Landing Gear Doors

The gear doors look trivial and hide two facts a pilot should know: the door-leg sequence (which drives the L/G DOORS indications and the success of a gravity extension), and which door is open and which is closed once the gear is down. There is also a ground door-opening mechanism that maintenance uses every day, built around a neat fail-safe ("baulk") design.

The doors do two opposing jobs: seal the bay in flight (restore the aerodynamic shape, protect the bay equipment from high-speed airflow) and clear the way during transit (the leg has to pass through). Per FCOM DSC-32-10-10:

All gear doors open during landing gear transit. The hydraulically operated doors close at the end of each retraction and extension sequence.

That sentence holds the most counter-intuitive point in the article: once the gear is down and locked, the hydraulically-operated door is closed, not open. The hydraulic sequencing that the LGCIU orchestrates around this is in Normal Extension and Retraction.

1. The door layout — three on the main gear, four plus a fairing on the nose

   MLG bay (3 doors per side)            NLG bay (4 doors + 1 fairing)
   ─────────────────────────            ─────────────────────────────
        fuselage belly                       forward-fuselage belly
   ┌───────[uplock]───────┐            ┌──[dual-hook uplock]──┐
   │ (1) hydraulic MAIN    │           │ (1) fwd L  (2) fwd R  │ ← 2 hydraulic
   │     door — opens      │           │     forward doors     │   forward doors,
   │     OUTWARD           │           │     each own actuator │   close INWARD,
   │     (CFRP + NOMEX)    │           │     (CFRP + NOMEX)     │   cutout clears dragstay
   │     access steps+ramp │           └───────────────────────┘
   ├───────────────────────┤           ┌───────────────────────┐
   │ (2) mechanical HINGED │           │ (3) aft L  (4) aft R   │ ← 2 mechanical
   │     door, hinged to   │           │     rear doors,        │   rear doors
   │     the wing surface, │           │     move with the leg  │   (move with the leg)
   │     rod-linked to leg │           │   + fixed FAIRING      │   fairing aft of leg
   ├───────────────────────┤           └───────────────────────┘   (aluminium alloy)
   │ (3) FAIRING door on   │
   │     the MLG leg,       │           Drive split (same on both):
   │     moves with the leg │           · hydraulic door  = door actuator (green)
   └───────────────────────┘           · mechanical/fairing = adjustable rod tied to the leg

2. The door-leg sequence — doors first, leg next, doors close after lock

The hydraulic doors and the leg do not move together; there is a strict order. Per AMM 32-12-00:

The door opens in sequence before movement of the main gear, but closes when the main gear is uplocked or downlocked.

As an action chain: open the hydraulic door first → the leg then moves (up or down) → the leg reaches its position and locks (up or down) → only then does the hydraulic door close. Why this order: the door sits across the leg's only path, so moving the leg before the door is open would drive the leg into the door. The LGCIU orchestrates this timing (detail in Normal Extension and Retraction).

3. Gear down — the hydraulic door is closed

Many people picture the bay hanging open once the gear is down. It is not. Per AMM 32-12-00:

When the MLG is extended the hydraulically operated door closes and the mechanically operated door stays open.

The reasoning:

• The hydraulic main door closes — once the leg is out, that large door is no longer needed (the leg does not pass through it). Closing it restores the belly's aerodynamic shape, reduces drag, and keeps airflow out of the bay.
• The mechanical/fairing doors stay open (with the leg) — these are tied to the leg, so they swing open as it extends, and the leg passes through that opening to stand on the ground. They cannot close, and should not (the leg occupies the gap).

The nose gear is the same: down and locked, the two hydraulic forward doors are closed and the two mechanical rear doors stay open. So from underneath, a normally-extended A330 shows the large hydraulic doors shut and only the small leg-hugging mechanical doors open — that is the correct down configuration. This matters directly for reading the L/G DOORS indications: with the gear down, a closed hydraulic door is normal; a hydraulic door that has not closed triggers an indication.

4. The ramp — a hidden safeguard for gravity extension

Inside the MLG hydraulic door is a ramp, unremarkable in normal use but key during a gravity (free-fall) extension. Per AMM 32-12-00:

Ramps on the door make sure that the main gear does not get caught on the door during a free-fall extension.

In a normal extension the door is precisely sequenced by hydraulics and never fouls the leg. But in a gravity extension the door simply hangs under gravity and the leg drops freely; if the leg happened to land on the door edge it could jam short of lock. The ramp's slope guides the leg clear, so it slides down to lock. This is one link in the reliability of gravity extension — see Gravity Extension.

5. The ground door-opening mechanism — a fail-safe bypass

To enter the bay, maintenance must open the hydraulic door on the ground — where green pressure may not be available. The A330 provides a purely mechanical, gravity-driven ground-opening mechanism built around a bypass valve. On opening (handle's first movement), per AMM 32-12-00:

...isolate the door actuators from the door-close hydraulic line (the Green hydraulic system does not have to be pressurized) / connect together the two chambers of each door actuator to transfer fluid during the actuator extension.

