Aircraft Landing Gear – Comprehensive Glossary

Definition and Overview

Aircraft landing gear (or undercarriage) is the complete assembly of wheels, shock absorbers, brakes, struts, retraction mechanisms, and related systems that support an aircraft during ground operations and enable safe takeoff and landing. The landing gear bears the aircraft’s weight, absorbs and dissipates landing shocks, enables ground steering, and provides braking and stability on various surfaces, including runways, grass, water, snow, or rough terrain.

Key functions of landing gear include:

• Weight Bearing: Supports the static and dynamic loads of the aircraft during all phases on the ground.
• Shock Absorption: Dissipates kinetic energy during landing or aborted takeoff, protecting the airframe.
• Ground Handling Stability: Ensures directional control and stability during taxi, takeoff, and landing.
• Braking/Deceleration: Equipped with brakes to stop the aircraft safely after landing or during an aborted takeoff.
• Adaptability: May feature floats, skis, or other adaptations for diverse terrains.

Landing gear design is regulated by standards such as EASA CS-25 and FAA FAR Part 25, ensuring safety, reliability, and performance. Modern systems may include electronic health monitoring and advanced status indicators.

Historical Context

The landing gear has evolved alongside aviation history. Early gliders and the Wright Flyer used wooden skids. As aircraft weight and speed increased, wheels replaced skids, and the 1920s-30s saw robust undercarriages with rubber tires and basic shock absorbers. World War II ushered in retractable landing gear for improved aerodynamics and specialized systems like floats and skis.

The jet age brought hydraulic retraction, advanced materials, and multi-wheel bogies for heavier aircraft. Today, landing gear supports weights exceeding 500,000 kg, incorporates fail-safe features, and leverages predictive maintenance and composites for further improvements.

Types of Aircraft Landing Gear

Aircraft landing gear varies by operational mode (fixed or retractable) and configuration (tricycle, tailwheel, tandem).

Fixed Landing Gear

Fixed gear remains extended during all flight phases. Common on light aircraft and trainers, it is simple, robust, and reliable—ideal where speed is secondary and ruggedness is vital. Fixed gear may include streamlined covers (“wheel pants”) to reduce drag.

Examples: Piper PA-18 Super Cub, Cessna 172.

Retractable Landing Gear

Retractable gear withdraws into the airframe, reducing drag and improving speed and fuel efficiency. Found on high-performance general aviation, commercial, and military aircraft, it features complex hydraulic or electric actuation, locking mechanisms, and manual backup systems.

Examples: Cirrus SR22, Boeing 737, F-16 Fighting Falcon.

Landing Gear Configurations

Tricycle Undercarriage

The tricycle setup has two main wheels behind the center of gravity and a steerable nose wheel. It offers:

• Better ground stability and forward visibility
• Reduced nose-over risk
• Easier handling in crosswinds

Prevalent on most airliners, business jets, and modern light aircraft.

Tailwheel (Conventional) Undercarriage

The tailwheel arrangement places two main wheels forward and a small wheel at the tail. It is lighter, provides better propeller clearance, and excels on rough fields. However, it is more challenging to handle and prone to “groundlooping.”

Popular with aerobatic, bush, and vintage aircraft.

Tandem and Outrigger Gear

Tandem gear aligns main wheels along the fuselage, with small outrigger wheels or skids for lateral stability. Used in special cases like the Lockheed U-2 or B-52, where fuselage constraints or wing design dictate unique arrangements.

Alternative Types

• Floats/Pontoons: For water operations (seaplanes)
• Skis: For snow or ice (ski planes)
• Skids: Used by helicopters and gliders

Key Components of Aircraft Landing Gear

Wheels and Tyres

Aircraft wheels are made from high-strength aluminum or magnesium alloys. Tires withstand high loads and speeds, built with multiple reinforced plies and inflated to high pressures (sometimes over 200 psi). Chined tires on nose wheels deflect water on wet runways.

Tire health is critical; features like fuse plugs prevent blowouts by venting pressure if overheated. Wheels may include heat shields, anti-skid brake discs, and robust bearings.

Shock Absorbers (Struts)

Most aircraft use oleo-pneumatic struts, combining compressed gas and hydraulic fluid to absorb landing energy. The strut compresses on impact, dissipating force and smoothing landings. Proper maintenance—checking fluid, gas pressure, and for leaks—is vital.

Brakes

Disc brakes (steel or carbon composite) are used on main wheels. Hydraulic actuation is standard, with anti-skid systems preventing lockup. Brakes endure high thermal loads; advanced systems monitor temperature and wear.

Steering Mechanisms

Steering is provided by the nose or tail wheel, linked to rudder pedals or a tiller. Power steering (hydraulic/electric) is used on large aircraft. Differential braking supplements tight turns and ground maneuvering.

