LGCIU and Position Warning

In Normal Extension and Retraction the LGCIU was always working behind the scenes, sequencing the doors and legs. This article brings it to the front — what it reads, what it computes, who it outputs to. And the most under-appreciated point: the LGCIU is not only the landing gear's computer; it is the source from which dozens of aircraft systems learn whether the aircraft is on the ground or in the air.

The under-rated word in "Landing Gear Control and Interface Unit" is interface. The LGCIU has two roles: inward, it is the gear's brain (reads the proximity sensors, sequences extension/retraction, drives indications and warnings); outward, it is the aircraft's ground/flight signal source. Per FCOM DSC-32-10-20:

Two LGCIUs receive landing gear position information from the proximity sensors... The LGCIUs send the landing gear position data to other aircraft systems. In case of a LGCIU failure, the landing gear is controlled by the remaining healthy LGCIU.

And per AMM 32-61-00:

The system has three functions: - to give L/G position data to interfaced systems - to give L/G position indication - to give warnings and cautions that are related to L/G system failures.

1. Two output types — discrete and ARINC 429

The LGCIU does not pass raw sensor voltages on; it processes them into meaningful logical conclusions, then outputs in two forms. Per AMM 32-61-00:

The LGCIU's process the data and give two types of output. They are: - discrete logic signals - ARINC 429 data.

• Discrete logic signals — a wire that is on/off, high/low, expressing one true/false conclusion (such as "on the ground / not on the ground"). Given to systems that need a single switch state, and provided only when the specific condition is met.
• ARINC 429 data — a digital data bus, broadcast continuously, carrying gear and door positions and system status — given to systems that need detailed information (the EIS display, for instance).

That the LGCIU outputs conclusions rather than raw signals is exactly why it is called an interface unit.

   Sensor layer            Computing layer (2 systems, isolated)    Output layer
  ──────────────          ──────────────────────────────────       ────────────
  proximity sensors                                          ┌─► EIS: WHEEL page (SD) + E/WD warnings
  (2 per position:         ┌──────────────┐                  │
   SYS1 / SYS2)──────────► │ LGCIU 1 (SYS1)│──── discrete ───┤─► FWS: aural/visual warnings (+chime)
   · leg up/downlock      │              │      + ARINC 429  │
   · absorber comp/ext    │  ◄ alternate ►│                  ├─► dozens of systems (ground/flight discrete):
   · door open/uplock     │              │                  │   pressurisation · EFCS · normal braking ·
   · shortening/pitch trim │ LGCIU 2 (SYS2)│──────────────────┤   anti-ice · fire · TCAS · GPWS · FADEC …
   · bogie trailed        └──────────────┘                  │
                                                            └─► downlock green lights
   3rd dedicated downlock sensor ──── via LGCIU 1 ──────────────► NLG / each MLG downlock light (independent)

2. The ground/flight criterion — why not a single sensor

How does the LGCIU decide the aircraft is "on the ground"? Not from shock-absorber compression alone, but from a compound criterion. Per AMM 32-61-00:

the L or the R MLGs are in a ground condition (the applicable gear(s) is(are) locked down and the bogie(s) not trailed)

Why both "locked down and bogie not trailed"? Recall article 01: the moment the aircraft leaves the ground, the pitch trimmer turns the bogie to the trailed position; on touchdown the bogie is pressed level (not trailed). So "downlocked + bogie pressed level" genuinely represents "the wheels are bearing weight on the ground" — more reliable than absorber compression alone or downlock alone. This is the physical basis of the A330's ground/flight (weight-on-wheels-equivalent) logic.

3. The aircraft-wide ground/flight source

This is the soul of the article. The FCOM lists, in a whole table, which systems the LGCIU feeds. Per FCOM DSC-32-10-20:

The two LGCIUs provide following discrete logic signals to various aircraft systems.

A sample of those systems:

A separate group takes a signal from the lever position (not from wheel state): lever DOWN → refuel-on-battery, FCMS; lever UP → normal braking (the BSCU brakes the wheels on retraction — see the Overview), cabin emergency lighting.

So a dual LGCIU failure is a major event — it does not only paralyse the gear indication; it removes a trusted ground/flight judgement from dozens of systems. This is why the LGCIU is dual-channel and why L/G LGCIU 1+2 FAULT is a serious failure.

4. The downlock green lights — three independent, three states

Per AMM 32-61-00:

Each light is in two parts and can show: - the color red and the legend UNLK in the top half (L/G not in selected position) - the color green in the bottom half (L/G downlocked) - no lights (L/G uplocked). The green lights for the NLG and each MLG have different proximity sensors, logic circuits and power supplies.

Reading the three states:

• Green (lower half) = downlocked, safe to land on this leg.
• Red UNLK (upper half) = the gear is not in the selected position (in transit, or jammed).
• No light = uplocked (stowed).

Why each green light has its own sensor, circuit, and power supply (a third dedicated sensor set via LGCIU 1): so the three green lights are mutually independent — even if a main channel or one LGCIU has a problem, each leg's "I am locked" green light can still illuminate independently and credibly. The three greens are the single most important thing to confirm before landing, so their power and logic are specifically isolated.

