Aircraft Landing Gear Functions

• Landing gear systems use hydraulic and pneumatic power to function.

Landing Gear System Scope

• Describe different aircraft landing gear configurations
• Describe the operating principle of the main and nose landing gear
• Describe the functions of landing gear components: struts, torque links, drag links, side struts, shimmy dampers, axles, wheels, and tires
• Describe the construction and operation of the shock-absorbing element
• Describe the aircraft steering
• Describe normal and emergency extension/retraction
• Describe safety devices, indication, and warning systems

Main Landing Gears

• Provide primary aircraft support by absorbing large download forces during ground operations
• Brakes installed on main wheels enable the aircraft to slow down or stop
• The number of landing gears, wheels, and brakes depends on the aircraft's weight and load

Nose Landing Gears

• Support aircraft weight and load, equipped with a steering mechanism for ground manoeuvring

Landing Gear Arrangement Types

• Tail or conventional configuration
• Tandem configuration
• Tricycle configuration

Tail Wheel or Conventional Configuration

• Used on older aircraft for landing on rough field operations

Tricycle Configuration

• Used over conventional landing gear aircraft
• Allows more forceful brake application, preventing nosing over and enabling higher landing speeds
• Improves visibility from the flight deck during landing and ground maneuvering
• Prevents ground-looping by keeping the aircraft moving forward due to the center of gravity placement

Aircraft Wheels

• Support the aircraft during taxi, takeoff, and landing
• Typically lightweight, strong, and made of aluminum alloy

Aircraft Tyres

• Support the aircraft's weight, absorb shock from landing and taxying, provide runway grip, and discharge static electricity

Trunnion

• Part of the landing gear assembly attached to the airframe
• Supported by bearing assemblies, allowing the gear to pivot during retraction and extension

Strut

• The vertical member of the landing gear assembly

Drag Link or Drag Strut

• Provides support to the shock strut and stabilizes it longitudinally

Side Strut or Side Brace Link

• Stabilizes the landing gear laterally

Overcenter Link

• Prevents the link from pivoting at the joint when the gear isnt retracted, thus preventing gear collapse during ground operation
• This locks the main gear only in the down position
• It's sometimes called a "Downlock," and it is hydraulically retracted for gear retraction

Lock Mechanism

• A "downlock" locks the landing gear in the down position
• The main landing gear is held in the up position by the uplock mechanism

Axles

• The main wheels are supported and installed on the axles

Shock Absorbing and Non-Shock Absorbing Landing Gear

• Absorbs impact forces during taxiing and landing
• Shock energy is altered and transferred throughout the airframe
• Shock is absorbed by converting energy into heat
• Some aircraft use flexible spring steel, aluminum, or composite struts
• These struts receive landing impact and return it to the airframe at a non-harmful rate

Shock Absorbed Pneumatic/Hydraulic Strut

• Uses nitrogen gas with hydraulic fluid to absorb and dissipate shock loads
• Constructed with two telescoping cylinders or tubes, closed on the external ends
• The upper cylinder is fixed; lower cylinder (piston) slides in and out
• Two chambers are formed: the lower always filled with hydraulic fluid, the upper with nitrogen
• An orifice between cylinders allows fluid passage from the bottom to top chamber when the strut compresses

Shock Strut Operation

• The compression stroke begins when the aircraft wheels touch down
• As the center of mass moves downward compresses the strut, forcing the lower cylinder/piston into the upper cylinder
• The metering pin moves up through the orifice
• This causes the gas volume to decrease, increasing the pressure while the hydraulic fluid volume remains constant
• The initial landing shock is cushioned by hydraulic fluid forced through the metered opening
• As temperature and pressure increase in the cylinder, vertical speed decreases
• When cylinder pressure is sufficient to stop vertical motion, gas pressure energy recoils the aircraft upwards
• During recoil, the strut extends until gas pressure supports aircraft weight, and compressed air acts as a shock absorber during taxiing

Fixed vs Retractable Landing Gear

• Aircraft with fixed landing gear expose gears to airflow, increasing drag as speed increases
• Retracting mechanisms reduce drag but add weight
• Aircraft commonly use retractable gears; the reduced drag is worth the added mass

Retraction and Extension of Main Landing Gear

• Main gear extends/retracts via a handle in the flight deck, mechanically connected to the selector valve
• Aircrew/ground crew sets handle to “UP”, “OFF”, or "DOWN"
• Setting gear to “UP”: circuit supplies hydraulic pressure to unlock/open wheel well doors unlocks landing gears via downlock actuator, retracts gears, closes wheel well doors
• Gears are kept in “UP” position via an up-lock mechanism
• Setting gear to “OFF”, "UP" components and "DOWN" components are connected to the hydraulic return line, gears remain in “UP” position via up-lock

Putting Landing gear handle to “DOWN”

