System Interface Map

This is the chapter's synthesis finale — no new facts. It gathers the cross-system interfaces scattered through articles 00–23 into one map, so you can see how the landing gear meshes with hydraulics, electrics, flight controls, autoflight, indicating, navigation, maintenance, and the engines. It is systems-thinking training: faults propagate along these interfaces, and capability degrades along them too.

The landing gear is an interface-heavy system — it uses all three hydraulic systems, connects to nearly every electrical bus, and couples deeply with the flight controls, autoflight, and indicating. Understanding this web is what lets you anticipate "how a fault elsewhere reaches the gear" and "what a gear fault affects in turn".

                          ┌─────────────── ATA-32 Landing Gear ───────────────┐
   ATA-29 Hydraulics       │                                                   │   ATA-31 Indicating
   ┌─────────┐  green──────┼─► normal extension · normal braking · normal steering ├──► ECAM (E/WD + WHEEL SD)
   │ G  B  Y │  blue───────┼─► alternate braking (with/without antiskid) · parking accumulator │   FWS (warning family)
   └─────────┘  yellow─────┼─► ALTERNATE nosewheel steering (ALTN N/W STRG, ground <70 kt) ★ │   SDAC/CMC (flight phase → TPIS)
                          │                                                   │
   ATA-24 Electrical       │   ┌──────────┐         ┌──────────┐              │   ATA-22 Autoflight
   ┌─────────┐  DC ESS─────┼──►│  BSCU    │         │  LGCIU   │◄─────────────┼── FMGEC ◄── NWS self-test (0.7°)
   │ HOT BUS │  85GA───────┼──►│ dual ch  │◄───────►│  1 / 2   │              │   decides CAT III B capability ★
   │ 701PP   │  115 V AC───┼──►│ brake +  │ config   │ ext/retr │              │   autopilot yaw → secondary steering
   │ 105PP   │  (brake fan)│   │ steering │ source   │ control  │              │
   └─────────┘  28 V (BTMU/TPIS) └────┬───┘         └────┬─────┘              │   ATA-34 Navigation
                          │           │                  ├── config broadcast ─┼──► ADIRS ──┐
   ATA-27 Flight controls │           │                  │                     │   · reference speed (antiskid/autobrake)
   ┌─────────┐  ground spoilers──────┤  autobrake trigger  pressurisation /    │   · steering speed-decay law
   │PRIM/FCPC│  (need L/G compressed │  (≥2 spoiler signals)  cabin / F/CTL law│   · ground speed (manual-braking availability)
   │ pedals  │◄──── + wheel speed)───┘                                         │   GPWS: TOO LOW GEAR (overruled by green triangle)
   └─────────┘            │                                                   │
                          │   ATA-45 Maintenance: BITE → CMS/CMC               │   ATA-70/73 Engines
   ATA-26 Fire/36 Bleed   │   ATA-78 Reverse: full reverse on brake failure    │   · take-off power → PARK BRK ON 🔴
   hot brake retracted →  │   differential taxi with asymmetric thrust         │   · abnormal landing ENG MASTER OFF timing
   wheel-well hyd-fluid fire └──────────────────────────────────────────────────┘

1. The three hydraulic systems — the full division (including the yellow point)

Yellow is the chapter's biggest correction (17): the early belief that "yellow plays no part in the landing gear" is inaccurate — it drives the alternate nosewheel steering. Each of the three has a "normal + back-up" or dedicated role; none is unrelated to the gear.

2. ADIRS — a much-depended-on ground-speed truth source

Braking and steering depend on the ADIRS in several places (10 / 11 / 16):

• Antiskid reference speed: normally computed from ADIRU horizontal acceleration; on a total loss it falls back to the highest wheel speed.
• Autobrake reference speed: ground tacho + ADIRS, and ADIRS-only once braking is applied.
• Steering speed-decay law: the angle decays with ground speed.
• Manual-braking availability: includes "ADIRS ground speed below a specified value".

A total ADIRS loss cascades: reduced antiskid accuracy (fall back to highest wheel speed), affected autobrake, and an inaccurate steering law — a textbook case of how the inertial reference reaches into ground handling. The gear's dependence on the ADIRS is greater than intuition suggests.

3. FMGEC — how a gear fault affects approach capability

The most counter-intuitive cross-system link (15 / 23):

NWS pre-landing self-test (0.7°) → 4 BSCU validity discretes → FMGEC → decides whether CAT III B is possible

So a BSCU/NWS drop to a single channel → CAT 3 SINGLE ONLY — the health of a "ground steering" system decides whether a low-visibility approach can be flown. This breaks the "the gear is only for the ground" intuition and is the best teaching case for systems thinking.

