11.13 Landing Gear

• Landing gear supports the aircraft's weight during ground operations, dampens vibrations, absorbs landing impact and braking loads, provides steering, and operates in harsh conditions.

Landing Gear Configurations

• Three main types of landing gear arrangements exist: tail wheel, tricycle, and tandem.
• Main gears are located at or around the aircraft's center of gravity (CG).
• Supporting gear can be a nose wheel, tail wheel, or outrigger gear for tandem configurations.

Tail Wheel Configuration

• Commonly used in propeller-driven and light aircraft.
• Main gear is located forward of the aircraft CG with the tail supported by a wheel or skid.
• Tailwheel is articulated and levered for self-centering, with steering achieved through differential braking or rudder pedals.

Tricycle Configuration

• Most widely used landing gear arrangement.
• Aircraft CG is positioned ahead of the main wheels, with a single or double nose wheel supporting the nose.
• Advantages include preventing ground looping, allowing forceful braking without nosing over, providing unobstructed forward vision, and reducing drag during takeoff.
• Number and location of wheels on the main gear can vary (single, double, or multiple "bogie").
• Larger aircraft may have additional gear assemblies supporting the body.

Tandem Configuration

• Relatively uncommon layout with main and tail gear aligned on the longitudinal axis.
• Gear is positioned forward and aft of the CG.
• Commonly used on aircraft gliders.
• Some military bombers such as the B-47 and B-52 also use tandem gear.

Fixed and Retractable Landing Gear

• Fixed landing gear is typically found on low-speed light aircraft.
• It is lighter and cheaper to produce than retractable gear but causes more drag.
• Higher-speed aircraft use retractable gear to reduce drag.

Main Landing Gear (MLG) Components

• Consists of a shock strut, side strut, drag brace, downlock/uplock mechanisms, trunnion, actuator, torsion link, and wheel/tire assemblies.
• MLG assembly transfers operational loads to the aircraft structure.
• The trunnion connects the leg to the wing structure, allowing rotation into and out of the gear bay.
• The shock strut features outer and inner telescopic cylinders filled with hydraulic oil and nitrogen.
• Upper and lower torsion links articulate, connecting the inner cylinder to the fixed outer cylinder, limiting extension, holding wheel alignment.
• Side strut resists lateral movement, connecting to the aircraft structure and shock strut via a universal joint.
• Drag strut braces the leg against longitudinal movement, connecting the shock strut to the trunnion.
• A gear down-lock mechanism prevents folding when the gear is extended.
• Extend and retract actuator connected to the shock or drag strut and wing structure.

Retraction Mechanism

• Articulated downlock strut pulled into an over-center position by springs locks the side strut, and a ground lock pin prevents inadvertent retraction.
• A roller pin engages with a spring-loaded uplock hook when the gear is fully retracted.
• Hydraulic actuators disengage locks for gear extension and retraction.

Shortening Link

• Used to shorten large landing gears during retraction due to limited space in the main landing gear bay.
• Components include an adjustable link, bellcrank lever, connecting link, and upper/lower links.
• Decreases the gear leg length by pulling the shock absorber into the main fitting during retraction.

Nose Landing Gear (NLG)

• Typically has two wheels, is steerable, and does not have brakes.
• Lighter than main gears and not designed to support initial landing loads.
• Retracts forward into the fuselage, enclosed by doors.
• Centering cams align nose wheels for retraction.
• A typical NLG includes a lock stay, drag strut, shock strut, shock absorber, nose gear actuator, torque link, and down/uplock mechanisms.
• Torque link dampers (shimmy dampers) counteract wheel shimmy caused by uneven tire pressure, wear, or runway surface.

Shock Absorbing

• Controls impact forces during landing, absorbing shock and dampening recoil.
• Alters shock energy distribution throughout the airframe, converting energy into heat.
• Two main classes: solid (steel or rubber springs) and oleo-pneumatic (fluid spring with gas or oil).

