on the A330 — by design, never by oversight.

[!note]- Q5. The cavitation failure mode is fundamentally about a mismatch between which two pressures, and how does the A330 architecture prevent it?

Cavitation occurs when the pump inlet pressure drops below the fluid's vapour pressure at the operating temperature. Vapour bubbles form at the inlet, then collapse violently as they enter the high-pressure side, eroding pump internals. The A330 architecture prevents cavitation through three mechanisms working together: (1) the reservoir gas cushion at 4.5 bar absolute keeps the suction line above vapour pressure across all operating temperatures and altitudes; (2) each pump has an inlet boost impeller that adds additional pressure right at the pump inlet; (3) the LO AIR PRESS caution at 1.5 bar relative warns the crew when the cushion has decayed substantially, before the actual vapour-pressure limit is approached. Together, the three layers maintain the inlet pressure margin under all operating conditions.

[!note]- Q6. Maintenance reports a hydraulic-fluid stain on the airframe near the wing root. The captain asks if it can be wiped off and dispatched. What is the issue?

Phosphate-ester fluid is corrosive to airframe paint and many composite primers, and skin contact is harmful. More importantly, a visible stain means the source must be identified — the fluid may indicate a developing leak, contamination across system boundaries, or a wrong-fluid event. None of these are dispatched by cleaning the visible residue. Maintenance traces the source before any dispatch decision is made. Hydraulic-fluid stains are never a cosmetic issue.

References

Generic transport-aircraft hydraulic theory consistent with standard texts (ASA Turbine Pilot's Flight Manual, Ch. 4 — reference waterwheel model, four-system mapping, Pascal's law, fluid incompressibility, hydraulic fuse concept, standard component definitions). A330 specifics per FCOM DSC-29-10 (General, Generation, Distribution). Certification basis per CS-25.1309 / FAR 25.1309 (public regulatory text).

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.