• Pressure from the hydraulic system is released via an internal circuit in the selector valve which is used for the wheels to unlock and opening the well doors, unlocking the up-lock, extending the landing gear, and ultimately closing the well doors

Landing Gear Force Analysis

• Maximum retractor actuator force: 53,000N, actuator stroke: 700mm
• Gear retracted in 10 seconds
• Two main landing gears, one identical nose gear
• The maximum system pressure is 207 bar
• The Cap end diameter: 500mm, piston rod diameter 300mm
• The Power is at 640KW

Downlock Mechanism

• Prevents undesired gear retraction in the “DOWN” position
• Overcenter links (between strut and side brace) prevent side brace pivoting if in the “overcenter” position
• Springs force the overcenter link to the remain in the overcentre position
• Maintenance personnel lock overcenter mechanism to ensure the landing safety
• Retracting the landing gear involves the downlock actuator pulling the overcenter links from the “overcenter” position
• After that the actuator side brace can pivot when the landing gear gets pulled up by the retract cylinder

Uplock Mechanism

• Hook secures landing gear in retracted position
• When unlocked, the gear extends due to mass and bungee springs to obtain the 'down' then locked position
• Hydraulic fluid slows retraction actuator, reducing "down" forces

Nose Landing Gear Steering System

• Steered using the nose wheel steering system and/or differential braking during taxiing
• Flight deck controls via a small wheel, tiller, or joystick (usually left side wall),
• The aircraft hydraulic the pressure that flows through the open safety which shuts the valve the fluid goes through the line into the metering valve
• The metering valve routes pressurized fluid comes out of port A goes through the right, the line goes into steering cylinder A
• The One-port cylinder and pressure will force the piston begins extension, then the rod will pivot at point Y, the extension will turn the steering gradually towards the right Fluid is pumped to a port for the aircraft to steer

Nose Landing Gear Alignment

• Most shock struts use torque links or arms
• One end and one of the end that have connected to the cylinder will remain the wheels aligned
• Aligns wheels with the longitudinal aircraft axis before landing
• Nose gear shock struts have aligning cam assembly
• A cam protrusion on the lower cylinder with a mating lower cam recess on the upper cylinder
• Cams align the wheel and axle assembly in a straight-ahead position when the shock strut is extended, that allows the nose wheel for an easier entering the well without structural damage
• Many nose gear struts have attachments for an external shimmy damper

Nose Landing Gear Steering System

• Pressure from the aircraft hydraulic system passes through an open safety shutoff valve to a line to the metering valve
• The metering valve routes the pressured fluid out through point A, passes this to right turn alternating line and steering cylinder A
• If pressure forces the piston to begin extending that enables aircraft to the turn, the rod that connects the spindle to the lower the piston gradually steers towards the right
• As the wheel turns, cylinders are pumped through a alternating line, metering valve directs a compensator that routes to the hydraulic return line

Shimmy Dampers

• Due to the oscillating force of the links, the oscillations causes a significant force that require dampening
• The damp cylinder will shimmy through hydraulic dampening that are caused by the upper strut cylinder
• Then the shaft is attached to the lower shock strut as they attempt to shimmy, it's forcing the air through a tiny hole
• Then the process of forcing compressed air is limited, causes the dampening effect

Emergency Extension Systems

• Emergency systems lower the landing gear if the main system fails
• Some planes use an emergency release handle in the flight deck that mechanically releases the gear uplocks
• When released, the gear free-falls to the extended position via gravity
• Some aircraft may implement the form of non-mechanical backup from pneumatic release

Safety Switch (Landing Gear Safety Devices)

• A landing gear squat switch, or safety switch, is found on most aircraft
• This switch can be either open/close, and it's mainly depending on the extension or compression of the gearstrut.
• Squat switch is wired into several system operating circuits, that will prevent gear from retracting when on ground
• At takeoff, the gear strut extends which causes the switch will close circuit, a signal energizes retracts solenoid, allowing gear raised

Ground Locks Safety Device

• Most aircraft have additional safety devices to prevent gear collapse on the ground
• Ground locks may implement pre drilled holes where gears are placed to avoid a form of gear collapsing
• Have streamers must available so visible during flight
• Usually flight carrys after walks around

Gear Indicator Safety Device

• Typically has micro switches, that provides feedback if the gearlocks are connected
• The gauges indicate the different for the pilot or ground member
• Green lights indicate the gears and they are in are in a locked position, red means are in transit
• There will be no light if they are up

Warning Horn

• Horn sounds to the aircrew and the landing gears are not downlocked to the ground.
• A very great important to hear so that any safety implications are heard.

This quiz tests understanding of aircraft landing gear systems, focusing on the functions of tires, trunnions, drag links, and overcenter links. It covers roles of the uplock mechanism, components, material composition, and impact force dissipation. Also tests knowledge about the pneumatic/hydraulic shock strut.