Oleo-Pneumatic Shock Struts

• Dampens impact from landing, taxiing and uneven surfaces.
• The struts have outer and inner telescopic cylinders.
• Types include those utilizing oil compressibility or combinations of oil and nitrogen.
• Hydraulic dieseling occurs when air mixes with hydraulic oil, causing combustion and cylinder failure.

Oleo-Pneumatic Strut without a Gas or Oil Separator

• Contains an outer cylinder with a fixed piston and orifice, and an inner cylinder with a metering pin.
• Partially filled with hydraulic oil and inflated with compressed nitrogen.
• On the ground, gas compresses to balance weight. During takeoff, expansion forces fluid transfer through a restricted orifice.
• On landing impact, fluid is forced through the metering orifice, compressing the gas until it balances the load.
• Recoil is controlled by restricted fluid flow through the orifice.

Oleo-Pneumatic Strut with Separator

• Features a piston separating oil and gas in separate chambers.
• The inner cylinder contains a valve head with a flutter plate functioning as a one-way restrictor.
• On landing impact, the outer cylinder pushes down, the fluid transfers to the lower chamber faster through the flutter plate, compressing gas until pressure balances the load.
• During recoil, the flutter plate shuts, reducing fluid flow, and controlling extension.

Liquid Spring

• Oil passes through damping orifices into an upper chamber.
• Large shocks compress oil until the pressure exceeds the loading of the piston’s non-return valve spring.
• Oil released around the valve to damp out shocks.
• On recoil, oil forced back through small damping orifices.

Servicing

• Main gear shock struts serviced with nitrogen gas and specified hydraulic fluid.
• Incorrect fluids cause seal deterioration.
• Follow maintenance manual procedures, and with the proper oil service and the aircraft’s weight on wheels, dimension ‘X’ and gas pressure can be measured.
• Dimension ‘X’ measures inner cylinder extension.
• It is checked against temperature-adjusted values in the maintenance manual to determine if gas needs to be added or released.

Extension and Retraction Systems

• Hydraulic systems are commonly used for retraction, with electrical and pneumatic systems on light aircraft.
• Landing gears use positive mechanical locks for retracted or extended positions.
• Gear doors are operated by mechanical rods or hydraulic actuators.
• A control lever on the flight deck manages the system, including a latch to prevent accidental ground retraction and an override for emergencies.
• Indication systems provide alerts on gear/door positions, and an emergency system lowers gear in case of system failure.
• Selector lever has up, off and down positions.
• A solenoid-operated latch prevents accidental retraction while the aircraft is on the ground.
• Systems may use proximity switches, controlled by a computer-programmed logic, to provide information about: gear door sequencing, hydraulic pressure valve operation, and position information to flight crew displays.

Normal Retraction and Extension

• Landing gear and doors are sequenced to prevent jamming.

Main Gear Retraction

• Down-locks are unlocked, and gear actuators receive fluid to retract the gear.
• Sequence valves control gear door operation to occur after gear stowage, then the door actuators close the doors.
• Hydraulic pressure may be applied to the wheel brakes to stop their rotation during retraction.

Nose Gear Retraction

• Inner cylinder extends, and internal cams the centre nose wheels.
• Hydraulic pressure retracts and engages the up-lock hook.

Main Gear Extension

• Hydraulic fluid unlocks the nose gear, fluid opens the doors engaging plungers and main gear up-locks then unlocks and extends the gear.
• Restrictors slow extension to prevent impact damage.

Nose Gear Extension

• Uplock actuator disengages, and up-lock releases, while fluid from the nose actuator extends the gear.
• The restricted return flow helps gear extension, while centring cams allow the NLG to centre.

