Landing Gear Overview

The A330 landing gear is a system that has to do two almost contradictory jobs — vanish in flight, then carry, stop, and steer two hundred tonnes on the ground. Before the dedicated articles on extension, braking, and steering make sense, the global model of the system has to be in place: which computer owns what, which hydraulic system powers what, and how the architecture falls back, layer by layer, onto gravity and a stored-energy accumulator when electrics, hydraulics, and computers fail one at a time.

This article builds that model. The remaining articles then take each block of it system by system.

1. Two contradictory jobs

No other system on the aircraft is asked to do two such opposite things. In flight the gear must disappear — fold into its bays, sit flush with the fuselage skin, and add as little drag as possible. On the ground the same structure must bear, stop, and steer — carry the full landing weight, absorb the touchdown impact, decelerate the aircraft from well over 200 km/h, and turn it where the crew commands.

The design answer to "how does one structure do both" is given in a single phrase. Per FCOM DSC-32-10-10:

Gears and doors are electrically-controlled and hydraulically-operated.

That phrase is the soul of the chapter, and it is the same design philosophy as the fly-by-wire flight controls: every lever moved and pedal pressed in the cockpit becomes an electrical signal first, is processed by a computer, and the computer commands hydraulics to provide the force. Wires move the information; fluid moves the gear. Hold that division in mind — it is why the system needs both a control layer (computers) and a power layer (hydraulics), and why a fault in one does not automatically take the other down.

2. Two computers: LGCIU and BSCU

The control layer is split cleanly across two computers. Per FCOM DSC-32-10-10:

Gears and doors' actuation are electrically-signalled by two Landing Gear Control and Interface Units (LGCIUs). The LGCIUs process gears and doors positions, sequencing control and gear lever selection. They also provide landing gear information on the ECAM, and ground/flight signals for other aircraft systems.

So the LGCIU (two of them) owns extension/retraction and position/warning, and — almost as a side effect — serves as the whole aircraft's ground/flight signal source. Many unrelated systems (flight controls, pressurisation, ADIRS, configuration warnings) ask the LGCIU a single question: is the aircraft on the ground or in the air?

The braking and steering side belongs to a different computer. Per FCOM DSC-32-30-10:

A dual channel Brake and Steering Control Unit (BSCU) controls all braking modes and functions...

The name is the memory aid: Brake and Steering. The BSCU owns braking (normal, alternate, antiskid, autobrake) and nosewheel steering. It is one unit with two channels — only one channel is active at a time, the other on standby, swapping after each landing so a dormant fault in the standby channel is forced to the surface.

The two computers do not back each other up, and they do not need to: lose the LGCIUs and braking still works; lose the BSCU and the gear still extends. They are independent control chains over independent functions.

3. The four sub-systems

The maintenance documentation splits the whole system into four official blocks — the skeleton of this chapter. Per AMM 32-00-00, the L/G system includes the gears and doors, the extension and retraction systems for the gears and doors, the braking and related systems, and a steering system. Mapped to owner and power source:

Two things to read from this table now, before the detail arrives. First, each sub-system has its own fallback path — none of the four critical functions (get the gear down, stop, steer, know the configuration) depends on a single chain. Second, the owner column shows why a BSCU fault is heard about in two unrelated places (braking and steering), while an LGCIU fault shows up in extension and in every system that needed the ground/flight signal.

4. The hydraulic split — green, blue, yellow

The A330 has three independent hydraulic systems (the subject of ATA-29). Their division of labour inside the landing gear is worth committing to memory, because it is the common root of most failure cases:

• Green powers everything normal: normal extension/retraction, normal braking, and normal nosewheel steering. Green is the landing gear's workhorse — which is exactly why a green-hydraulic problem degrades extension, braking, and steering at the same time.
• Blue powers the braking back-ups: alternate braking, and the parking / ultimate emergency braking that sits behind the accumulator.
• Yellow powers exactly one landing-gear function: the alternate nosewheel steering (the ALTN N/W STRG source, ground-only, inhibited above 70 kt).

[!warning]- Misconception — "alternate braking is yellow" and "yellow plays no part in the gear" Two widespread errors, both corrected by the source library. Alternate braking is Blue, not Yellow (FCOM DSC-32-30-10; corroborated by AMM 32-43 and 32-45) — the accumulator that backs it is on the Blue system. And Yellow is not absent from the landing gear: per FCOM DSC-32-20-20 it supplies the alternate nosewheel steering. Nosewheel steering therefore has two hydraulic sources — Green (normal) and Yellow (alternate). The clean memory model is: Green = everything normal; Blue = braking back-up; Yellow = steering back-up.