So the bypass valve short-circuits the actuator's two chambers into one free passage, the uplock hook is released, and gravity pulls the door open while fluid transfers from one chamber to the other. Because the fluid has to squeeze across, a little cavitation occurs. Per AMM 32-12-00:

The main door opens slowly because some cavitation occurs in the door actuator.

"Opens slowly" is a feature, not a fault — do not expect the door to spring open.

The safety detail worth dwelling on is the baulk. Per AMM 32-12-00:

...there is a baulk in the bypass valve. This prevents movement of the ground door-opening handle to the doors close position, unless the Green hydraulic system is pressurized.

What danger does it prevent? Imagine the door left open, the handle moved back to "close", and no hydraulics at the time — the next time the system is pressurised, the door would slam shut with no warning, possibly trapping someone working in the bay. The baulk blocks that path: the handle cannot reach "close" unless green is already pressurised — so when the handle does reach "close", the door closes under controlled hydraulic pressure rather than as a primed trap waiting for the next pressurisation. Combined with a bi-stable mechanism (spring + crank, so the handle can only rest fully open or fully closed, never mid-travel), the whole mechanism gives "either firmly fully open or controlled fully closed, with no dangerous in-between state".

6. Proximity switches to the LGCIU

Each hydraulic door has a proximity switch (sensor on the airframe, target on the door) that feeds door position to the LGCIU, used both to sequence the transit and to supply door-state data for cockpit indications and warnings. So if a door is not in position, the LGCIU knows and the ECAM shows it.

[!warning]- Five misconceptions this article corrects (1) The bay is not open once the gear is down — the hydraulic door closes (restoring shape); only the leg-hugging mechanical door stays open. (2) The door and leg do not move together — strict sequence: open door → move leg → lock leg → close door. (3) Ground opening does not need green hydraulics (the bypass joins the two actuator chambers and gravity opens the door); it is closing that requires green first, forced by the baulk. (4) A slow ground opening is not a fault — it is cavitation in the actuator, the expected behaviour. (5) The leg is not at risk of snagging the door on a free-fall — the door ramp is built to guide it clear.

Self-test

[!note]- Q1. Name the three MLG doors, how each is driven, and whether each is open or closed with the gear down.

(1) The hydraulic main door — driven by a green door actuator, closed with the gear down (restores the aerodynamic shape). (2) The mechanically-hinged door and (3) the fairing door on the leg — both rod-linked to the leg, open with the gear down (they move with the leg and the leg passes through the opening). The takeaway: with the gear down the large door is shut; look at the three green lights, not at the door.

[!note]- Q2. Why is the door-leg sequence "open the door first, close it only after the leg locks"?

The door sits across the leg's only path. Moving the leg before the door is open would drive the leg into the door, so the door opens first. The door is closed only after the leg is uplocked or downlocked, because until then the leg is still in transit through the opening. The LGCIU enforces this order.

[!note]- Q3. Why does ground opening not need green hydraulics, while closing must have it, and what does the baulk prevent?

Opening: the bypass valve joins the actuator's two chambers and releases the uplock, so gravity opens the door with no green pressure (fluid just transfers between chambers, slowly, due to cavitation). Closing must be controlled and synchronised, so the baulk blocks the handle from reaching "close" unless green is pressurised — preventing the hazard of a handle left at "close" with no pressure, which would slam the door shut the moment the system is next pressurised, possibly on someone in the bay.

[!note]- Q4. In what situation does the door ramp matter, and what does it do?

During a gravity (free-fall) extension. With no hydraulic sequencing, the door hangs under gravity and the leg drops freely; if the leg caught the door edge it could jam short of lock. The ramp's slope guides the leg clear so it slides down to lock — one link in the reliability of gravity extension.

[!note]- Q5. How do the NLG hydraulic forward doors differ in opening direction from the MLG main door, and what is the forward-door cutout for?

The NLG forward doors close inward; the MLG main door opens outward. The forward doors carry a cutout shaped to clear the dragstay as the nose gear extends, so the door geometry does not foul the dragstay.

Key takeaways

References

A330 specifics per FCOM DSC-32-10-10 (door sequencing — open in transit, close at sequence end), AMM 32-12-00 (Main Gear Doors — three-door configuration, hydraulic door closed / mechanical door open with gear down, door-leg sequence, free-fall ramp, CFRP + NOMEX, ground door-opening bypass valve / bi-stable / baulk / cavitation, proximity switches), and AMM 32-22-00 (Nose Gear Doors — four doors + fairing, inward-closing forward doors with dual-hook uplock and dragstay cutout, ground-opening handle 75° / valve 60°, same baulk and bi-stable design). The door-layout diagram is an integrative synthesis of the AMM text, not a redraw of a single source figure. CFRP door-structure repair belongs to ATA-52.

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.