ATA 32 LEARNING PATH
Read the chapter in sequence
01 · MECHANICAL FOUNDATION
Start with the landing gear as a load-carrying structure
The Main Landing Gear (MLG) and Nose Landing Gear (NLG) do more than hold the aircraft off the ground. They absorb vertical and longitudinal loads during landing, taxi and braking, transmit those loads into the airframe, provide stable geometry for ground operation, and create the mechanical platform for braking and steering functions.
Each A350 MLG is built around an oleo-pneumatic shock-absorber leg formed by a main fitting and sliding tube. A bogie beam carries the wheels, while torque links keep the main fitting and sliding tube aligned. Forward and aft side stays, lock links, downlock springs and lock-stay actuators establish the extended-and-locked condition. A retraction actuator provides the main movement force during normal operation.
The A350-1000 introduces the larger six-wheel bogie arrangement and has variant-specific details around the bogie pitch-trim system and braking distribution. The important maintenance habit is therefore to confirm effectivity before turning general system knowledge into a maintenance action.
The NLG follows the same broad shock-absorber principle but uses a drag stay and lock-link arrangement suited to the nose installation. Twin steering actuators connect the steering system to the NLG, making the nose gear both a load-supporting and directional-control assembly.
02 · ENERGY ABSORPTION
Shock absorbers, changeover seals and MLG bogie pitch control
The MLG and NLG shock absorbers are single-stage oleo-pneumatic units. The combination of oil and high-pressure nitrogen absorbs energy as the sliding tube moves relative to the main fitting. Gas compression provides the spring effect; controlled fluid movement provides damping.
The source material also describes a changeover-valve arrangement that allows a secondary sealing configuration to be selected if the primary dynamic seal becomes unserviceable. From a learning perspective, this is a good example of local maintainability built into a heavily loaded component: the shock strut is not simply a sealed cylinder, but a managed pressure vessel with servicing points and a backup sealing strategy.
The MLG bogie pitch-trim actuator positions the bogie so that the wheel group is correctly aligned for retraction and also damps bogie pitch oscillations during landing and taxi. The -900 uses a hydraulically supplied arrangement, while the -1000 source describes an independent fluid-and-nitrogen actuator with monitoring for fluid quantity and nitrogen pre-charge condition.
03 · DOORS & LOCKS
Why the doors are part of the sequence, not a cosmetic cover
Each MLG bay uses a main door plus additional fairing/hinged door elements. The hydraulically operated main door must open at the correct time to give clearance for gear movement and then return to the required configuration. The NLG bay uses forward hydraulically operated doors and rear doors mechanically linked to gear movement.
Uplock assemblies hold the gear and applicable doors in the retracted/closed state. Downlock mechanisms, including lock links and springs, secure the gear after extension. The system therefore alternates between two kinds of mechanical certainty: an uplocked flight configuration and a downlocked landing configuration.
Maintenance safety is especially important around these doors. The training source notes that safety pins and actuator collars are required for specific ground conditions and that some safety-pin states are not electronically monitored. The lesson is broader than ATA 32: a safe maintenance configuration can depend on a physical device that the aircraft itself cannot verify.
04 · NORMAL EXTENSION / RETRACTION
LGCIS turns a lever command into a verified sequence
In normal operation the landing gear system is electrically controlled and hydraulically operated. The Landing Gear Control and Indication System (LGCIS) is arranged in two redundant sides. Each side uses CPIOM-hosted Landing Gear Extension and Retraction System (LGERS) application logic, CRDCs, SSPCs and the AFDX network. One side controls while the other continues monitoring, with changeover logic designed to reduce dormant-failure exposure.
The hydraulic power sources are split by gear group: the NLG normal extension/retraction function uses the Yellow hydraulic system and the MLG uses the Green hydraulic system. Solenoid-operated isolation, selector and uplock valves route pressure to the correct actuator chambers at the correct time.
Retraction logic
Before retraction begins, the system checks prerequisite states such as NLG centering, shock-absorber extension and the required MLG bogie condition. Once the lever is selected UP, hydraulic isolation is opened, door uplocks are released, doors are commanded open, the gear is retracted and uplocked, and the doors are then closed and uplocked before the isolation function is returned to its non-energized state.
Extension logic
For extension, the same architecture works toward the opposite final state: hydraulic supply is enabled, doors are released and opened, gear extension is commanded, downlock is confirmed, and the doors are restored to the required position. The important point is that the system does not merely command movement; it waits for state confirmation.
