The landing gear system of the A320 consists of two main landing gears that retract inboard and one nose landing gear that retracts forward. The doors, which are fitted to the landing gear struts, are operated mechanically by the gear and close at the end of the gear retraction.
Each main landing gear has twin wheels and an oleo-pneumatic shock absorber. Additionally, each main wheel is equipped with an anti-skid brake system. The nose landing gear, which also features a two-wheel design, is equipped with an oleo-pneumatic shock strut and includes a nose wheel steering system.
Things to study:
- Landing Gears and Doors Operation
- Nose Wheel Steering
- Brakes and Anti-Skid System
Landing Gear and Doors Operation
Two Landing Gear Control and Interface Units (LGCIUs) oversee the operation of the landing gear and doors. Additionally, they provide crucial information about the landing gear status to the ECAM system.
One LGCIU controls one complete gear cycle, then switches over automatically to the other LGCIU at completion of retraction cycle. It also switches over automatically in case of failure. Green hydraulic system is used for actuation of all gear and doors. A safety valve automatically cuts off the hydraulic supply when the aircraft exceeds 260 knots. Below this speed, the hydraulic supply remains cut off as long as the landing gear lever is in the up position.
Landing Gear Gravity Extension
In the cockpit, there is a crank lever for manual extension of the landing gear. This lever is used in the event of a normal system failure. When a crew member turns the crank, it performs the following actions:
– Isolates the landing gear hydraulics from the green hydraulic system.
– Unlocks the landing gear doors, as well as the main and nose gear.
– Allows gravity to drop the gear into the extended position.
In this process, the gear doors remain open (For training flights, a gear reset is possible provided that the green hydraulic system is available).
LGCIU additional info
Landing Gear Controls and Indicators
- Landing gear indicator panel is connected to LGCIU 1. Triangle light comes on green if the gear is locked down otherwise (transit or non-commanded position) red UNLK light is displayed. If one UNLK indication remains on, refer WHEEL SD page. WHEEL SD page has info from both LGCIU. Only one green triangle on each landing gear is sufficient to confirm that landing gear is down and locked.
- During landing gear extension or retraction, the normal brake system automatically brakes the main landing gear wheels as the landing gear doors open. Additionally, a brake band in the nose landing gear well stops the nose landing gear wheels as the doors close. After takeoff, an internal cam mechanism centers the nose wheel.
Nose Wheel Steering
Nose Wheel Steering is controlled by BSCU (Brake and Steering Control Unit) via yellow hydraulic system. BSCU receives orders from CM1 and CM2 steering hand wheels, or Rudder Pedals, or Autopilot. Steering hand wheels are part of fly by wire concept, so their input is algebraically added up. Additionally, RUDDER PEDAL DISC pb on either handwheel can be used to remove control of nose wheel steering from rudder pedals. Nose wheel steering angle varies with ground speed and following are maximum limits:
- orders from rudder pedals – up to 6 degrees
- orders from hand wheels – up to 75 degrees
- after deactivation of steering system from towing, wheel can be turned up to 95 degrees
The NW STRG DISC memo appears green when the nose wheel steering selector is in the towing position and the engines are not running. It appears amber when the nose wheel steering selector is in the towing position and the engines are running.
Brakes and Anti-Skid System
The main wheels are equipped with carbon multi-disc brakes, while the nose wheel does not have any brakes. Brakes are actuated by two independent systems. The normal system uses green hydraulic pressure, while the alternate system uses yellow hydraulic pressure backed by a hydraulic accumulator. An anti-skid and autobrake system is also provided.
Braking commands come either from the brake pedals or the autobrake system. Under normal operations, the BSCU controls normal braking and anti-skid functions. The green hydraulic system is used for normal braking, while the yellow hydraulic system is used for alternate braking. The BSCU has two channels, and the changeover between these channels occurs with each DOWN selection of the landing gear lever or in the event of a channel failure.
Depending on the level of failure, braking may revert to either of the following configuration:
- Alternate braking with anti-skid. This mode is managed by the Alternate Braking Control Unit (ABCU), while the anti-skid function remains controlled by the BSCU.
- Alternate braking without anti-skid. Only ABCU controls the braking.
- Alternate braking with anti-skid on accumulator (case of G+Y Hydraulic Failure). Only ABCU controls the braking and limited number of brakes application is available.
Additional Info:
- The BSCU checks residual pressure in the brakes, monitors brake temperature, and provides discrete wheel speed information to other aircraft systems.
