Page 1: Aircraft Structures and Landing Gears - Landing Gears

• Landing gear supports the aircraft during landing and while it is on the ground

• Simple aircraft have fixed gear, while complex aircraft have retractable landing gear

• Landing gear must be strong and light

• Two basic configurations of airplane landing gear: Conventional gear and Tricycle gear

Conventional gear or Tailwheel Gear

• Dominated early aviation

• Two main wheels under the weight of the aircraft

• Smaller wheel located at the aft end of the fuselage

• Tailwheel can be steered when the tail is off the ground

• Stable at higher speeds

• Offers increased clearance for non-paved runways

Tricycle Gear

• Most prevalent landing gear configuration

• Main wheels and a shock-absorbing nose wheel

• Center of gravity is forward of the main wheels

• Benefits:

  • Allows more forceful application of brakes without nosing over

  • Provides better visibility from the flight deck

  • Prevents ground-looping of the aircraft

Page 5: Landing Gears Nomenclature

• NLG DOORS: Lock collar can be installed on each door actuator to prevent closure

• NGL bay: Four doors and a fixed fairing close the NGL bay

• MGL DOORS: Three doors close each MGL bay and protect the equipment

• Lock Stay Assembly, Downlock Actuator, Retraction Actuator, Side Stay Assembly, Integrated Drag Stay, Pitch Trimmer Actuator, Leg Assembly/Main Fitting, Bogie Beam Assembly, Sliding Tube, Right Landing Gear/Main Landing Gear (MLG), Side Retraction Stays Actuator, Lock Stays, Lock Stay Actuators, Hinged Downlock, Oleo-Pneumatic Door Springs, Shock Absorber, Fixed Hydraulic Fairing, Pitch Trimmer Door, Door Actuator, Bogie Beam Assembly, Main Door

Page 8: Aircraft Landing Gears - Introduction

• Some early aircraft designs use a skid instead of a tail wheel

• Tail wheel-type landing gear offers increased clearance for non-paved runways

• Conventional gear is used for weight savings

Page 9: Landing Gear Arrangement

• Three basic arrangements of landing gear: tail wheel-type, tandem, and tricycle-type

• Tail wheel-type landing gear has a third wheel for support

• Tandem landing gear aligns the main gear and tail gear on the longitudinal axis of the aircraft

• Tricycle-type landing gear is the most commonly used arrangement

Page 11: Tricycle-Type Landing Gear

• Comprised of main gear and nose gear

• Benefits of tricycle-type landing gear:

  • Allows more forceful application of brakes without nosing over

  • Provides better visibility from the flight deck

  • Prevents ground-looping of the aircraft

• Nose gear can be steerable or castering

• Control is achieved through mechanical linkage or hydraulic power

Page 13:

• Landing gear assemblies can have multiple wheels attached to a bogie

  • Number of wheels in the bogie depends on the gross design weight

• Tricycle-type landing gear can be attached to reinforced wing structure or fuselage structure

• Main gear can have two or more wheels

  • Multiple wheels spread the weight of the aircraft over a larger area

  • Provides safety

Page 14:

• Tricycle-type landing gear consists of various parts and assemblies

  • Air/oil shock struts, gear alignment units, support units, etc.

• Landing gear can be classified as fixed or retractable

• Fixed landing gear is attached to the airframe and remains exposed to the slipstream

• Retractable landing gear stows in fuselage or wing compartments while in flight

  • Close-fitting panel or separate doors are used to fair the gear with the aircraft skin

Page 15:

• Landing gear must control the forces of impact during landing

• Shock energy is altered and transferred throughout the airframe

• Shock can be absorbed by converting the energy into heat energy

• Retractable landing gear does not cause parasite drag when stowed

• Retractable gear can have close-fitting panels or separate doors for fairing

Page 16:

• Landing gear must absorb the forces of impact during landing

• Shock energy is altered and transferred throughout the airframe

• Shock can be absorbed by converting the energy into heat energy

Page 17:

• Leaf-type spring gear utilizes flexible spring steel, aluminum, or composite struts

• Gear flexes initially and forces are transferred as it returns to its original position

• Rigid landing gear struts directly transfer shock load to the airframe

• Pneumatic tires aid in softening the impact loads

• Bungee cords and rubber cushions are used on non-shock absorbing landing gear

Page 18:

• Bungee cords are positioned between the rigid airframe structure and the flexing gear assembly

• Bungees take up the loads and return them to the airframe at a non-damaging rate

• Shock struts are self-contained hydraulic units that support the aircraft and protect the structure during landing

• Shock struts use compressed air or nitrogen combined with hydraulic fluid to absorb and dissipate shock loads

• Struts have two telescoping cylinders, with the lower cylinder called the piston

• Orifice and metering pin control the rate of fluid flow in the strut

Page 19:

• Heat energy is converted from the pressure build-up caused by strut compression and hydraulic fluid flow

• Metering pin or metering tube controls the flow of fluid from the bottom chamber to the top chamber during compression

• Damping or snubbing devices prevent sharp impacts at the end of the stroke

• Recoil valve or recoil tube restricts the flow of fluid during the extension stroke to prevent damaging impact forces.

