Gravity Extension

Normal extension (article 04) needs green hydraulics and a working LGCIU. Both can be lost. Gravity (free-fall) extension is the fallback that puts the legs down when everything else is gone, and its whole character is that it depends on almost nothing that can fail: a battery-powered electric actuator, the Earth's gravity, and a spring-driven geometric downlock.

Put the two modes side by side and the design philosophy is obvious:

Per FCOM DSC-32-10-30:

The two motorized actuators are electrically-powered to close the hydraulic cut-off valve and to disengage door and gear uplocks which permit the nose... and main landing gear to deploy by gravity and to automatically lock down.

1. Three things it does not depend on

The reliability of gravity extension comes from deliberately not depending on the things that fail. Per AMM 32-33-00:

When the related electrical switches are set in the cockpit, electrical rotary-type actuators release the MLG and the NLG door and gear uplocks. A mechanically operated valve isolates the hydraulic supply. Other mechanically operated valves let fluid move in the hydraulic components... This prevents cavitation and hydraulic locks. When the uplocks release, gravity extends these landing gears. Springs pull the downlock links of each landing gear (L/G) into the locked position and the landing gear doors stay open.

Three independences:

1. Not on green hydraulics for force — the leg falls under gravity (the nose gear also gets airflow help — see article 02).
2. Not on the LGCIU for sequencing — the actuators release the uplocks mechanically and directly, with no step-by-step LGCIU control.
3. Not on the normal electrical network — power comes from the battery through the hot buses (below).

The downlock then uses the overcentre lock stay and springs from article 01 / article 02 — a purely mechanical geometric lock that tightens under load.

  Power (emergency)   Cockpit       Free-fall actuators (2 motors each)    Mechanism        Result
 ──────────────────  ──────────     ────────────────────────────────      ──────────       ──────
  battery            LDG GEAR       5GF (L MLG)  6GF (R MLG)               linkage          (1) cut-out valve closes
  └ 2 hot buses ───► GRVTY EXTN ──► 7GF (NLG)                              (output shaft     → isolates green
    701PP / 702PP    selector       each = twin permanent-magnet motors    drives)          (2) vent valve opens
    28 V DC          3 positions:   + reduction/differential/epicyclic     │                → fluid free to move
     │               OFF/DOWN/RESET + self-locking output shaft            ├─ release door uplocks (no cavitation/lock)
     │               (guarded)        │ (cannot be back-driven)            ├─ release gear uplocks → all uplocks released
     └ 2 independent circuits,        │ torque/thermal protection          │
       energised together             └ one circuit lost → other at        ▼
       (one lost → other at             half speed                   gravity drops the legs
        half speed)                                                  springs pull lock stay overcentre → auto downlock
                                                                     (doors stay open · NWS isolated)

2. Power — battery hot buses, dual circuits

Per AMM 32-33-00:

The aircraft batteries supply 28V DC through two hot bus bars (701PP and 702PP). There are two independent circuits between the bus bars and the free-fall actuators... Usually the circuits are energized at the same time, but each circuit will operate the free-fall actuators (at half speed) if the other circuit is not available.

A hot bus is a bar tied directly to the battery and live almost permanently — even with the whole network lost and every generator stopped, it still has power. This guarantees that under a total electrical failure the gravity-extension actuators still have power. Two independent circuits, energised together, either one capable of driving the actuators at half speed — that is the redundancy on the power side.

3. The free-fall actuator — two motors, self-locking, dual protection

Per AMM 32-33-00:

The free-fall actuator is a rotary-type with two permanent-magnetic-field motors... Each motor can turn the output shaft if the other motor is stopped, although it takes more time to get to the full travel position.

The redundancy stacks: two motors (one fails → half speed) × two power circuits (one fails → half speed) — lose half of both and the leg still extends, only slower. The actuator also carries two protections: a torque limit switch (cuts power as torque nears its limit, preventing a jam from burning the motor) and a thermal switch (cuts power on overheat).

One key safety feature. Per AMM 32-33-00:

The output shaft can not turn the worm shaft.

This is a self-locking worm drive — once the actuator has turned to the extended position, an external force (a mechanism rebound, say) cannot drive it back. So the unlocking action is a locked, one-way move that will not retreat and re-engage the uplock. Limit switches stop the motors at 0° and 90°, with a mechanical stop a further 5° beyond.

4. Cut-out and vent valves — letting the leg move

Releasing the uplocks is not enough. As the leg falls, fluid remains in the actuator and lock chambers; if it is trapped (a hydraulic lock) the leg cannot move, and if a chamber is starved (cavitation) the motion stalls. Two valve types solve this:

• Cut-out valve 5285GA (fitted to the right MLG bay only) isolates the gear hydraulics from green, so the leg's chambers are disconnected from the green system and not held by it. Per AMM 32-33-00: The left MLG configuration is almost the same as the right MLG, but does not include a cut-out valve. (One cut-out valve isolates the whole gear hydraulic supply, so only the right bay needs it.)
• Vent valves 5206/5230/5231GA (one per bay) connect the chambers (B↔A) of the leg, lock, and actuator and route excess fluid through the manifold back to green return — giving the fluid somewhere to go and preventing cavitation and hydraulic locks.

A neat fail-safe. Per AMM 32-33-00:

The mechanism includes a shear pin which breaks if the spool becomes unserviceable and can not move. This lets the linkage in the mechanical system continue to move to operate the uplocks and vent valves.

