Main Landing Gear

Each main landing gear (MLG) is the first thing to touch the ground at landing, the structure that takes the largest impact, and the assembly that has to stop a hundred-tonne-plus aircraft. Its design has to satisfy four demands that pull against each other: carry the weight and absorb the touchdown impact; fold into a wing-root bay that is shorter than the extended leg; set a touchdown and rotation geometry that puts the rear wheels down first and allows a large nose-up angle; and lock down so firmly that a gravity extension with no hydraulics still holds under landing load.

Per FCOM DSC-32-10-10:

Each main gear is a four wheel, twin tandem bogie assembly having an oleopneumatic shock absorber. Each main wheel is fitted with an antiskid brake.

This article takes that one-line description apart — main fitting, shock absorber, shortening mechanism, bogie beam, pitch trimmer, and side/lock stay — and explains the mechanism behind each. It is a purely mechanical-structure article; the hydraulic sequencing that drives these parts is in Normal Extension and Retraction.

1. Four load paths

Everything else follows from how four kinds of load reach the airframe. The MLG attaches at four points, and each load travels its own route — they do not share paths.

        WING REAR SPAR  ════════════════════════════════════════════
            │               │                    │
      [retraction         ┌─SHORTENING─┐    [REAR ATTACH LUG]
       bellcrank]         │  mechanism  │      │ (spherical bearing:
            │             │ (inside leg)│      │  vertical + drag load)
      [FWD TRUNNION ──────┤             │   ┌──CROSS MEMBER──┐
       PIN (pintle)]      │             │   │  + DRAG STRUT form a
            │             │             │   │  triangle (fore/aft load)
   ┌────────┴────────┐    │      ╔══════╧════╧══════╗
   │ SIDE STAY ASSY: │    │      ║   MAIN FITTING    ║  ← no screw threads
   │ · lock-stay actu│◄───┤      ║   = barrel        ║    (crack prevention)
   │ · 4 LOCK SPRINGS│lock│      ║   holds the       ║
   │ · LOCK STAY     │side│      ║   shock absorber  ║
   │ · SIDE STAY     │load│      ╠═══════════════════╣
   └─────────────────┘    │      ║   SLIDING TUBE    ║ ← oleo outer cylinder
            ▲             │      ║   (extends/retracts)
       overcenter        │  [PITCH TRIMMER]──┤  [TORQUE LINKS]
       geometric lock    │   + ARTICULATING──┘   (stop tube rotation,
                         │     LINKS              allow vertical travel)
                         ▼
                 ┌────────────────────┐
                 │    BOGIE BEAM       │ ← pivots near sliding-tube base
              ◯══╪══◯            ◯══╪══◯
             front pair          rear pair    ← four wheels, twin tandem
              └─ BRAKE RODS (carry brake torque straight to sliding tube) ─┘

The one-line model: the main fitting is the backbone, the rear spar is the main beam, the side stay handles sideways load, and the brake rod carries brake torque — each on its own path, none crossing into another.

2. The main fitting — a thread-free backbone

The main fitting is the backbone of the leg, built from three pieces: the barrel (a cylinder housing the shock absorber), a cross member, and a drag strut. The cross member reaches forward and forms a triangle with the drag strut — a triangle being the stiffest arrangement against fore/aft load.

One design choice is worth dwelling on. Per AMM 32-11-00:

The main fitting is without screw threads or screw thread inserts. This decrease the risk of stress areas that can cause cracks.

The main fitting takes the full landing load, repeatedly, for the life of the aircraft. A screw thread root is a natural stress concentration and a nursery for fatigue cracks. The design answer is to cut no thread at all into this part — every connection is made through lugs, pins, and bearings instead. This is the structural reason main-fitting inspection is a focus item and why dedicated overheat/crack checks exist for it. The main fitting is also the mounting hub for almost everything else on the leg — retraction actuator, lock stay, brake manifold, shortening bellcrank, uplock pin, side stay, articulating links, pitch trimmer, and the upper torque link.

3. The shock absorber — why recoil must be slow

The shock absorber is an inverted, capsule-type oleo-pneumatic unit inside the main-fitting barrel. Its outer cylinder is the sliding tube that extends and retracts.