- The main gear wheels are equipped with fusible plugs to protect against tire bursts in the event of overheating. Additionally, they are fitted with brake cooling fans that enable high-speed cooling of the brakes.
ANTI-SKID SYSTEM
The anti-skid system is designed to provide maximum braking efficiency by maintaining the wheels at the threshold of skidding. It optimizes brake usage by releasing brake pressure just before the wheels are about to skid on the runway. Under normal conditions, the anti-skid system is available both with and without autobrakes and automatically deactivates below 20 knots of ground speed.
The speed of each main gear wheel, measured by a tachometer, is continuously compared to the aircraft’s reference speed. The reference speed is determined by the BSCU using the horizontal acceleration data from ADIRU 1, 2, or 3. If all ADIRUs fail, the reference speed is set to the maximum of either main landing gear wheel speeds.
If a wheel’s speed falls below approximately 87% of the reference speed, the system issues brake release commands to maintain that slip ratio.
AUTO BRAKE
The purpose of the autobrake system is to reduce braking distance in the event of an aborted takeoff and to establish and maintain a selected deceleration rate during landing. This improves passenger comfort and reduces crew workload. The autobrake system offers three modes: LO, MED, and MAX, which can be selected via a pushbutton panel. Autobrakes can be armed at any time, even with the parking brake on. However, the MAX mode cannot be armed in flight.
Auto brake arming conditions:
- Green Hydraulic System available
- Anit-skid electrically powered
- No failure in the braking system
- At least one ADIRU is available
Auto brake activation logic:
Automatic braking is activated when a command for ground spoiler extension is detected, and wheel speed is above activation threshold for different modes.
| LO and MED Mode | MAX Mode |
| Main gear wheel speed needs to be above 80 knots during landing phase. Source: AMM 32-42-00
This prevents activation of auto brakes during partial extension of ground spoilers. |
Main gear when speed needs to be above 40 knots. |
* If the aircraft makes an acceleration stop and begins to decelerate when the wheel speed is under 72 knots, automatic braking will not activate because the ground spoilers will not extend. At least two SECs must be operative for the autobrake system to activate. Depending on the aircraft’s MSN, the AUTO BRK FMA may be available on the PFD.
Ground Spoiler Extension Logic |
| Full Extension – Rejected Take-off Phase
– Ground spoilers armed + wheel speed > 72 kt: Automatically extend when both thrust levers set to idle. – Ground spoilers not armed + wheel speed > 72 kt: Automatically extend when reverse thrust selected on one engine (other thrust lever at idle). * Speed brake not retracted is not mentioned in this case as Takeoff Config warning will warn crew against attempting takeoff with speed brakes not in retracted condition. |
| Full Extension – Landing Phase (both main landing gears on ground)
– Speed brake lever not retracted or ground spoilers armed: Both thrust levers at or below idle, or reverse selected on one engine (other thrust lever below MCT) – Speed brake lever retracted but ground spoilers not armed: Reverse selected on one engine (other thrust lever below MCT) |
Safety First – Evolution of Ground Spoiler Logic
AUTO BRK PANEL
The auto brake panel has pushbutton switches for MAX, MED, and LO autobrake selections:
| MAX Mode | MED and LO Modes |
| – Normally selected for takeoff. – In case of an aborted takeoff, maximum pressure is applied to the brakes as soon as the system generates the ground spoiler deployment order. |
– Typically selected for landing. – Both modes send progressive pressure to the brakes 2 seconds after ground spoilers deploy. – MED Mode: Decelerates the aircraft at 3 m/s². – LO Mode: Decelerates the aircraft at 2 m/s². * in some MSNs LO Mode gets activated 4 seconds after ground spoilers deployment and its decelerates the aircraft at 1.7 m/s2. |
- ON light on push button switches indicate positive arming of AUTO BRK mode
- DECEL light on push button switches is displayed ON when actual deceleration is 80% of the selected rate (On a slippery runway, the predetermined deceleration may not be reached due to anti-skid operation. DECEL light will remain OFF. This does not indicate that the autobrake system is not functioning.)
PARKING BRAKE
The parking brakes are supplied by the yellow hydraulic system or by an accumulator, which maintains parking pressure for at least 12 hours. If the parking brake is activated and there is no yellow hydraulic or accumulator brake pressure available, normal braking can be applied via the brake pedals. A triple indicator on the center instrument panel displays the pressure delivered to the left and right brakes, as well as the accumulator pressure.
