Aircraft Landing Gear Systems

• Landing gear configuration involves stating and describing options for aircraft.
• Main and nose landing gear operating principles involve describing the functions of parts.
• Landing gear component functions include struts, torque/drag/side links, shimmy dampers, axles, wheels, tires.
• Construction and operation of the shock-absorbing element in the landing gear involves the construction and operation of key components.
• Aircraft steering involves steering the aircraft.
• Normal and emergency landing gear extension/retraction systems involve the normal and emergency landing gear extension/retraction systems.
• Landing gear safety devices, indication, and warning systems involve safety features.

Main Landing Gears

• Provides primary aircraft support and absorbs download forces during ground operations like landing and taxiing
• Brakes installed on main wheels enable the aircraft to decelerate or stop.
• The number of landing gears, wheels, and brakes is based on the aircraft's weight and load capacity.

Nose Landing Gears

• Supports aircraft weight and load.
• Equipped with a steering mechanism for ground maneuvering.

Types of Landing Gear Arrangement

• Tail or conventional configuration.

• Tandem configuration.

• Tricycle configuration.

• Older aircraft use conventional configuration for rough field operations.

• The tricycle-type landing gear is common with various benefits.

• Allows forceful brake application without nosing over, enabling higher landing speeds.

• Better visibility from the flight deck during landing and ground maneuvering.

• Prevents ground-looping due to the aircraft's center of gravity being ahead of the main gear.

Sub-components of the Main and Nose Landing Gear

Aircraft Wheels

• Import components of the landing gear.
• Support entire aircraft weight during taxi, takeoff, and landing while tires are mounted.
• Typically lightweight, strong, and made of aluminum alloy.

Aircraft Tyres

• Support weight, absorb shock during landing/taxiing, provide runway grip and discharge static electricity.

Trunnion

• Part of landing gear assembly attached to the airframe.
• Supported at its ends by bearing assemblies.
• Allows gear to pivot during retraction and extension.

Strut

• Strut is the vertical member of the landing gear assembly.

Drag Link or Drag Strut

• Provides support to shock strut and stabilizes it longitudinally.

Side Strut or Side Brace Link

• Stabilizes the landing gear laterally.

Overcenter Link

• Prevents link pivoting at the joint, avoiding gear collapse during ground operation.
• Locks main gear in down position.
• Sometimes called "Downlock."
• 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

• Main wheels are supported and installed on it.

Shock Absorbing and Non-Shock Absorbing Landing Gear

• To support the aircraft for taxi, the forces of impact on an aircraft during landing must absorbed by the gears.
• Impact is absorbed in two ways:
• Shock energy is altered, transferred throughout the airframe at a different rate/time than the one strong pulse of impact.
• The shock absorbed by converting energy into heat energy.
• Many aircraft use flexible spring steel, aluminum, or composite struts to receive landing impact.
• Struts return the gear to the airframe to dissipate energy at a non-harmful rate.

Shock Absorbing Elements

• A pneumatic/hydraulic shock strut uses nitrogen gas with hydraulic fluid to absorb and dissipate shock loads.
• The shock strut consists of two telescoping tubes that are closed on the external ends.
• The upper cylinder is fixed to the aircraft and does not move.
• The lower cylinder (piston) slides in/out of the upper cylinder, forming two chambers.
• The lower chamber is filled with hydraulic fluid, and the upper chamber is filled with nitrogen.
• An orifice is located between the two cylinders allowing fluid to flow from one cylinder to the other when the strut is compressed.

Shock Strut Operation

• Compression stroke starts, and aircraft wheels touch the ground.
• As the center mass moves downwards, the strut compresses, and the piston is forced upwards into the upper cylinder.
• Metering pin moves up via the orifice.
• This causes the gas volume to decrease, which increases pressure but the hydraulic fluid volumes remains.
• The initial landing shock is cushioned by the hydraulic fluid being forced through the metered opening.
• As pressure and temperature increase in the cylinder, the vertical speed of the aircraft decreases.
• Cylinder pressure increases until it stops vertical motion.
• The gas energy pressure recoils upward.
• During recoil, the shock strut extends until gas pressure supports aircraft weight.
• Compressed air acts as a shock absorber while the aircraft is taxiing.

Fixed and Retractable Landing Gear

• Aircraft with fixed landing gear have gears exposed to airflow while flying, increasing drag as speed rises.
• Retracting mechanisms reduce drag but add weight.
• Airplanes fit retractable gears since the added weight is a minimal sacrifice.

Retraction and Extension of Main Landing Gear

• Main gear extends/retracts via flight deck handle.

• The handle connects mechanically to the selector valve.

• Set to "UP", "OFF" (neutral), or "DOWN" position by crew.

• In "UP" position, the internal circuit in the selector valve supplies hydraulic pressure to:

  • Unlock and open wheel doors via the unlatch and door actuator.
  • Unlock landing gears via downlock actuator.
  • Retract landing gears via retract actuators.
  • Close wheel doors via door and unlatch actuators.
  • Landing gears kept "UP” by a lock mechanism.