Its governing idea is one sentence. Per AMM 32-11-00:

The recoil stroke is slow, which makes sure that the aircraft does not become airborne again when it touches down on landing.

This is the single most important point for a pilot: the shock absorber is not a spring. It is a one-way damper — compression can be fast (to absorb the impact), but recoil must be slow. A fast recoil would release the energy stored in compression like a trampoline and throw the aircraft back into the air — a bounced landing. The A330 makes recoil "bleed out slowly" through two internal valves:

• A bistable damping valve at the piston base, with a two-way restrictor. On compression, fluid is forced through the restrictor and the gas charge absorbs the energy; if the compression is violent (a hard landing), the restrictor piston lifts to increase flow area — effectively opening a larger bleed under heavy impact to avoid bottoming. On recoil, the chamber pressure pushes fluid back.
• A snubber at the piston/cylinder interface, sensitive to flow rate. As recoil approaches the end of travel, seals close some ports and reduce flow in the recoil chamber — the final brake that makes the end of the recoil stroke slow.

A second redundancy point. Per AMM 32-11-00:

The shock absorber has a gland that includes two dynamic seals (installed one above the other). Usually only the top seal seals the gland. If a hydraulic leakage occurs around the top seal, the lower seal can become the in use seal.

Changeover is a manual maintenance action (the shock-absorber pressure is released first). A pilot need not know the procedure, only the fact: the shock-absorber gland is a two-seal, backed-up design.

4. The shortening mechanism — the leg is longer than the bay

A contradiction: the extended leg is long (it needs ground clearance plus shock-absorber stroke), but the wing-root bay cannot hold a leg that long. The answer is not a bigger bay; it is a leg that shortens itself as it retracts. Per AMM 32-11-00:

A shortening mechanism at the top of the MLG decreases the length of the MLG leg during retraction. This lets the main gears go into the space available in the MLG bays.

The mechanism is a linkage (adjustable link, bellcrank, connecting link, upper and lower links), with the lower link tied to the shock-absorber piston. On retraction, the linkage breaks an overcentre lock and pulls the lower link up, drawing the shock absorber into the main fitting so the leg becomes shorter. On extension, the linkage opens out under the bogie's weight; as the leg reaches full extension the lower link passes overcentre, and the adjustable link drives it into the locked position and holds it there — so once extended, the leg is "propped" at full length and cannot collapse under landing load.

The mental model: retract = the linkage folds and pulls the leg short; extend = the linkage passes overcentre and props the leg long.

5. The bogie beam and torque links

The bogie beam is the four-wheel carrier across the bottom of the leg, pivoting near the base of the sliding tube. Each end is a hollow twin-wheel axle with a jacking dome below. The axle is hollow because it houses the tachometer (the wheel-speed source for antiskid, in a later article).

A detail with high pilot relevance. Per AMM 32-11-00:

The axles and the body of the bogie beam have a band of temperature sensitive paint which is as an overheat indicator.

This is what lets ground engineering tell, at a glance, whether a brake or bearing has overheated — and it is the same phenomenon at the root of the heat that shows up after gear-up (developed in the brake-temperature article).

The torque links (upper and lower, joined by a centre pin) do one specific job: keep the main fitting and sliding tube rotationally aligned (stop the sliding tube turning) while still allowing it to extend and retract. Without them, the bogie could skew as the shock absorber compresses. Brake torque is taken by brake rods between each brake and the sliding-tube base, carrying the torque straight into the sliding tube rather than twisting the bogie pivot.

6. The pitch trimmer — one jack, three jobs

This is the cleverest part of the MLG: a hydraulic jack between the main fitting and the articulating links, kept pressurised by the green system. Per AMM 32-11-00, it has three functions:

The pitch trimmer has three functions, with the help of the articulating links, it: - turns the bogie beam to keep the rear wheels on the ground so that the aircraft can rotate at a large angle during takeoff - puts the bogie beam into its correct position for the retraction sequence when the aircraft is airborne - decreases the speed at which the bogie beam moves during aircraft movement on the ground.