Page 21

• Most shock struts have an axle as part of the lower cylinder for installing aircraft wheels

• Shock struts without an integral axle have provisions on the end of the lower cylinder for installing the axle assembly

• Suitable connections are provided on all shock strut upper cylinders to attach the strut to the airframe

Page 22

• The upper cylinder of a shock strut contains a valve fitting assembly for filling the strut with hydraulic fluid and inflating it with air or nitrogen

• A packing gland is used to seal the sliding joint between the upper and lower telescoping cylinders

• A packing gland wiper ring is installed to keep dirt, mud, ice, and snow from entering the packing gland and upper cylinder

• Regular cleaning of the exposed portion of the strut piston helps the wiper ring do its job and prevents damage to the packing gland

Page 23

• Most shock struts are equipped with torque links or torque arms to keep the piston and wheels aligned

• Torque links are attached to the fixed upper cylinder and the lower cylinder to prevent rotation and maintain wheel alignment

• Torque links also retain the piston in the end of the upper cylinder when the strut is extended

• Nose gear shock struts have a locating cam assembly to keep the gear aligned and prevent structural damage to the aircraft

• The cam assembly aligns the wheels with the longitudinal axis of the aircraft prior to landing

Page 24

• Many nose gear shock struts have attachments for an external shimmy damper

• Nose gear struts have a locking or disconnect pin for quick turning of the aircraft while towing or positioning

• Nose and main gear shock struts have jacking points and towing lugs for maintenance and towing purposes

• Shock struts contain an instruction plate near the filler inlet and air valve assembly, specifying the correct type of hydraulic fluid and pressure for inflation

Page 25

• Landing gear maintenance includes checking emergency control handles and systems, inspecting wheels for cleanliness and damage, checking tire condition and inflation, inspecting landing gear mechanism and steering system, and inspecting shock struts for cracks, corrosion, and breaks

• Lubrication of landing gear points of friction and wear is necessary using specific products recommended by the manufacturer

• Grease should be applied to clean fittings, excess grease should be wiped off, and piston rods should be kept clean

Page 26

• Wheel bearings need periodic removal, cleaning, inspection, and lubrication

• Recommended cleaning solvent should be used for wheel bearing cleaning, and dry air should be used to dry the bearing

• When inspecting the bearing, check for defects that render it unserviceable and replace if necessary

• Tapered roller bearings should be lubricated using a bearing lubrication tool or by packing grease between the rollers and cone

• Landing gear rigging and adjustment may be necessary for proper operation of the landing gear system and doors

• Overtravel must be checked when replacing actuating cylinders or making length adjustments

Page 27

Adjusting Landing Gear Latches

• Latches are used in landing gear systems to hold the gear up or down and/or to hold the gear doors open or closed.

• All latches must operate automatically at the proper time and hold the unit in the desired position.

• Example of a landing gear door latch on a particular aircraft:

  • The door is held closed by two latches.

  • Both latches must grip and hold the door tightly against the aircraft structure.

  • Components of each latch mechanism: hydraulic latch cylinder, latch hook, spring-loaded crank-and-lever linkage with sector, and latch hook.

  • Clearances and dimensional measurements of rollers, shafts, bushings, pins, bolts, etc., are common.

Additional Adjustments

• Rigging and adjustment procedures vary from aircraft to aircraft.

• Uplock and downlock clearances, linkage adjustments, limit switch adjustments, and other adjustments must be confirmed by the technician in the manufacturer's maintenance data before taking action.

• Examples of various adjustments are given to convey concepts, not actual procedures for any particular aircraft.

Page 28

Landing Gear Retraction Test

• Landing gear system and components can be checked by performing a landing gear retraction test.

• Also known as swinging the gear.

• The aircraft is properly supported on jacks for this check.

• The landing gear should be cleaned and lubricated if needed.

• The gear is raised and lowered as though the aircraft were in flight while a close visual inspection is performed.

• All parts of the system should be observed for security and proper operation.

• The emergency back-up extension system should be checked whenever swinging the gear.

• Retraction tests are performed during annual inspection, after replacing landing gear components that could affect system functioning, after hard or overweight landings, and while attempting to locate a malfunction within the system.

• Consult the manufacturer's maintenance manual for the specific inspection points and procedures for each landing gear system.

General Inspection Items while Swinging the Gear

• Check the landing gear for proper extension and retraction.

• Check all switches, lights, and warning devices for proper operation.

• Check the landing gear doors for clearance and freedom from binding.

• Check landing gear linkage for proper operation, adjustment, and general condition.

• Check the alternate/emergency extension or retraction systems for proper operation.

• Investigate any unusual sounds caused by rubbing, binding, chafing, or vibration.