4. Brake temperature — a three-system entanglement

A "take-off brake-temperature limit" looks like an ATA-32-internal matter but involves three systems (14): ATA-32 brakes (the carbon makes heat); ATA-29 hydraulics (a leak could drip fluid onto a hot brake); and the wheel well (structure/fire) (a hot brake retracted into the bay + leaked fluid → a wheel-well fire). So the real purpose of the take-off brake-temperature limit is to prevent a hot brake igniting potentially-leaking hydraulic fluid in the wheel well — an ATA-32 number rooted in a cross-system fire risk.

5. The spoiler-compression-wheelspeed-autobrake interlock

The ground deceleration chain is a set of mutually-dependent signals (11):

L/G compressed (LGCIU) + wheel speed (tacho) → PRIM commands ground spoilers → ≥2 spoiler signals → BSCU activates autobrake
                                                      │
                          MAX also needs wheel speed >40 kt + nose compressed; spoilers do not auto-extend below 72 kt → low-speed RTO autobrake not activated

Each link depends on the last: no compression → no spoilers → no autobrake. This explains the 72 kt RTO trap in Autobrake — the gear's "compression signal" is the start of the whole ground deceleration chain.

6. LGCIU — the broadcast station for gear configuration

As LGCIU and Position Warning covered, the LGCIU broadcasts the gear configuration (up/down/compressed) to a host of users: F/CTL (flight-control laws), pressurisation (ground/air logic), ADIRS, ECAM, autobrake, the oversteer-protection power, and more. "On the ground or in the air" is one of the most fundamental states on the aircraft, and the LGCIU is its source. A single LGCIU fault leaves a swath of downstream systems unsure whether the aircraft is on the ground.

[!warning]- Five misconceptions this article corrects (1) Yellow hydraulics is not unrelated to the gear — it drives the alternate nosewheel steering. (2) The gear does not only affect the ground — the NWS self-test → FMGEC affects CAT III B approach capability. (3) Brake temperature is not an ATA-32-internal matter — it involves hydraulic leaks and a wheel-well fire. (4) Autobrake does not activate on its own — it depends on the spoilers (which depend on L/G compression + wheel speed). (5) The gear does not operate independently of the navigation systems — antiskid/autobrake/the steering law all depend on the ADIRS.

Self-test

[!note]- Q1. What does each of the three hydraulic systems drive in the landing gear?

Green: normal extension/retraction, normal braking, normal steering. Blue: alternate braking (with/without antiskid) and parking-brake accumulator charging. Yellow: alternate nosewheel steering (ALTN N/W STRG, ground only). None of the three is unrelated to the gear.

[!note]- Q2. What does a total ADIRS loss cascade into for the landing gear?

Antiskid reference speed falls back to the highest wheel speed (reduced accuracy), autobrake is affected, and the steering speed-decay law becomes inaccurate. The gear depends on the inertial reference more than intuition suggests.

[!note]- Q3. How does a gear fault affect CAT III B?

The NWS pre-landing self-test result feeds the FMGEC through four BSCU validity discretes. A BSCU/NWS drop to a single channel loses CAT III B capability, leaving CAT 3 SINGLE ONLY — a ground-steering system deciding a low-visibility approach.

[!note]- Q4. Where does the take-off brake-temperature limit really come from?

From preventing a hot brake, once retracted into the wheel well, igniting potentially-leaking hydraulic fluid there — entangling the brakes (ATA-32), hydraulics (ATA-29), and the wheel well (structure/fire).

[!note]- Q5. Give the autobrake activation signal chain.

L/G compressed (LGCIU) + wheel speed (tacho) → PRIM commands the ground spoilers → ≥2 spoiler signals → the BSCU activates autobrake. No compression → no spoilers → no autobrake (the 72 kt RTO trap).

Key takeaways

References

This is a synthesis article with no new source facts — every interface comes from the dumped source library cited in articles 00–23: the three-hydraulic division (AMM 32-42/43/44 + FCOM DSC-32-20-20/30-10), the ADIRS dependence (FCOM DSC-32-30-10 + AMM 32-42/51), NWS → FMGEC → CAT III B (AMM 32-46-00), brake temperature → wheel-well fire (FCTM PR-NP-SOP-100), the spoiler-compression-autobrake interlock (FCOM DSC-32-30-10 + AMM 32-42), and the LGCIU broadcast (AMM 32-21 + FCOM). The interface map and link chains are integrative topologies over the facts already cited in those articles, introducing no new source assertion. The far side of each cross-chapter interface (ATA-22/24/27/29/31/34/45/70) belongs to its own chapter; this article establishes only the ATA-32 side.

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.