So even if a valve spool jams, the mechanism is not stopped by it — the shear pin breaks and the linkage continues to drive the uplock release. A deliberately breakable pin keeps the whole unlock chain from being held hostage by a single jammed valve — the "get the legs down even if something local fails" philosophy of the whole mode.

5. The NLG door-wheel anti-clash delay

Some configurations add a 15-second delay relay in the NLG actuator supply. Per AMM 32-33-00:

...15 second interval between motor 1 and 2 activation... This prevents contact between the nose landing-gear doors and the nosewheel during freefall extension.

Triggered by a microswitch on the NLG output shaft, it briefly holds the two motors between the door-uplock release and the gear-uplock release — letting the door open fully before the leg drops, opening up clearance between door and nosewheel. A normal hydraulic extension is precisely timed and never clashes; a gravity extension is "coarse", so this delay is added as protection against the nosewheel brushing the door.

6. Indications and the 200 kt limit

Per AMM 32-33-00:

There is no special indicating system for the position of the landing gear in the Free Fall Extension mode... If the WHEEL page is not shown on the SD at the start of a free fall extension, it automatically shows after 30 seconds. When a free fall extension is completed the Engine Warning Display shows this data: - L/G DOOR NOT CLOSED - MAX SPEED 200.

Two things a pilot must know:

1. L/G DOOR NOT CLOSED is normal — after a gravity extension there is no hydraulics left to close the hydraulic doors, so the doors stay open as expected (see Landing Gear Doors). Do not treat it as a new fault.
2. MAX SPEED 200 — with the doors open and the legs in the airstream, the aerodynamic limit drops; after a gravity extension the maximum speed is 200 kt. This is tighter than the 250 kt VLE and must be observed.

[!warning]- Six misconceptions this article corrects (1) Gravity extension does not need green hydraulics for force — the leg falls under gravity, and the cut-out valve actually isolates green. (2) It does not need the normal network — it runs on the battery hot buses (live even under a total electrical failure), dual circuits, half speed on one. (3) A single failed actuator motor does not stop extension — two motors and two power circuits, it still extends (slower) with half of each lost. (4) L/G DOOR NOT CLOSED after a gravity extension is not a new fault — the hydraulic doors are expected to stay open. (5) You may not keep flying at VLE 250 afterwards — MAX SPEED 200, tighter than VLE. (6) It is not freely reversible — the worm drive is self-locking and one-way; a RESET is required to return to the normal mode.

Self-test

[!note]- Q1. What are the "three independences" of gravity extension?

It does not depend on green hydraulics for force (the leg falls under gravity), it does not depend on the LGCIU for sequencing (the actuators release the uplocks mechanically and directly), and it does not depend on the normal electrical network (power is from the battery hot buses). The downlock is then the spring-driven overcentre geometric lock. Each removed dependency is one more layer of "everything else has failed and I can still get the legs down".

[!note]- Q2. How many motors and power circuits does the actuator have, what happens if half fail, and why can the output shaft not be back-driven?

Two permanent-magnet motors and two independent power circuits. If one motor or one circuit fails, the leg still extends at half speed; lose half of both and it still extends, only slower. The output shaft cannot turn the worm shaft — a self-locking worm drive — so once it has unlocked, an external force cannot drive it back to re-engage the uplock. It is a one-way, locked action.

[!note]- Q3. What do the cut-out valve and the vent valve each do, and what is the shear pin for?

The cut-out valve (right MLG bay only) isolates the gear hydraulics from green, so the legs are not held by green pressure. The vent valves connect the leg/lock/actuator chambers and route excess fluid to return, preventing cavitation and hydraulic locks so the legs can move. The shear pin breaks if a valve spool jams, letting the linkage continue to operate the uplocks and vent valves — one breakable pin protects the whole unlock chain from a single jammed valve.

[!note]- Q4. After a completed gravity extension, what two messages appear on the EWD, and what is the speed limit?

L/G DOOR NOT CLOSED and MAX SPEED 200. The first is normal (no hydraulics left to close the hydraulic doors). The second is a hard limit of 200 kt — tighter than the 250 kt VLE — because the open doors and exposed legs reduce the aerodynamic limit.

[!note]- Q5. Is there a dedicated position indication for gravity extension, and how do the doors differ from a normal extension?

No dedicated indication; the crew uses the WHEEL page and the downlock lights, and the WHEEL page auto-displays after 30 seconds if it was not already shown. After a gravity extension the hydraulic doors stay open (no hydraulics to close them), whereas after a normal extension the hydraulic doors are closed. The leg downlocks either way.

Key takeaways

References

A330 specifics per FCOM DSC-32-10-30 (Gravity Extension — three-position selector OFF / DOWN guarded / RESET, motorised actuators close the cut-off valve and release the uplocks for gravity deployment and automatic downlock) and AMM 32-33-00 (Free Fall Extension — description and operation: the three independences, hot-bus dual-circuit power, twin-motor self-locking actuator with torque/thermal protection, cut-out/vent valves with shear-pin fail-safe, check/restrictor anti-cavitation, NLG door-wheel 15-second anti-clash, no dedicated indication / 30-second WHEEL page / L/G DOOR NOT CLOSED / MAX SPEED 200, NWS isolation). The free-fall link diagram is an integrative synthesis of the AMM text. The full emergency-power architecture (batteries, hot buses, RAT) belongs to ATA-24; the lock-stay/spring downlock geometry is detailed in article 01 / article 02. The hydraulic-side rationale for a gravity extension with green still available is in the ATA-29 hydraulics chapter.

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.