Landing gear, brakes and tyres

Three jobs under extreme loads

Landing gear systems absorb the impact of landing, support the aircraft on the ground, provide steering and braking, and allow safe takeoff and landing operations. Although they are used for only a small portion of each flight, landing gear components endure some of the highest loads experienced by the aircraft. Understanding shock struts, retraction systems, brakes, antiskid protection and tyre maintenance is essential for both pilots and maintenance technicians. A landing gear system must perform three jobs:

• Support the aircraft's weight on the ground.
• Absorb landing loads safely.
• Provide braking, steering and directional control.

Oleo-pneumatic shock struts

When an aircraft touches down, enormous kinetic energy must be absorbed. Without a suspension system, those loads would be transmitted directly into the airframe structure. Most transport aircraft use oleo-pneumatic shock struts, which combine hydraulic fluid and compressed nitrogen gas to absorb landing impact efficiently. During landing:

• The landing gear compresses.
• Hydraulic fluid is forced through calibrated restrictions or orifices.
• Fluid flow absorbs energy.
• Nitrogen gas compresses and acts as a spring.

Damping and spring

This combination provides both damping and spring action. The system absorbs the impact and gradually returns the strut to its normal extension.

Compressed nitrogen forms the spring element of the shock strut, and the hydraulic fluid provides the damping. A useful memory aid is: oil absorbs the shock, nitrogen provides the spring. Nitrogen is preferred because it:

• Is dry
• Is inert
• Does not support combustion
• Experiences predictable pressure changes

Strut servicing

Shock struts must be serviced according to aircraft maintenance instructions, with the correct fluid quantity, gas pressure and strut extension. A strut that repeatedly bottoms out transfers landing loads directly into the aircraft structure, increasing the risk of damage. Improper servicing can lead to:

• Under-serviced struts: harsh landings, bottoming out, structural stress and poor ground handling.
• Over-serviced struts: excessive extension, rough taxi characteristics, incorrect geometry and increased loads.

Landing gear retraction systems

Retractable gear reduces aerodynamic drag during flight. Large transport aircraft use hydraulic power to move the landing gear between the extended and retracted positions. Because landing gear assemblies are large and heavy, hydraulic power is ideal for this application. Hydraulic actuators provide the force needed to:

• Retract the landing gear
• Extend the landing gear
• Operate gear doors

Landing gear sequencing

Landing gear movement follows a carefully controlled sequence. Sequence valves and control logic ensure components move in the proper order, because incorrect sequencing could result in structural damage. Typically:

• Gear doors open.
• Landing gear moves.
• Gear reaches its destination.
• Doors close when required.

Uplocks and downlocks

The landing gear must remain securely positioned both up and down.

• Uplocks hold the landing gear in the retracted position, preventing gravity-assisted gear movement, vibration-induced deployment and uncommanded extension. The uplock is often a mechanical hook or latch mechanism.
• Downlocks secure the gear in the fully extended position to prevent collapse during landing, taxi and ground operations. Once downlocked, the gear remains mechanically secure even if hydraulic pressure is lost.

Ground safety pins

Whenever the aircraft is parked or undergoing maintenance, landing gear safety pins are installed. These pins provide an additional mechanical safeguard against accidental gear retraction. Safety pins are among the most important maintenance safety devices on an aircraft: pins in before maintenance involving landing gear, pins out before flight.

Alternate extension systems

A transport aircraft cannot rely solely on hydraulic pressure for gear extension. If the normal system fails, crews need a backup method. Many aircraft use gravity-assisted extension. The landing gear free-falls into position and locks down mechanically. When activated:

• Uplocks release.
• Hydraulic restrictions are bypassed.
• Gravity and aerodynamic loads assist gear deployment.

Emergency extension methods

Different aircraft use different backup methods. The objective is always the same: get the gear down and locked even after multiple failures. Methods include:

• Free-fall systems
• Manual release mechanisms
• Nitrogen-assisted systems
• Alternative hydraulic sources

Air/ground sensing systems

Modern aircraft continuously determine whether they are airborne or on the ground. This information is provided by air/ground sensing systems, often called weight-on-wheels (WOW) systems. They typically use proximity sensors, compression sensors and landing gear position sensors to determine whether the landing gear is carrying aircraft weight.

Why air/ground sensing matters

Many aircraft systems depend on accurate air/ground information, and an incorrect air/ground signal can affect multiple aircraft systems simultaneously. Examples include:

• Flight controls: ground spoiler deployment and flight spoiler logic.
• Braking systems: autobrake activation and brake logic.
• Pressurization: cabin pressure scheduling.
• Thrust reversers: deployment interlocks.
• Flight guidance: mode logic changes.