Emergency Extension

• Alternative systems lower gear in case of hydraulic or system failures.
• May involve mechanical linkages, electrical actuators, dedicated electro-hydraulic power packs, the use of compressed gas, or an electrically operated alternate hydraulic system.
• Large commercial transport aircraft systems may use T-handles connected to uplocks allow gear to freefall.
• Landing gear selector goes to the off position with hydraulic pressure connected via return lines.
• Position and warning indicators and gear down-lock viewers confirm gear is down and locked with proper alignment marks.
• Some small aircraft require manual extension.
• Aircraft Maintenance Manual (AMM) needs to be consulted for system details.

Indication and Warning

Landing Gear Position Sensing System

• Gives the flight deck crew visual indication of gear positions.
• Position sensors are located on each gears' downlock, up-lock, and door locks.
• Most lights have dual filament bulbs. Large aircraft indications are duplicated with a lights test facility.
• Lights indicate position on flight deck and via Electronic Centralised Aircraft Monitor (ECAM [Airbus]) / Engine Indicating Crew Alerting System (EICAS [Boeing]).

Light Signals

• A GREEN light indicates when gear is DOWN and LOCKED.
• RED light indicates when gear is UNLOCKED or travelling.
• NO lights indicates when gear is UP and LOCKED.
• The system shows door positions as open or closed.

Microswitches

• Located on the landing gear control lever, in circuit with the gear position sensors.
• Illuminate the red gear disagreement light if there is a disagreement between the control lever position and the landing gear position.

Proximity Switches

• Used instead of normal microswitches:
  • A reed switch consists of two spring-loaded, normally open contacts embedded into glass which contains a gas.
  • The contacts are closed by magnetic flux created when a permanent magnet is moved adjacent to them.
• An electronic proximity switch contains a sensing coil excited by an electronic unit.

Warning Systems

• A warning system detects and alerts when the landing gear is not down and locked when the aircraft is configured for landing.
• Uses visual and auditory warnings detecting unsafe landing conditions.
• A horn auditory warning sounds if flaps are at a mid position with the throttles retarded.
• Systems incorporate an isolation switch silences warnings, but if the landing flap is selected with the gear NOT down, it cannot be silenced until the landing gear is selected down.

Wheels

• Support the aircraft's weight during taxi, takeoff, and landing.
• Subject to repeated loading, high temperatures from brakes, cold temperatures at altitude, and exposure to moisture/debris.
• Construction materials: aluminum alloy (forged or cast) and magnesium alloy.

Modern Aircraft Wheels

• Constructed from two half hubs carrying a tubeless tyre; older constructions might have a tubed tyre assembly fitted.
• Halves are bolted with a groove containing an O-ring seal for tubeless tyres.
• Tyre contacts wheel at the bead seat which accepts tensile loads during landing.
• Current wheel hubs have inner and outer tapered roller bearings.

Wheel Balancing and Brakes

• Wheels are balanced statically and dynamically to preserve the balance after tyre assembly
• Typically only the main gear wheels are fitted with brakes on the inboard half of the wheel.
• Rotating brake disks driven are by steel drive keys bolted on the inside of the main wheels and mate with slots on the periphery of the brake disks.
• A heat shield prevents brake heat from damaging the wheels and tyres during normal braking.
• Extreme temperatures can affect material properties so the Aircraft Maintenance Manual (AMM) should be consulted for inspection criteria after an overheat event.
• Fusible plugs are fitted to the hubs of wheels with tubeless tyres, and automatically release the tyre pressure.

Bearings

• All aircraft wheels have tapered roller bearings (protected by a seal and spacer tube).
• Tapered roller bearings support axial and radial loads simultaneously.

Brakes

• Purposes:
• Keep the aircraft stationary for parking and engine running
• Slow the aircraft during normal ground manoeuvres
• Stop the mainwheel rotation after take‐off
• Shorten landing run
• Emergency stop after a Rejected Take Off (RTO)
• Types:
• Single disc
• Dual disc
• Multiple disc
• Located on each of the main wheels

When brakes are used to slow the aircraft, kinetic energy is converted into heat energy, designed to disperse rapidly into the airflow.

• Overheated or damaged wheel assemblies should be approached from the rolling direction.

