Normal Extension and Retraction

Normal extension and retraction can be reduced to one sentence: you give an "I want UP / I want DOWN" intent, and the LGCIU handles the rest, orchestrating an unbreakable sequence. Per AMM 32-31-00:

The System is electrically controlled and hydraulically operated. Two independently wired electrical systems control the operation of the hydraulic components.

This system is the art of sequencing — the doors and the leg cannot move at random; they must follow a fixed order. It also carries a protection a pilot must understand: above 280 kt the system locks the hydraulics out, so the gear cannot be extended even if you select it (protecting the doors and legs from the high-speed airflow). This article takes the mechanical parts of the main gear, nose gear, and doors and makes them move. The LGCIU computer itself is in LGCIU and Position Warning.

1. The control chain — two electrical systems, alternating LGCIUs

  Cockpit         Electrical control (2 systems, isolated)    EH valves       Green actuators
 ─────────       ──────────────────────────────────────      ──────────      ───────────────
                ┌──────────── SYSTEM 1 ─────────────┐
  L/G lever ───►│ LGCIU 1  ◄── proximity sensors     │
   6GA          │   │       (2 per position, 1/system)│
   UP / DOWN    └───┼────────────────────────────────┘
     │  ▲baulk      │ only one LGCIU is master at a time
     │  │(lock)     │ swaps after each retraction cycle / on failure
     │  └─baulk     ▼
     │   solenoid ┌──────────── SYSTEM 2 ─────────────┐    ┌────────────────┐
     └───────────►│ LGCIU 2  ◄── proximity sensors     │───►│ 85GA safety vlv │──► isolate/pass green
                  └───────────────────────────────────┘    │ 81GA door selr  │──► doors open/close
                         ▲ ADIRS speed                      │ 82GA gear selr  │──► gear ext/retract
                         │ (>280 kt → 85GA isolates)        └────────────────┘
                    ┌────┴────┐                                    │
                    │  ADIRS  │                      ┌─────────────┴──────────────┐
                    └─────────┘                      ▼                            ▼
                                          actuators: gear / gear uplock / gear downlock
                                                     / door (1–2) / door uplock

Three points:

Two electrical systems, SYS 1 and SYS 2, fully isolated, each with one LGCIU and a complete set of proximity sensors. Both connect to the lever (6GA), the door selector (81GA), the gear selector (82GA), and the safety valve (85GA).
Only one LGCIU is master at a time, swapping automatically after each cycle. Per AMM 32-31-00: Control changes from one LGCIU to the other after each retraction/extension cycle (when the L/G control lever is moved away from the DOWN position) or when one becomes unserviceable. This is alternation, not a fixed primary/standby.
Each position has two proximity sensors (one per system); the downlock has a third, on an independently wired, independently powered circuit in SYS 1, dedicated to lighting the downlock green light. So even if the main control channels fail, the downlock green light can still illuminate on its own — an important redundancy.

2. The hydraulic architecture

  GREEN ──► [85GA safety valve] ──► [5280GA selector + manifold assembly]
  (29-11)    two-position           ├─ 81GA door selector ──┬─ door OPEN line
             solenoid; de-energised  │  (independent)        └─ door CLOSE line ─[door bypass valves]
             = isolated; >280 kt no   └─ 82GA gear selector ──┬─ gear EXTEND line ─[restrictor 5287/5288]
             extension                  (independent)         └─ gear RETRACT line ─[MLG delay valve]

  bypass parts: NLG change-over valve 5215GA (on retraction, routes NLG return to steering return → faster)
                safety valves 5281/5282GA (prevent upstream fluid loss on a large downstream leak)
                continuous bleed manifold 5436GA (one-way continuous flow between RH MLG door-lock reset/unlock)

Green pressure passes the 85GA safety valve, then the 5280GA selector-and-manifold assembly splits it across four lines — door open, door close, gear extend, gear retract — through two independent selector valves.

3. The 85GA safety valve — locked out above 280 kt

The single most important protection. Per AMM 32-31-00:

The L/G isolation safety valve 85GA is a two-position valve that isolates the L/G hydraulic supply from the Green hydraulic system when its solenoid is de-energized. This prevents extension of the L/G when the aircraft speed is more than 280 kt, to prevent damage.