Translated into what the pilot feels:

1. Take-off rotation. As you rotate, the main gear is still on the ground. Without bogie trim, the rear of the bogie would lift and leave the ground early, reducing the usable rotation angle. The pitch trimmer holds the rear wheels down, letting you rotate to 14 degrees with the rear wheels still planted.
2. Positioning after lift-off. Once airborne, the ground load disappears, the shock absorber extends, the pitch trimmer retracts, and through the articulating links the bogie is turned to the angle needed to enter the bay.
3. Ground damping. During taxi, surface bumps try to swing the bogie about its pivot; the pitch trimmer's hydraulic damping holds it back, reducing the swing rate and smoothing the ride.

If green pressure falls, a control manifold in the wing keeps the pitch trimmer working for a while through three valves: a check valve traps residual pressure in the trimmer when green drops (so it can still trim briefly); a relief valve vents excess when the articulating links mechanically drive trimmer pressure above green; and a safety valve (with a push-to-reset button) cuts off automatically on a large downstream leak to protect green fluid. The check valve is the key part — it is the small redundancy that lets the pitch trimmer ride out a brief loss of green.

7. Side stay and lock stay — the overcentre geometric lock

The side stay assembly sits between the wing rear spar and the main fitting, connected through Cardan joints. It comprises the side stay, the lock stay, a downlock actuator, four lock springs, and proximity sensors. The lock stay is the MLG's mechanical downlock, and it locks by going overcentre. Per AMM 32-11-00:

The upper link of the lock stay continues below the center pivot to give an overcenter stop. This gives an overcenter, geometric lock if there is a failure of the internal stop of the downlock actuator.

Overcentre is the key mechanical idea: when two hinged links pass just beyond the line joining their ends and hit a stop, external load (the aircraft weight) presses them tighter rather than folding them — like a straightened elbow that someone pressing down on your hand cannot bend. Once the lock stay is overcentre it is held by geometry alone, needing no continuous hydraulics.

Why design it this way: at landing the MLG takes a huge upward load. If the downlock relied on hydraulics to hold, a loss of hydraulics would let the leg fold under load — a catastrophe. A geometric overcentre lock means the downlock is mechanical and does not depend on hydraulic pressure, and the heavier the landing load, the tighter it locks.

The lock is driven home by four lock springs, with redundancy built in. Per AMM 32-11-00:

If one lock spring is unserviceable, the remaining three springs are sufficient to pull the lock stay into the locked position.

This is exactly why gravity extension is reliable: the downlock does not need hydraulics — it needs the springs and the geometry, and a broken spring still leaves three. A ground-lock pin can be inserted in the lock stay for physical safety during maintenance.

[!warning]- Six misconceptions this article corrects (1) The shock absorber is not a spring — it is a one-way damper; compression can be fast but recoil must be slow, or the aircraft bounces. (2) The downlock is not held by hydraulics — it is the lock stay's overcentre geometric lock, tightening under load. (3) A broken lock spring does not prevent extension — four springs, three are enough. (4) The MLG leg is not fixed length — the shortening mechanism shortens it on retraction and props it long on extension. (5) The pitch trimmer is not a minor damper — it does three jobs (rotation, repositioning, ground damping). (6) Brake torque does not pass through the bogie pivot — it goes straight to the sliding tube through dedicated brake rods.

8. Retraction and extension sequence

Retraction (UP). Per AMM 32-11-00:

Because the downlock actuator is more lightly loaded, it retracts first to release the overcenter lock of the lock stay.

Order: select UP → pressure reaches the retraction actuator and the downlock actuator → the lightly-loaded downlock actuator moves first, releasing the lock stay's overcentre lock → the lock stay folds against the spring tension and folds the side stay with it → the retraction actuator raises the leg → the shortening mechanism simultaneously draws the shock absorber into the main fitting and shortens the leg → the leg enters the bay.

Extension (DOWN). Per AMM 32-11-00:

The uplock actuator opens quickly (to keep a minimum load-time on the uplock hook).