Mechanical or hydraulic linkages to the rudder pedals allow the pilot to control the brakes.

• Different sized aircraft use different types of brakes.

Single disc brakes

• Used on small general aviation aircraft. A hydraulically operated piston provides braking friction.

Small Light Aircraft Brakes

• Apply pressure to both sides of the disc bolted to each wheel. Pressure provided by master hydraulic cylinders connecting to the rudder pedals.

Cylinders

• Operate with a piston, a return spring, and an automatic adjusting pin. Brake has six brake linings or pucks.
• The brake disk is keyed to the wheel can move laterally in the key slots. The self-adjusting feature of the brake maintains the same clearance. Each cylinder accepts an actuating piston assembly comprising of a piston, a return spring, and an automatic adjusting pin.

Adjusting Pin

• Ensures same travel is required regardless of wear, stem of pin protruding serves as a wear indicator with minimum length needed per AMM.

Calliper Housing

• Houses a bleed port to remove unwanted air to be per AMM.

Dual-disc Brakes

• Have two discs keyed to the wheel used on aircraft that require greater braking friction.
• The center carrier is located between the two discs, with friction linings on either side.

Multiple-Disc Brakes

• Required by large aircraft, provides increased friction surface maximising braking forces and heat dispersal.
• The main components are piston housing, torque tube, and the heat pack.

Heat Pack

• Made of alternating rotors and stators, a torque tube, a pressure plate, a pressure ring/torque plate, a piston assembly housing

Brake Pressure

• Causes friction between the rotors and stators and turns kinetic energy into heat.

Automatic Adjusters

• Causes springs to the pressure plate to return to the normal position making discs turn freely.

Pack Construction

• Modern heat brake pack assemblies have carbon fibre rotors and stators, saving weight and withstanding high temps

Carbon Heat Packs

• Advantages and disadvantages:
• Advantage: approximately 40% lighter than conventional brakes saving significant amounts of weight,
• Advantage: withstand temperatures 50% higher than steel components, lasting longer, thus cheaper maintenance.
• Disadvantage: Discs must be replaced if contaminated with oil or grease and are more expensive.
• Advantage: Discs ca be recycled

Steel Disc Construction

• Steel rotors are segmented to prevent bending from heat. Key slots engage with drive keys of the wheel with steel stators (Linings riveted)

Brake Piston Housing

• Made from forged aluminum alloy Pistons change hydraulic brake pressure. Shuttle or selector valve activates the alternate system in the event of a normal system hydraulic failure.

Brake Piston

• Functions To apply brake force via applied pressure, compensate brake disc wear, and automatically adjust brake clearance. Also incorporates a return spring which pushes the piston back.

Overall Wear

• Is is visually indicated by the wear indicator pin, usually attached to the pressure plate, and heat pack is fully worn when wear pin is aligned or flush with bracket. Inspect within wear limits specified in the AMM with the parking brake set with cold brakes.

Torque Tube

• Typically made of forged steel, transmits the torque of the stator discs to the piston housing, with splines/back plate.

Brake Systems

Independent Master Cylinders

• Small aircraft without hydraulic systems use independent master cylinders (braking).
• Pilot pushes on the tops of the rudder pedals with master cylinder mechanically connected to corresponding rudder pedal. A piston in the seal fluid chamber of the master cylinder forces hydraulic fluid pushing break linings generates slowing friction

Power Brakes

• Equipped on large aircraft to slow, stop and hold the aircraft. The activating systems use the aircraft’s hydraulic system as a power source. Pilots apply by pressing on the top of the rudder pedal.
• Instead of master cylinder power, a power brake control valve or brake metering valve provides the supply receiving pedal input.

Brake Control Valve / Brake Metering Valve

• Responding hydraulic fluid directing the brake via an influx corresponding to pedal exerted increasing the action. Built to facilitate graduated pressure control, pedal feel, and redundancy actions. Integrate with anti-skid.
• Hydraulic fuses stop any excessive flow of fluid.