5. The warning family and the lever down-arrow light

The LGCIU drives a family of warnings. Per AMM 32-61-00 these include L/G LGCIU 1 (2) (1+2) FAULT, L/G GEAR NOT DOWNLOCKED, L/G GEAR NOT UPLOCKED, L/G DOOR NOT CLOSED, L/G GEAR UPLOCK FAULT, L/G SYS DISAGREE, L/G RETRACTION FAULT, L/G LENGTHENING FAULT, and L/G GEAR NOT DOWN. When these appear the WHEEL page auto-displays; some trigger a Master Warning/Caution and chime. Each message's meaning and handling is in the ECAM Warning Family.

The lever down-arrow light is the last reminder of an unsafe configuration. Per AMM 32-61-00:

The 'down arrow' light in the L/G control lever comes on if: - the L/G is not locked down - the aircraft is below 750 feet - the aircraft is in a landing configuration.

Read it: you have lowered the flaps, brought the thrust back, and descended below 750 ft to land — but the gear is not locked down. The red arrow shouts "you are about to land and the wheels are not ready". It is the same class of gear-up-landing protection as the GPWS TOO LOW GEAR, from the landing-gear side. (The LGCIU also runs a BITE — power-up self-test, continuous monitoring, fault recording to the central maintenance system — but for the pilot it is enough to know the LGCIU finds and reports its own faults.)

[!warning]- Five misconceptions this article corrects (1) The LGCIU does not only serve the landing gear — it is the aircraft-wide ground/flight source for pressurisation, EFCS, braking, anti-ice, TCAS/GPWS, FADEC and more. (2) "On the ground" is not decided by absorber compression alone — the criterion is downlocked and bogie not trailed (compound, more reliable). (3) The three downlock green lights do not share one circuit — NLG and each MLG have independent sensor, logic, and power. (4) A dark green light does not mean "not extended" — green = downlocked, red UNLK = not in the selected position, no light = uplocked (three states). (5) The down-arrow light does not mean the gear is broken — it is the unsafe-configuration reminder (landing config + below 750 ft + not downlocked), protecting against a gear-up landing.

6. WHEEL page auto-display

The WHEEL page auto-displays at engine start (flight phase 2) until the first engine reaches 80 kt (phase 3); with the gear selected DOWN and downlocked below 10000 ft, held until the last engine stops (phase 9); and when specific L/G warnings appear. At other times it can be selected manually on the ECP. The FCOM adds a parallel approach trigger: it also auto-displays at landing-gear-down selection or below 800 ft radio height — so even without a gear-down selection, descending below 800 ft radio height brings it up. The two are an OR: 10000 ft is the window for the gear-down trigger, 800 ft is the low-height backstop (developed in Controls and Indications).

Self-test

[!note]- Q1. What are the LGCIU's two output types, and what kind of system does each serve?

Discrete logic signals — a single true/false switch state, given only when a specific condition is met, to systems that need one conclusion (e.g. "on the ground"). And ARINC 429 data — a digital bus broadcast continuously with gear/door positions and system status, to systems that need detailed information (e.g. the EIS display). The LGCIU outputs processed conclusions, not raw sensor voltages — hence "interface unit".

[!note]- Q2. What is the "on the ground" criterion, and why two conditions instead of one?

Downlocked and the bogie not trailed. After lift-off the pitch trimmer turns the bogie to trailed; on touchdown the bogie is pressed level (not trailed). So "downlocked + bogie pressed level" genuinely represents weight on the wheels — more reliable than absorber compression alone or downlock alone.

[!note]- Q3. Name five systems that rely on the LGCIU ground/flight signal, and state the impact of a dual LGCIU failure.

Any five of: pressurisation, EFCS, normal braking, anti-ice, engine/APU/cargo fire detection, TCAS, GPWS, FADEC, electrical generation. A dual LGCIU failure does not just lose the gear indication — it removes a trusted ground/flight judgement from dozens of systems, which is why it is a serious failure and why the LGCIU is dual-channel.

[!note]- Q4. What do the three downlock light states mean, and why is each green light independently powered?

Green (lower) = downlocked; red UNLK (upper) = not in the selected position; no light = uplocked. Each green light (NLG and each MLG) has its own sensor, logic, and power so they are mutually independent — even if a main channel or one LGCIU fails, each leg's "locked" green can still illuminate credibly. The three greens are the key pre-landing confirmation.

[!note]- Q5. What three conditions light the lever down-arrow, and what is it protecting against?

Gear not locked down, below 750 ft, and in a landing configuration (from flap/slat position and thrust selection). It protects against a gear-up landing — the same class of protection as the GPWS TOO LOW GEAR, from the landing-gear side.

Key takeaways

References

A330 specifics per FCOM DSC-32-10-20 (LGCIU and aircraft-system interface — dual LGCIU with single-failure takeover, the table of discrete signals to aircraft systems, the lever-position signals) and AMM 32-61-00 (Indicating and Warning — description and operation: three functions, discrete + ARINC 429 outputs, the compound ground/flight criterion, WHEEL-page auto-display, three-state downlock lights with independent circuits, the warning family, the down-arrow light conditions). The WHEEL-page low-height (800 ft radio) trigger is per FCOM DSC-32-10-30, detailed in Controls and Indications. The LGCIU BITE is per AMM 32-69-00 (concept only). The information-flow diagram is an integrative synthesis of the AMM text and the FCOM interface table. The ARINC 429 bit-label tables and BITE test menus are maintenance-layer and are not included. How each interfaced system uses the ground/flight signal belongs to its own ATA chapter; the cross-system web is in System Interface Map.

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.