How does it know the speed? From the ADIRS. Per AMM 32-31-00:

A signal from the Air Data/Inertial Reference System (ADIRS) and the L/G control lever controls the operation of the safety valve.

The complete logic:

Above 280 kt: the ADIRS signals overspeed → 85GA de-energises and isolates → green cannot reach the gear → selecting DOWN does not extend the gear (protecting legs and doors from the airflow).
Back below 280 kt with the lever at DOWN: 85GA energises and passes → green is restored → normal extension follows.

Keep the two speeds distinct: VLO/VLE 250 kt is the crew operating limit (a red line you should not reach); 280 kt is the system mechanical backstop (if you do reach it, the system stops you for you) — a 30 kt margin between them.

4. Independent door and gear selectors

Per AMM 32-31-00:

The selector valve and manifold assembly 5280GA controls the flow of hydraulic fluid to these parts of the system: the door open lines / the door close lines / the gear extend lines / the gear retract lines... The L/G door selector valve 81GA and the L/G selector valve 82GA operate independently from each other.

Doors and gear use two independent selector valves precisely so the "doors first, then leg, then doors" timing can be orchestrated — doors on 81GA, leg on 82GA, the LGCIU moving each on its own schedule.

5. The five-step retraction sequence

Per AMM 32-31-00:

(a) The door uplocks open (b) The doors open (c) The gear downlocks unlock (d) The gear retracts and locks in position (e) The doors close and lock.

 UP:   (1) door uplocks open → (2) doors open → (3) gear downlocks unlock → (4) gear retracts + locks → (5) doors close + lock
 DOWN: (1) doors open → (2) gear extends (+ downlocks) → (3) doors close

Why this order is mandatory: the door sits across the leg's only path, so it is always "open the door, then move the leg; lock the leg, then close the door." Each step's completion is reported by a proximity sensor to the LGCIU, which confirms the position before moving to the next — that is the physical meaning of "sequence". Extension is the mirror image: open the doors, extend and downlock the leg, then close the hydraulic doors (the mechanical doors stay open with the leg — see Landing Gear Doors).

6. The lever baulk — no UP on the ground

The lever carries a baulk that blocks movement to UP. Per AMM 32-31-00:

The L/G control lever has a baulk device which stops movement of the lever to the UP position: when the aircraft is on the ground / when the L/G is not in the correct configuration for retraction.

The baulk is released by a baulk solenoid under LGCIU control. The LGCIU energises it to unlock only when the gear configuration is correct for retraction, and "correct" is a chain of hard conditions:

the left MLG bogie is trailed (or the left MLG is not downlocked), and the same for the right MLG;
the left MLG shortening mechanism is downlocked (or the left MLG is not downlocked), and the same for the right MLG;
the NLG shock absorber is fully extended and the nose wheels are centred.

In plain terms: the aircraft has genuinely left the ground (absorbers extended, bogies positioned) and the nose wheel is centred before the system lets you retract. On the ground — bogies not trailed, absorbers not extended — the baulk is locked, preventing an inadvertent retraction that would settle the aircraft onto its bays. This is the weight-on-wheels inadvertent-retraction protection, realised in hardware on the landing-gear side.

7. Smoothing and speeding — delay, restrictor, change-over valves

The circuit carries a few small valves whose only job is "not too violent, not too slow":

MLG delay valve (retract line). Per AMM 32-31-00: A delay valve in each MLG retract line makes sure that the initial movement in the retract cycle is smooth. This reduces the loads on the MLG and structure. A heavy leg snatched up violently would shock the structure; the delay valve gives it a gentle start.
Restrictor valve 5287/5288GA (gear extend line): meters extension flow so the leg does not drop too fast.
NLG change-over valve 5215GA (on retraction): routes the NLG actuator return to the steering-system return line to speed up the NLG retraction (some configurations) — the forward-retracting nose gear must work against the airflow, so it cannot afford to be slow.

These are the snubbing idea at system level: using restriction and delay to keep mechanism motion within what the structure can take.