Order: select DOWN → pressure to the uplock actuator and both sides of the retraction and downlock actuators → the uplock opens quickly to release the leg → because the retraction actuator's extend side has greater volume than the retract side, the net force drives the leg down (helped by spring tension and the leg's own weight) → the side stay and lock stay open to overcentre → the downlock actuator confirms overcentre and helps the springs hold → the shortening mechanism extends the shock absorber, with the shortening links passing overcentre to carry load at full extension.

9. Bogie alignment through a take-off and landing

• Take-off rotation: ground load decreases → shock absorber extends, pitch trimmer retracts → articulating links turn the bogie; once the trimmer piston reaches its outer stop, the shock absorber extends further to turn the bogie to its trim angle → the longer bogie-plus-absorber raises the overall MLG length, letting the aircraft rotate to a large angle.
• Touchdown: per AMM 32-11-00, as the aircraft lands, the rear wheels touch the ground and the first loads are transmitted to the tires and the shock absorber. As the load increases, the shock absorber compresses, which causes the bogie beam to become level until the front wheels touch the ground. This is the mechanical origin of the A330's rear-wheels-first touchdown — it is set by the pitch trimmer's preset bogie angle.
• Taxi: further shock-absorber compression forces the pitch trimmer to extend, but green pressure makes it want to retract — so it resists the bogie swinging about its pivot and reduces the swing rate, damping the ride.

Self-test

[!note]- Q1. Trace the vertical impact load from the wheel to the wing rear spar. Which parts does it pass through?

Wheel → bogie beam → sliding tube → shock absorber → main fitting → the spherical bearing on the rear attachment lug → wing rear spar. The forward trunnion/pintle pin takes a share of the vertical load (its spherical bearing floats fore/aft to absorb structural deflection), the side stay takes side load, and brake torque takes a separate path through the brake rods to the sliding tube. Four loads, four paths, none shared.

[!note]- Q2. Why must shock-absorber compression be fast but recoil slow, and what would a fast recoil cause?

Compression must be fast to absorb the touchdown impact without bottoming. Recoil must be slow so the energy stored in compression is not released suddenly — a fast recoil would throw the aircraft back into the air, a bounced landing. The slow recoil is produced by the bistable damping valve (which also opens a larger bleed under a hard landing) and the snubber that further slows the final part of the recoil stroke.

[!note]- Q3. The lock stay's overcentre geometric lock does not depend on hydraulics. Is a heavy landing load good or bad for the lock, and why?

Good. Once the lock stay passes overcentre against its stop, external load presses it tighter rather than folding it — like a straightened elbow that cannot be bent by pressing down on the hand. So the heavier the landing load, the more firmly the downlock holds. Because it is geometric, the lock survives a total loss of hydraulics — which is exactly why gravity extension can be trusted to hold under landing load.

[!note]- Q4. One lock spring fails. Is gravity extension still reliable, and why?

Yes. The downlock is driven by four lock springs, and the source states that if one is unserviceable the remaining three are sufficient to pull the lock stay into the locked position. Combined with the overcentre geometry (which needs no hydraulics to hold), this is why a no-hydraulics gravity extension still locks down reliably.

[!note]- Q5. Name the pitch trimmer's three functions and the flight phase each serves.

(1) It holds the rear wheels on the ground during take-off so the aircraft can rotate to a large angle. (2) After lift-off it positions the bogie correctly for the retraction sequence. (3) On the ground it reduces the bogie's swing rate about its pivot, damping the taxi ride. The first explains why the A330 can rotate to 14 degrees without the rear wheels lifting early.

Key takeaways

References

A330 specifics per FCOM DSC-32-10-10 (MLG configuration — four-wheel twin-tandem bogie, oleo-pneumatic, antiskid brake per wheel) and AMM 32-11-00 (Main Gear — description and operation: four load paths, thread-free main fitting, bistable damping + snubber + slow recoil + dual seal, shortening mechanism, bogie temperature paint, torque links, pitch trimmer three functions + three-valve manifold, side/lock stay overcentre + four lock springs, retraction/extension sequence, bogie alignment). The side-view structure diagram in §1 is an integrative synthesis of the AMM component figure and the AMM text, not a redraw of a single source figure. Numerical values (charge pressures, strokes, angles) are collected in the Landing Gear Overview.

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.