Retraction Systems

Hydraulic, electric, or pneumatic systems actuate retraction/extension. Safety features include uplocks, downlocks, squat switches (to prevent retraction on the ground), and manual or gravity drop for emergencies.

Doors and Fairings

Gear doors enclose landing gear bays for aerodynamic smoothing. Designs range from simple to complex, with seals to prevent air leaks. Door integrity is essential for safety.

Position Indication and Warning Systems

Lights on cockpit panels indicate “gear up,” “gear down,” or “in transit.” Audible warnings alert pilots if the gear is not down when landing is set (flaps extended, throttle idle). Advanced systems provide real-time monitoring of gear position and lock status.

Undercarriage Material and Design Considerations

Structural Materials

• High-strength steel: Main load-bearing parts, fatigue-resistant, corrosion-protected.
• Aluminum: Used for lower-stress parts and some wheels.
• Titanium: Increasingly used for strength and weight savings.
• Composite materials: Carbon fiber for secondary structures, reducing weight.

Design Criteria

Landing gear must withstand extreme loads (hard landings, high braking, crosswinds). Regulations specify “limit” and “ultimate” load requirements.

Other design considerations:

• Fatigue Life: Rigorous non-destructive testing to prevent cracks.
• Corrosion Resistance: Protective coatings, regular inspection.
• Aerodynamics: Streamlined fairings and tight-sealing doors.
• Redundancy: Manual or backup extension to ensure the gear deploys for landing.

Operational Use and Scenarios

How Landing Gear Is Used

• Taxiing: Supports full weight, absorbs shocks, enables steering/braking.
• Takeoff: Transmits acceleration to runway, resists crosswind and uneven surfaces.
• Landing: Absorbs vertical/horizontal forces, provides braking and steering.
• Ground Handling: Enables towing, pushback, parking, and withstands ramp activities.

Real-World Examples

• Cessna 172: Fixed tricycle gear, simple and reliable for training/general use.
• Boeing 737: Retractable tricycle gear, multi-wheel bogies, sophisticated brakes.
• Twin Otter (floats): Amphibious gear for both runway and water operations.

Special Use Cases

• STOL aircraft: Fixed oversized gear for rough, unprepared surfaces.
• Fighter jets: Complex retractable gear for short, high-speed landings.
• Cargo planes: Multi-bogie gear for heavy loads and varied airstrips.

Challenges in Undercarriage Operation and Maintenance

Operational Stresses

• Landing Impact: High vertical loads, especially in hard landings.
• Taxiing Loads: Side, torsional, and dynamic forces from terrain/debris.
• Environmental Exposure: Moisture, chemicals, and temperature extremes.

Common Failure Modes

• Tyre Blowouts: Due to FOD, underinflation, or overheating.
• Strut Leaks/Collapse: From worn seals or hydraulic loss.
• Brake Overheating: Caused by heavy/repeated braking.
• Shimmy: Oscillation of steerable wheels, managed by dampers.

Design Solutions

• Oleo Struts: Reliable shock absorption.
• Shimmy Dampers: Prevent damaging oscillations.
• Redundant Retraction: Manual/gravity systems for backup.
• Brake Cooling: Ventilated/carbon discs, heat shields.

Maintenance and Inspection

Regular inspection and servicing are critical for safety. Maintenance protocols include:

• Checking tire pressure, tread, and condition
• Inspecting shock strut extension, leaks, and fluid levels
• Testing brake wear, temperature, and hydraulic integrity
• Verifying retraction/extension operation and emergency systems
• Monitoring for corrosion and fatigue cracks

Predictive maintenance and electronic health monitoring are increasingly used to schedule repairs and prevent failures before they occur.

Future Trends in Landing Gear

• Composite Materials: More use of carbon fiber for lighter, stronger gear.
• Smart Sensors: Advanced health monitoring for condition-based maintenance.
• Electric Actuation: Reducing hydraulic complexity and weight.
• Noise Reduction: Quieter gear and doors for urban operations.

Conclusion

Aircraft landing gear is a highly engineered system essential for safe flight operations. Its design and maintenance are governed by strict regulations and advanced engineering, ensuring reliability, efficiency, and safety in diverse operating environments. Ongoing innovations in materials, sensing, and actuation continue to improve performance, reduce weight, and enhance overall safety.

Related Terms

• Shock Absorber (Oleo Strut)
• Tricycle Gear
• Tailwheel Gear
• Multi-Bogie
• Anti-skid System
• Floats (Seaplane)
• Skis (Ski Plane)
• Retraction System
• Position Indication
• Shimmy Damper

Aircraft landing gear is foundational to aviation safety and performance—designed and maintained for maximum reliability, it continues to evolve alongside aircraft themselves.