Landing Gear: Support, Braking, and Steering

Every flight begins and ends with one critical system working flawlessly—the landing gear. But what happens when hydraulic pressure fails during approach? How does the aircraft ensure you can always extend the gear when needed? The landing gear system solves these challenges through mechanical backups, multiple hydraulic sources, and intelligent monitoring that keeps you informed of every component's status.

How the Landing Gear System Works

The A320/A321 landing gear consists of three main assemblies: two main landing gears that retract inward into the wing roots and a nose landing gear that retracts forward into the fuselage. Think of it as a carefully choreographed dance—when you move the gear lever, doors open in sequence, the gear extends or retracts, and then the doors close automatically to maintain the aircraft's aerodynamic efficiency.

The system operates on green hydraulic pressure under normal conditions, with two Landing Gear Control Interface Units (LGCIUs) managing the entire process. These units alternate control after each gear cycle or when one fails, ensuring redundancy. The beauty of this design lies in its simplicity: one lever controls everything, but the system handles the complex sequencing automatically.

Normal operation follows this sequence: All gear bay doors open, the landing gear moves to the selected position, and then all doors close. The system includes a crucial safety feature: a safety valve prevents hydraulic supply to the landing gear when airspeed exceeds 264 kt, protecting against inadvertent retraction at high speeds.

When Hydraulics Fail: The Gravity Extension System

Here's where the system's brilliance becomes apparent. If green hydraulic pressure fails completely, you're not stranded with gear that won't extend. The manual gravity extension system provides a mechanical backup that works regardless of hydraulic or electrical failures.

The gravity extension process is elegantly simple: Pull out the gear crank on the center pedestal and turn it clockwise three times. This action disconnects the landing gear from the green hydraulic system, depressurizes the system, and unlocks all gear doors and landing gear assemblies. Gravity does the rest, with locking springs and aerodynamic forces helping secure the gear in the down position.

Important operational note: After gravity extension, the gear doors remain open, creating additional drag. This affects your approach planning and fuel calculations, but it's a small price for the reliability this backup system provides.

Landing Gear Monitoring and Indications

The cockpit provides multiple ways to verify gear position, because knowing your gear status is critical for safe operations. The primary indication comes from the landing gear indicator panel, which shows green lights when each gear is locked down and red "UNLK" lights when the gear isn't locked in the selected position.

For complete verification, check the WHEEL synoptic page on the ECAM, which displays data from both LGCIUs. Look for at least one green triangle on each gear—this confirms the gear is down and locked. The system also provides an "LDG GEAR DN" green memo for additional confirmation.

The red arrow indicator becomes your attention-getter if the landing gear isn't locked down when the aircraft is in landing configuration. This triggers both a cockpit light and a red ECAM warning, ensuring you can't miss a gear problem during approach.

Nose Wheel Steering: Ground Maneuvering Control

Ground operations require precise control, and the nose wheel steering system delivers this through hydraulic actuation powered by the green or yellow hydraulic system. The Brake and Steering Control Unit (BSCU) processes inputs from multiple sources: the captain's and first officer's steering handwheels, rudder pedals, and even autopilot commands during certain operations.

Steering capability varies by input source: The interconnected steering handwheels provide up to 75 ° of nose wheel deflection in either direction—sufficient for most taxi operations. For towing operations, the ground crew can deactivate the steering system, allowing up to 95 ° of nose wheel movement.

System activation requires specific conditions: The A/SKID & N/W STRG switch must be ON, the towing control lever in normal position, at least one engine running, and the aircraft on the ground. This prevents inadvertent steering inputs during flight while ensuring the system is ready when you need it.

Braking Systems: Multiple Layers of Stopping Power

The braking system exemplifies the A320/A321's philosophy of redundancy and reliability. Carbon multi-disc brakes on the main wheels are powered by two independent hydraulic systems, ensuring you always have stopping capability.

Normal braking uses green hydraulic pressure and provides full functionality, including anti-skid and autobrake systems. The dual-channel BSCU manages these functions, automatically switching between channels at each landing gear lever DOWN selection or if a channel fails.

When green hydraulic pressure fails, the system automatically transitions to alternate braking using yellow hydraulic pressure. Anti-skid remains functional in this mode, though the autobrake becomes inoperative. The system includes a hydraulic accumulator that can support at least seven full brake applications even if yellow hydraulic pressure is lost.

In the most degraded state—alternate braking without anti-skid—brake pressure is limited to 1000 PSI to prevent wheel locking and minimize tire damage. The accumulator can maintain parking brake pressure for at least 12 h, ensuring the aircraft stays put during extended ground operations.

Anti-Skid and Autobrake: Maximizing Performance

The anti-skid system maximizes braking efficiency by keeping wheel speed just below the skid threshold. It continuously compares main gear wheel speeds (measured by tachometers) to a reference speed calculated from aircraft horizontal acceleration data. When wheel speed drops below 87 % of the reference speed, the system commands brake release to prevent skidding.

The system's intelligence shows in its reference speed calculation: It uses data from ADIRU 1, 2, or 3 to determine aircraft speed. If all ADIRUs fail, it defaults to using the maximum speed of the main gear wheels as a reference, ensuring the system remains functional even with multiple failures.

Autobrake reduces pilot workload during both rejected takeoffs and landings. The system arms when you select LO, MED, or MAX modes (provided green hydraulic pressure is available, anti-skid is powered, no braking failures exist, and at least one ADIRU is operational). MAX mode provides maximum deceleration for rejected takeoffs, while LO and MED modes offer consistent deceleration rates during landing.

What This Means for Your Operations

Understanding these systems helps you make better operational decisions. When you see alternate braking indications, you know anti-skid is still working, but autobrake isn't available—plan for manual braking. If you need to use gravity extension, factor in the additional drag from open gear doors for your approach planning.

The system's monitoring capabilities mean most problems are detected and displayed before they become critical. Brake temperature monitoring prevents overheating, while continuous component monitoring triggers ECAM warnings for any failures. This gives you time to plan and execute appropriate responses rather than dealing with sudden system failures.

Remember the operational limitations: The braking system cannot hold the aircraft stationary under high thrust, maximum brake temperature for takeoff is 300 °C with brake fans off, and specific speed limits apply with gear extended. These aren't arbitrary numbers—they’re based on the system's design capabilities and ensure safe operation within tested parameters.

The landing gear system represents aviation engineering at its finest—simple to operate, reliable in function, and robust in design. By understanding how these systems work together, you're better equipped to handle both normal operations and the unexpected situations that test your knowledge and decision-making skills.

A320 landing gear explained – normal and gravity extension, braking system layers, anti-skid logic, nose wheel steering, and what each indication actually means.