ADIRS speed information is part of the protection logic, and the landing gear selector includes a baulk function to prevent inappropriate UP selection on the ground. These interfaces show why ATA 32 cannot be understood by tracing hydraulic lines alone.
05 · MONITORING
Proximity sensors close the control loop
Proximity sensors monitor gear and door uplock/downlock states, door-open states, shock-absorber extension and bogie geometry. Redundant sensing supports both LGCIS sides, while some downlock and bogie-position functions use independent arrangements to strengthen confirmation logic.
Downlock indication is not treated as one sensor equals one truth. The training material describes consolidated logic using LGCIS 1, LGCIS 2 and independent downlock information. In normal conditions, a two-out-of-three style confirmation is used for a gear-downlocked result. This is a key modern-aircraft principle: cockpit indication is the result of sensing plus logic, not a direct mechanical pointer.
Weight on Wheels (WoW) is derived from a combination of downlock and shock-strut state. That derived air/ground information is then distributed to other aircraft systems. For troubleshooting, always separate three questions: did the mechanism move, did the sensor detect it, and did the aircraft logic accept the result?
For maintenance adjustment, the sensor-to-target gap is critical. The source describes mechanical adjustment with shims/fasteners and status verification through OMT functions showing NEAR/FAR conditions.
06 · GROUND DOOR OPENING
Maintenance access requires a dedicated door-opening architecture
The Ground Door Opening (GDO) system lets maintenance personnel open landing-gear doors from outside the aircraft. On the -900, mechanical handles operate a bypass function and release the door uplock. The bypass arrangement isolates the door actuator from the normal pressure path and connects actuator chambers so the door can open by gravity.
The -1000 source describes an electrically controlled E-GDO arrangement using Ground Door Opening Panels, electrical motor actuators and push-button/handle logic. Door opening can be achieved without hydraulic pressure, but hydraulic pressure is required for closing. The closing sequence also deliberately controls door speed before the system returns to the flight configuration.
GDO is therefore not just a convenience. It is a maintenance mode that deliberately changes the hydraulic and locking configuration of a flight-critical door system. Correct use of safety pins and actuator collars remains essential.
07 · ALTERNATE EXTENSION
An independent path when normal extension is unavailable
The alternate extension system is designed to release the gear and door uplocks when the normal electrical/hydraulic extension path cannot complete the job. It operates independently of the normal AFDX-controlled path and uses Alternate Extension Control Modules (AECMs), dedicated alternate power packs and vent valves.
The alternate power packs create hydraulic pressure for the alternate functions. The vent valves reconfigure the normal actuator circuits so trapped pressure cannot hydraulically lock the gear or doors. Door uplocks are released, then gear uplocks are released, after which gravity and downlock springs drive the assemblies toward the landing configuration.
After alternate extension the landing-gear doors remain open. This creates an important ground-safety condition. The reset sequence requires a controlled maintenance configuration, including gear safety pins, GDO isolation and door-actuator safety collars before hydraulic restoration/reset activities.
The reset is not instantaneous: the training source describes a period during which actuator chambers are refilled before the vent valves are returned to their normal state. That detail is a reminder that alternate-mode recovery is itself a system sequence and should not be treated as simply “switch back to normal.”
08 · CONTROL & INDICATION
What the crew sees during normal and degraded operation
The cockpit uses the landing gear control lever for normal operation and a dedicated gravity-extension control for the alternate path. The WHEEL System Display page and lower PFD area provide the principal landing-gear position cues.
Gear symbols distinguish downlocked, in-transit/unconfirmed and uplocked conditions; door symbols show door state. Fault logic can identify conditions such as doors not closed, uplock disagreement or the total loss of both normal landing-gear control sides. During alternate extension the WHEEL page indicates that gravity extension is in progress until the sequence is complete.
The most useful learning approach is to connect each cockpit cue to the underlying evidence: which sensors, which logic channel, and which mechanical state must exist for that indication to be valid?
09 · MAINTENANCE THINKING
How to troubleshoot Part 1 without jumping to component replacement
- Confirm configuration. Aircraft on ground or in flight, hydraulic state, gear lever position, GDO state, safety devices.
- Identify the stopped step. Door release, door open, gear movement, lock confirmation or door closure.
- Separate command from response. Was the correct command generated? Did pressure reach the actuator? Did the mechanism move?
- Verify sensing. Compare actual mechanical position with proximity-sensor status and system indication.
- Use redundancy. Determine whether one LGCIS side disagrees with the other or whether an independent sensor confirms the condition.
- Check the applicable approved data. Use the current aircraft-specific troubleshooting and maintenance instructions for the actual task.