[!warning]- Five misconceptions this article corrects (1) The gear is not lost above 250 kt (VLO) — that is the operating limit; the system mechanical backstop is 280 kt (85GA isolation), with a 30 kt margin. (2) The 85GA does not sense speed mechanically — it acts on an ADIRS airspeed signal plus lever position. (3) Doors and leg do not move roughly together — a strict five-step sequence, each step confirmed by a proximity sensor. (4) A lever that won't move to UP on the ground is not faulty — it is the baulk, locked on the ground and when the configuration is wrong, preventing inadvertent retraction. (5) The two LGCIUs are not a fixed primary/standby pair — they alternate as master after each cycle (and swap on failure).

Self-test

[!note]- Q1. Trace the signal from lever to leg, and describe how the two LGCIUs share the work.

Lever 6GA → the master LGCIU (one of SYS 1 / SYS 2) → the door (81GA) and gear (82GA) selector valves → green hydraulics → the actuators. Each position is reported by proximity sensors. The two LGCIUs alternate as master after each retraction/extension cycle (when the lever leaves DOWN), or swap if one becomes unserviceable — alternation, not fixed primary/standby.

[!note]- Q2. Why can't the gear be extended above 280 kt, what measures the speed, and when is extension restored?

Above 280 kt the 85GA safety valve de-energises and isolates the gear hydraulics from green, so the gear cannot be extended — protecting the legs and doors from the airflow. The speed comes from the ADIRS (with lever position). Extension is restored once speed is back below 280 kt with the lever at DOWN. Note 250 kt (VLO/VLE) is the crew limit; 280 kt is the system backstop.

[!note]- Q3. Give the five-step retraction sequence and explain why the door must open before, and close after, the leg moves.

(a) door uplocks open, (b) doors open, (c) gear downlocks unlock, (d) gear retracts and locks, (e) doors close and lock. The door sits across the leg's only path, so the leg cannot move until the door is open, and the door is not closed until the leg is locked clear. Each step is confirmed by a proximity sensor before the LGCIU proceeds.

[!note]- Q4. Why can the lever not be moved to UP on the ground, and what configuration must the baulk solenoid see to release?

The baulk blocks UP on the ground and whenever the configuration is wrong for retraction, preventing an inadvertent retraction. The baulk solenoid releases only when both MLG bogies are trailed (or the MLG is not downlocked), both MLG shortening mechanisms are downlocked (likewise), and the NLG shock absorber is fully extended with the nose wheels centred — i.e. the aircraft has genuinely left the ground and the nose wheel is straight.

[!note]- Q5. What problem does each of the delay valve and the change-over valve solve?

The MLG delay valve smooths the initial movement of the retraction cycle, reducing loads on the MLG and structure (a heavy leg should not be snatched up violently). The NLG change-over valve routes the NLG actuator return to the steering return line on retraction, speeding up the NLG retraction — necessary because the forward-retracting nose gear works against the airflow.

Key takeaways

Theme	The one thing to remember
Control philosophy	You give intent (UP/DOWN); the LGCIU orchestrates the sequence on proximity feedback
Two electrical systems	SYS 1 / SYS 2 isolated; LGCIUs alternate as master each cycle
85GA safety valve	Isolates green above 280 kt (ADIRS signal) — no extension; distinct from the 250 kt VLO/VLE
Sequence	Five steps up, three down — door before leg, door after lock, every time
Lever baulk	No UP on the ground / wrong configuration — weight-on-wheels inadvertent-retraction protection
Smoothing/speeding	Delay valve smooths retraction start; change-over valve speeds the NLG against airflow

References

A330 specifics per AMM 32-31-00 (Normal Extension and Retraction — description and operation: two electrical systems and LGCIU alternation, proximity sensors including the independent third downlock circuit, 85GA safety valve with ADIRS / 280 kt, 5280GA selector with independent 81GA/82GA and four lines, five-step retraction sequence and extension, lever baulk and baulk-solenoid configuration logic, delay/restrictor/change-over/safety valves) and FCOM DSC-32-10-10 (280 kt safety valve, green actuation — corroborating). The control-chain and hydraulic-architecture diagrams are integrative syntheses of the AMM text (the sequence is logical timing, better expressed as a chain than as a schematic redraw). The LGCIU computer internals and BITE are in LGCIU and Position Warning; the gravity extension cut-out/vent valve is in Gravity Extension.

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.