5. The master architecture

Cockpit input → computer → hydraulics → mechanism. The diagram below compresses the AMM description into one top-level topology (a synthesis of several paragraphs, not a redraw of a single source figure):

  Cockpit input            Computer (electrical)      Hydraulic actuation        Mechanism / result
 ──────────────           ──────────────────────     ────────────────────       ───────────────────
                       ┌──────────────────────┐
  L/G lever ──────────►│ LGCIU 1  ◄──►  LGCIU 2│─────► Green EHV block ────────► retract jacks + 3 doors
   (UP / DN)           │ one master at a time; │                                 shortening / up-lock / down-lock
      │                │ swaps after retract   │
      │                └──────────────────────┘
      └─ free-fall handle ─► electric rotary actuator ─► releases uplocks + isolates Green ─► gear free-falls
         (3 turns, bypasses normal system)                                                  springs drive down-locks home

  brake pedals ───────┐   ┌──────────────────────┐   ┌─► Green ───────────────► normal braking (piston set 1)
  autobrake LO/MED/MAX┤──►│ BSCU  ch1  ◄──►  ch2  │───┤
  rudder pedals (FCPC)┘   │ antiskid / autobrake / │   └─► Blue / accumulator ──► alternate braking (piston set 2)
                         │ steering              │                              parking / ultimate (≥ 12 h hold)
                         └──────────────────────┘
  tillers ×2 ─────────┐          ▲
  rudder pedals (FCPC)┤──────────┘  ─► Green (normal) ──────────────────────► nosewheel steering (±72 degrees)
  autopilot (FCPC) ───┘             └─► Yellow (alternate ALTN N/W STRG, ground < 70 kt)

  tyre-pressure sensors ─► TPIC ──────────────────────────────────────────────► WHEEL SD page (tyre pressures)

Read five things off it. (1) Two computers, two main lines, no mutual substitution. (2) Each line runs the same three stages — electrical control, hydraulic power, mechanical output. (3) Green is the workhorse of three of those lines at once. (4) Each line keeps a dashed fallback — gravity for extension, blue-then-accumulator for braking, yellow for steering. (5) Every brake carries two piston sets — green pushes set 1, blue pushes set 2, physically separate so a dirty or leaking system cannot contaminate the other. That last point is the physical basis of the whole braking fallback chain and is developed in Brake System Overview.

6. Extension and retraction — green, with a gravity fallback

In normal operation the lever signals the LGCIU, which drives the green electro-hydraulic valve block through a fixed sequence: open the doors, extend or retract the legs, close the doors. Of the two LGCIUs only one is master at a time; they swap after each retraction.

A speed protection sits on top. Per FCOM DSC-32-10-10:

All gears and doors are hydraulically-actuated by the green hydraulic system. Hydraulic supply is automatically isolated by closing a safety valve above 280 kt.

Above 280 kt the safety valve cuts the hydraulics so the gear cannot be moved — protecting the doors and legs from being torn off by the airstream — and stays cut until the lever is at DOWN and speed has fallen back below 280 kt. (Note the deliberate gap from the 250 kt VLO/VLE crew limit: the valve is a system backstop, 30 kt beyond the operating limit, not the limit itself.)

When green is gone, or the normal extension system has failed, a path remains that needs neither green hydraulics nor the normal electrical control. Per AMM 32-00-00:

When the uplocks release, gravity extends these landing gears. Springs pull the downlock links of each L/G into the locked position and the landing gear doors stay open.

Three turns of the free-fall handle drive an electric rotary actuator that mechanically releases the uplocks and isolates green; gravity drops the gear and springs snap the down-locks home. The doors are left open — there is no hydraulic power left to close them. Full detail in Gravity Extension.

7. Braking — a four-layer fallback

Braking is the thickest block of the chapter, and its essence is layered fallback. Per FCOM DSC-32-30-10:

The normal system uses green hydraulic pressure, whilst the alternate system uses the blue hydraulic system (backed up by the hydraulic accumulator). An antiskid and autobrake system is also provided.

The four layers, in order of descent:

1. Normal (green). Pedals or autobrake → BSCU → green pressure on each brake's piston set 1. Full antiskid and autobrake (LO/MED/MAX).
2. Alternate (blue). On loss of normal braking or low green pressure, an automatic changeover sends pedal demand to piston set 2 on blue pressure, still with antiskid.
3. Alternate without antiskid. One step lower — pure pedal-travel-proportional pressure from blue or the accumulator, needing no electrical power. This is the towing mode.
4. Parking / ultimate emergency (accumulator). The last resort, on the blue system's accumulator. Per AMM 32-45-00, the accumulators have sufficient capacity to hold the brakes on for a minimum time of twelve hours.

The physical foundation of all four layers is the two-piston-set design: green on set 1, blue on set 2, no shared fluid path. Detail in Alternate Braking and Parking and Ultimate Emergency Braking.

8. Steering — BSCU plus green, with free-castoring on loss

Nosewheel steering belongs to the BSCU. Per AMM 32-51-00:

Two hand wheel transmitters in the cockpit supply the primary steering inputs to the BSCU. The rudder pedals and the autopilot supply secondary steering inputs to the BSCU through the Flight Control Primary Computers (FCPC).