Multi-disc brakes

Transport aircraft use highly sophisticated braking systems capable of absorbing enormous amounts of energy. Most large aircraft use multiple steel or carbon discs arranged in stacks. When hydraulic pressure is applied:

• The discs clamp together.
• Friction converts kinetic energy into heat.
• The aircraft slows.

Carbon brakes and wear indicators

Modern airliners increasingly use carbon brakes, which have become the industry standard for many transport aircraft. They offer lower weight, longer service life, better high-temperature performance and improved heat capacity.

Brake wear is monitored using wear pins. As brake material wears, the wear pins retract and the remaining brake life decreases. When wear reaches maintenance limits, brake replacement is required.

Antiskid systems

A locked wheel provides less braking effectiveness and rapidly damages tyres. To prevent this, aircraft use antiskid systems. The system compares individual wheel speeds, aircraft ground speed and wheel deceleration rates. If a wheel is about to skid, hydraulic pressure decreases, the wheel accelerates again, and pressure is restored. This process repeats many times each second. The principle is similar to automotive ABS but designed for aircraft operations.

Antiskid is especially valuable on wet and contaminated runways. It:

• Reduces stopping distance
• Improves directional control
• Prevents tyre damage
• Reduces flat spotting

Autobrake systems

Autobrakes automate braking during landing and rejected takeoffs.

• Landing autobrakes: the crew selects a desired deceleration rate, and the system applies braking automatically to achieve it. This gives consistent braking, reduced workload and predictable stopping performance.
• Rejected takeoff (RTO): during a high-speed rejected takeoff, maximum braking is commanded automatically. Energy absorption is enormous, and these events produce some of the highest temperatures encountered in aircraft brakes.

Hot brakes and fusible plugs

Following heavy braking, brake temperatures can become extremely high, and wheel temperatures rise accordingly. This creates a significant safety hazard.

Aircraft wheels contain fusible plugs made of materials with precise melting temperatures. If wheel temperature becomes excessive, the plug melts and tyre pressure is safely released, reducing the risk of explosive wheel failure.

Approach precautions

A severely overheated wheel assembly can fail violently. For safety, approach from the front or rear, never directly from the side. The side position places personnel in the potential trajectory of wheel or tyre fragments.

Aircraft tyres and nitrogen inflation

Aircraft tyres are designed to withstand high loads, high speeds and repeated impacts. Despite their appearance, they are highly engineered components. They are typically inflated with dry nitrogen, which also minimizes internal moisture that could freeze at altitude. Reasons include:

• Reduced moisture content
• More stable pressure changes
• Reduced oxidation
• Less fire risk

Cold pressure checks

Tyre pressure should be checked when the tyre is cold. A warm tyre produces an artificially high pressure reading, and servicing based on warm readings can result in underinflation.

Underinflation

The most damaging tyre condition is underinflation. Tyre failures often originate from prolonged operation at low pressure. Consequences include:

• Sidewall flexing
• Excessive heat generation
• Accelerated wear
• Structural damage
• Increased failure risk

Tyre inspection items

Acceptable limits are specified by the aircraft maintenance documentation. Technicians inspect for:

• Cuts
• Bulges
• Flat spots
• Cord exposure
• Tread wear
• Sidewall damage
• Foreign-object damage

Common maintenance considerations

Because landing gear systems are safety-critical, many tasks require independent inspections and strict procedural compliance. Landing gear maintenance often includes:

• Shock strut servicing
• Gear retraction tests
• Gear swing procedures
• Brake inspections
• Wheel bearing checks
• Tyre replacement
• Proximity sensor verification
• Antiskid operational tests

Key takeaways

• Oleo-pneumatic shock struts use hydraulic fluid for damping and nitrogen gas as the spring.
• Most transport aircraft use hydraulic systems to retract and extend landing gear.
• Uplocks secure gear up. Downlocks secure gear down.
• Ground safety pins protect against accidental gear retraction during maintenance.
• Alternate extension systems allow gear deployment after hydraulic failures.
• Air/ground sensing affects many systems, including spoilers, autobrakes, thrust reversers and pressurization.
• Multi-disc brakes, increasingly made from carbon, absorb landing and rejected-takeoff energy.
• Antiskid systems prevent wheel lockup and improve stopping performance.
• Fusible plugs protect against wheel explosions caused by excessive brake heat.
• Aircraft tyres are inflated with nitrogen and should be checked cold.
• Underinflation is one of the most common and damaging tyre conditions.

The bottom line

Landing gear systems do far more than support the aircraft on the ground. Shock struts absorb landing energy, retraction systems reduce drag, air/ground sensors provide critical aircraft logic, brakes and antiskid systems ensure safe stopping, and properly maintained tyres protect against catastrophic failures. Because these systems operate under extreme loads and temperatures, careful inspection, servicing and adherence to maintenance procedures are essential for safe aircraft operation.