• In the “OFF” position, all components are connected to return line of hydraulic system, so, landing gears are kept "UP" by an up-lock mechanism.

• When the handle is set the “DOWN” position, pressure from hydraulic system released by internal circuit in selector valve.

  • Unlock and open wheel doors.
  • Unlock up-lock.
  • Extend the landing gear.
  • Close the wheel well doors

Calculation to retract landing gear

• Maximum force exerted by the retract actuator is 53,000N
• Actuator stroke is 700mm
• Gear must be fully retracted 10 seconds
• Two main gears and the same nose gear
• Maximum system pressure is 207 bar
• Cap end diameter is 500mm
• Piston rod diameter is 300mm
• The formula used is therefore:
  • Volume required to retract 3 actuators = [3 x (π/4 x (0.5)^2 - π/4 x (0.3)^2) x 0.7] = 0.263 m³
  • Flow rate for retract 3 acutators = 0.263 / 10 = 0.0263 m^3/s
  • power required = PQ/η = 0.0263 X 207X10^5 / 0.85 = 640KW

Downlock Mechanism

• Prevents un-wanted landing gear retraction in the DOWN position.
• "Overcenter links" between the strut side brace ensure it cannot pivot when in "overcenter" position.
• The overcenter link remain in "overcenter" position by "bungee springs".
• The overcenter mechanism is locked by landing gear lock pins for safety during ground maintenance.
• When landing gear retracts, the downlock actuator pulls the overcenter links from "overcenter" and side brace pivots as the gear is pulled up via retraction cylinder.

Uplock Mechanism

• Includes a hook securing the landing gear while retracted.
• When unlocked, landing gear extends via mass and bungee springs to the 'down and locked' position.
• Hydraulic fluid escaping the retraction actuator slows process to reduce "down" shock.

Nose Landing Gear Steering System

• Steered during aircraft taxiing using nose wheel steering and/or differential braking.
• Flight deck control with a small wheel, tiller, or joystick (typically left-side wall mounted).

Nose Landing Gear Alignment

• Shock struts have links or arms with one end attached to the upper cylinder and the other to the lower cylinder to prevent rotation and maintain wheel alignment。
• Provides locating cam assembly to maintain alignment.
• A cam protrusion is attached to the lower, and a recess at the upper.
• The wheel and axle line up when the shock strut is fully extended.
• This allows the nose wheel align into the wheel well during retraction.

Nose Landing Gear Steering System

• Directs pressure from the hydraulic system through a safety shutoff valve to the metering valve.
• The metering valve routes fluid through alternating lines.
• One port cylinder pressure extends piston.
• The piston rod connects to the nose steering spindle on the nose shock strut, which pivots at a point to gradually turn the steering spindle right.

Nose Landing Gear Shimmy Dampers

• Torque links connecting the upper cylinder a lower piston on a nose strut are usually not enough to prevent oscillations, or shimmy.
• A shimmy damper controls nose wheel shimmy through hydraulic damping.
• A piston-type shimmy damper attaches to the upper cylinder.
• The shaft attaches to the lower cylinder and piston inside the damper.
• As the lower strut cylinder attempts to shimmy, the damper controls hydraulic fluid forced through a bleed hole in the piston, restricting the flow through the bleed hold, causing the effect.

Emergency Extension Systems

• Lowers landing gear if the main power system fails.
• Some aircraft have a flight deck emergency release handle connected to gear uplocks via mechanical linkage.
• Handle operation releases uplocks. The gear free-falls to extended position via gravity.
• Some aircraft use non-mechanical backups (ex. pneumatic power) to unlatch the gear.

Safety Switch (Landing Gear Safety Device)

• The landing gear squat or safety switch is positioned to open or close.
• Squat switch wired into system operation
  • Prevents gear retraction on ground.
• On Takeoff, strut extends.
  • Switch closes, activating safety circuit
  • Solenoid retracts lock-pin (selector handle).
  • The gear can be raised.

Ground Locks (Landing Gear Safety Device)

• Most aircraft have gear collapse prevention devices while on the ground.
• Ground lock can be a pin placed in pre-drilled gear component holes.
• All ground locks have red streamers attached, so they are visible and also removed before flight.
• Ground locks are carried in the aircraft and fitted by flight crew.

Gear Indicator (Landing Gear Safety Device)

• Main components of the landing gear indicator are micro switches or proximity switches on the up and down locks.
• Connected to landing gear signal on the instrument panel
• Green light indicates that gear is in locked position.
• Red indicates that it is in transit.
• If all gears are up and locked, there are no lights.

Warning Horn (Landing Gear Safety Device)

• Sounds if landing gear is not down and locked.
• Alerts crew to not land with retracted gear.
• In the event of any incident, horn will sound.

Explores landing gear configurations and operating principles. Covers component functions like struts and steering. Details normal and emergency extension/retraction.