The tillers are the primary input; rudder pedals and autopilot are secondary, routed through the FCPC — which is the formal interface between ATA-27 flight controls and the gear. Normal steering runs on green; the alternate source is yellow (ALTN N/W STRG). If steering is lost entirely, it can be disconnected to let the nosewheel free-castor — the towing state — and the crew holds heading with differential braking and rudder. Detail in Nosewheel Steering.

9. The counter-intuitive part — wheels are braked on retraction

A detail many crews never picture: as the aircraft lifts off and the gear folds into the bays, the wheels are still spinning fast. Retracting them spinning would let centrifugal force damage the tyres and throw rubber debris around the bay. The A330 stops them — differently for main and nose wheels.

The main wheels are braked electrically. Per AMM 32-00-00:

During retraction, the BSCU automatically operates the brakes for a short time. This makes sure that the wheels do not turn as they go into the bays.

The nose wheels have no brakes at all; they are stopped mechanically. Per AMM 32-00-00:

As the NLG goes into the bay, the wheels touch an antispin brake to stop the rotation of the wheels.

So: main wheels stopped by BSCU electrical braking, nose wheels stopped by a mechanical touch-pad — because the nosewheel has no proper brake system to call on.

10. Hard numbers

These are the landing gear's "ID-card" figures, reused throughout the chapter. Get the magnitudes first; the dedicated articles supply the context. Per AMM 32-00-00 General Characteristics:

11. Pilot contact points across a flight

The landing gear is mostly invisible in normal operation; the crew meets it at specific points. Anchoring the architecture to these moments makes the abstract map operational:

The heavy events are at gear-up (retraction braking, sequencing) and at landing (braking, possible degraded chain). When everything is healthy the crew feels none of the electro-hydraulic machinery behind it — which is exactly the design intent.

Self-test

[!note]- Q1. The landing gear is "electrically-controlled and hydraulically-operated". What does each half mean, and why are both layers needed?

Electrically-controlled: every cockpit input (gear lever, brake pedals, tiller) becomes an electrical signal processed by a computer (LGCIU or BSCU) — not a direct mechanical or hydraulic command. Hydraulically-operated: the actual force to move gear, doors, brakes, and steering comes from the hydraulic systems, commanded by those computers. Both layers are needed because the control problem (sequencing, monitoring, protections, antiskid) is best solved electronically, while the force problem (moving heavy structures fast) is best solved hydraulically. Wires carry the information; fluid carries the force. A fault in one layer does not automatically disable the other.

[!note]- Q2. Which computer owns which functions, and why can each one fail without taking the other's job down?

The LGCIU (two units) owns extension/retraction and position/warning, and provides the ground/flight signal to other systems. The BSCU (one unit, two channels) owns braking and nosewheel steering. They control independent functions over independent chains, so loss of the LGCIUs leaves braking intact, and loss of the BSCU still leaves the gear able to extend. They are not redundant backups for each other — they simply own different parts of the system.

[!note]- Q3. What does each of the three hydraulic systems drive in the landing gear, and what is the single most common misconception?

Green drives everything normal — normal extension/retraction, normal braking, normal nosewheel steering. Blue drives the braking back-ups — alternate braking and parking/ultimate emergency braking (behind the accumulator). Yellow drives the alternate nosewheel steering only (ALTN N/W STRG, ground-only, inhibited above 70 kt). The most common misconception is that alternate braking is yellow — it is blue. A second is that yellow plays no part in the gear — it does: it is the alternate steering source.

[!note]- Q4. Green hydraulics is lost. Walk through how the gear still comes down, how the aircraft still stops, and how it stays parked.

Down: gravity (free-fall) extension — three turns of the handle drive a rotary actuator that releases the uplocks and isolates green; gravity drops the gear and springs lock the down-locks. The doors stay open. Stop: braking changes over automatically to the alternate (blue) system, still with antiskid; below that lies alternate-without-antiskid, then the accumulator. Parked: the parking brake holds on the blue accumulator, which has capacity for a minimum of twelve hours. None of these three fallback paths needs green.

[!note]- Q5. During retraction, how are the still-spinning main and nose wheels stopped, and why does it matter that they differ?

The main wheels are braked briefly and automatically by the BSCU during retraction. The nose wheels have no brakes and are stopped by touching a mechanical antispin brake pad as they enter the bay. It matters because the nosewheel has no proper brake system, so a different (mechanical) method is required; and because spinning wheels retracted into a closed bay would otherwise risk tyre damage and rubber debris. The crew sees neither action directly — both happen automatically on gear-up.

Key takeaways

References

A330 specifics per FCOM DSC-32-10-10 (Gears and Doors — description, LGCIU role, 280 kt safety valve), DSC-32-30-10 (Brakes and Antiskid — green/blue split, BSCU), DSC-32-20-20 (alternate nosewheel steering on Yellow), and AMM 32-00-00 (four-sub-system classification, General Characteristics hard numbers, retraction wheel-braking, gravity extension), AMM 32-45-00 (parking/ultimate emergency braking, twelve-hour accumulator hold), and AMM 32-51-00 (steering inputs). The master architecture diagram in §5 is an integrative synthesis of the AMM description, not a redraw of a single source figure.

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.