Emergency Brake Systems

• Configure emergency brake systems for independent operation from two different hydraulic power sources allowing fail safe and redundancy features
• The brake accumulator provides an emergency brake source w/pre-charged accumulator diaphragm; pre-charged fluid can also deliver.
• Simpler systems can gets emergency power directly while a shuttle valve accepts shift source.

Brake Debooster

• Reduces pressure from braking operation. Cylinder applies force over different sides pistons in case it needs it lower power with formula:
  • Pressure = Force/Area

Lockout Debooster

• Functions as and hydraulic fuse reducing brake pressure

Parking Brake Systems

• Breakin during parking involves permanent pressurisation of the brake hydraulic circuits, typically the hydraulic accumulator w/nitrogen. shut valve is closed from brakes back to hydraulic trapping fluid, holding stationary.

Brake Temperature Monitoring and Indication

• A large for multi-wheeled aircraft with abnormal functions. High conveys dragging and low means braking is not working, where brake temperature is indicated 0-9 and green wheel indicate the hottest 100c whereas amber is 300c

Anti-skid System

• Large with power brakes and protects tyres/efficiency. Tyre and ground contact is important.

Mechanical Hydraulic Anti-skid System

• Mechanical/hydraulic systems mechanically connected the wheel by hydraulic valve controlled controlled by energy stored small flywheel. By measuring the relative speed of two spinning disks causes the braking releases to reapply.

Electrohydraulic Anti-skid

• Releases to help keep it skidding below. The flight switches activates where it functions 30
• The components include the wheel sensor with anti skid control values and a control unit.
• For operation the anti skid, the position set. Braked wheel reference ref supplied

Non skid

• A pilot via the anti skids at which to apply the break at the area where it provides the no skid contition by the pilot.

Operation Skid Control

• Operation releases a brake by skid control to re apply pressure
• Operates at the applied pressure

Anti skid Valve With Parking Applied

• When electrical is running continues bleed

For anti-skid and hydroplaning protection

• For anti0skid BSCU relays with with brake at 30 kn

For electromechanical braking system

• Are responsible of r braking through including anti-skid with electric systems.

Locked Wheel Protection

• Recognizes its not rotating in a signal. On take off the system is off shutting wheel to brake for the stored

Touchdown and Locked Wheel Protection

• Most aircraft have touchdown the the detector circuitry closes during build of and compression where it ensures the braking is there. The brakes are not needed until contact with running and could blow tyres. The antiskid opens system shuts to stop build until safety closes.

Anti Skid contol

• Ensures the system has input

Auto-braking

Activation

• The auto-brake system can stop the aircraft at the selected deceleration lever without the pilots activating

Switch

• With which is a selected value to a constant one and not be exceeded.

Stages of Auto- Braking

• The "On ramp" stage shows a smooth build-up of brake pressure until the preselected deceleration rate is reached.

• The “off ramp” stage shows a smooth decrease in brake pressure.

Auto braking system

• Smooth until system switchs and not over done

Components

Pilot

• Selects the deceleration rate.

On

• Where deceleration is less compared selected increase the deceleration rate.

Deactivation

• At the application of it will be system as it disconnects if there, including main hydruali c and an e fault

Auto Breaking

• The auto functions regulated electrical of by from panels which by pedal system.

With system braking thrust

• The predetermines on

• Tyres

Function

The design for the role

• Characteristics

There classified by tubed or

• Types radial with identification There are seven tyres for modern general speed on.

The number width it will use.

The first width from use

Rating

rating the is where strength still has significance.

Ply

the capacity to This of.

Tube Versus

Benefits And Characteristics

Where tubed has puncture the air the of is The there are components of with long Type or The is 60 by Radial one with or fabric

• Is the with
• Strength
• To

The steel the the on

Piles

The

Fabric Strength

The the the of of.

tread

The the:

The

Liner

The. are Those Gas in