AFS Topic 3 MOD

Air-Oil Struts (Oleo Struts)

• Widely used in non-retractable and retractable undercarriage systems
• Operate on the principle of restricting oil displacement and air compression to cushion landing impacts and carry aircraft weight
• Two main types: metering pin and air-oil separated

Metering Pin Shock Strut

• Consists of a cylinder and piston tube assembly
• Contains hydraulic fluid and nitrogen
• Cylinder assembly is constructed of high tensile steel and is divided into upper and lower cylinders

Cylinder Assembly

• Upper cylinder contains a NATO inflation valve for inflating the oleo strut with nitrogen
• Snubber tube sits in the cylinder and has a flutter plate containing a small hole and a return leak hole in the lower end

Shimmy Damper

• Directly attached to both the lower (piston) and upper (cylinder) of the nose landing gear
• Slows oscillating forces produced from the wheel assembly through the restriction of fluid flow within the shimmy damper
• Types: vane type and piston type

Damper Inspections

• Check for leakage and effectiveness of operation
• Check fluid quantity in replenishment reservoir and add fluid of specified type if necessary
• Inspect mount bolts and fittings for wear

Ground Locks

• Devices that prevent collapse of the gear when the aircraft is on the ground
• Types: pin installed in aligned holes in two or more units of the landing gear support structure and distance tube designed to fit around extension/retraction actuator pistons

General Precautions

• Fit ground locks whenever the aircraft is released for maintenance
• Trip circuit breakers or remove fuses when working on the system
• Perform a retraction test after replacing or adjusting components in the retraction system
• Never remove components while the system is under pressure

Segmented Brake Assembly

• Rotor has multiple blocks riveted to both sides
• Provides more brake cooling due to the gap between rotor segments and space between brake lining sections

Brake Construction Materials

• Brake housings: aluminium or magnesium alloys
• Brake discs: high strength steel or carbon materials
• Organic composition linings: good friction, wear, and thermal characteristics
• Brass or copper particles may be embedded in the material to provide friction characteristics

Automatic Adjusters

• On single disc brakes, a retraction pin is often attached directly to the operating piston
• Amount of pin left visible indicates how much the linings are worn on the brake assembly

GenFly Brake System Emergency Operation

• In the event of No 1 system failure, the spool valve in the priority change over valve moves to connect the No 2 Emergency Brakes brake control valve to the brake units

Park Brakes

• Sub-assembly of the normal hydraulic brake system
• Always consider brake assembly temperature before setting the park brake
• Hot brakes should be allowed to cool before the park brake is applied

Classification of Landing Gear

• There are two basic classifications of landing gear: fixed or retractable.
• The arrangement of landing gear varies greatly under these classifications and can be tricycle, tailwheel, bicycle, skids, or floats.

Non-Shock Absorbing Landing Gear

• Non-shock absorbing landing gear dissipates the energy of the aircraft contacting the ground during landing by altering and transferring the shock energy throughout the airframe at a different rate.
• Types of non-shock absorbing landing gear include rigid, shock cord, and spring type gear.
• Rigid landing gear is commonly found on helicopters and sailplanes and is rigidly mounted to the aircraft with no specific component to cushion the ground contact.

Shock Absorbing Landing Gear

• Shock absorber operation involves the transmission of impact received by the wheel on landing to the shock absorber piston rod, generating a positive pressure in the chamber over the diaphragm and a lower pressure in the lower chamber.
• The difference in pressure produces the displacement of the piston, which opens the gate, and a part of the impact energy is absorbed during this first shock phase.
• During the bounce phase, the pressure direction is reversed, and the piston in the diaphragm tends to close the gate, restricting the flow of fluid from one chamber to the other, thus dissipating the largest part of the energy accumulated by the landing.

Maintenance

• Oleo pneumatic undercarriages should be subjected to regular inspections, including examinations for cracks or damage to mounting structure, corrosion, and wear at pivot points.

Components

• Landing gear assemblies are made up of various components designed to support and stabilize the assembly, including actuators, trunnions, drag, torque, and side links, shock struts/oleos, and fairings.
• Different manufacturers may use different terms for the same basic components.
• New components should be examined and cross-checked with the appropriate maintenance publication to ensure correct installation.

Wheel Assemblies

• Wheel assemblies are made up of two major assemblies: tyre and hub.
• Aircraft tyres provide a cushion of air that helps absorb shocks and roughness of landings and take-offs, support the weight of the aircraft while on the ground, and provide necessary traction for braking and stopping.

Aircraft Wheel Brakes

• The primary purpose of an aircraft wheel brake is to stop the aircraft within a specified distance.
• Braking involves converting the aircraft’s kinetic energy into heat energy by friction.
• Rapid cooling is of prime importance, as brakes will lose efficiency if the heat is not dissipated quickly.
• Types of aircraft brake systems include drum, single disc, multiple disc, and segmented rotor disc.
• Drum brakes are of the duo-servo type, using the weight of the aircraft to aid in the application of the brake.