AFS3 DIFF combined

Landing Gear Systems

• Landing gear systems are classified into tricycle, tailwheel, bicycle, skids, or floats.
• Non-shock absorbing landing gear relies on pilot skill for a soft ground contact, examples include helicopter landing gear skid and glider landing gear.
• Shock absorbing landing gear absorbs impact forces of landing by converting energy into heat energy, often by forcing a fluid through a restriction.

Non-Shock Absorbing Landing Gear

• Non-shock absorbing landing gear relies on the tyre to provide some cushioning effect.
• An example of non-shock absorbing landing gear is the landing gear skid of a helicopter.

Shock Absorbing Landing Gear

• Shock absorbing landing gear absorbs impact forces of landing by converting energy into heat energy.
• The energy is generated from landing and taxiing and is dissipated into the surrounding atmosphere.
• The movement of fluid generates heat, which is radiated into the surrounding atmosphere.

Components of Shock Absorbing Landing Gear

• Upper cylinder
• Wer cylinder
• NATO inflation valve
• Flutter plate
• Snubber tube
• Return leak hole
• Piston stop
• Gland packings O-ring
• Brass bearing
• Gland nut

Piston Tube Assembly

• The piston tube assembly is constructed of high tensile steel.
• It comprises of a piston head, metering pin, axle housing, and torque links.
• The piston tube assembly is shown in Figure 6.

Brakes Applied

• The two reservoirs supply fluid to each individual dual cylinder master cylinders.
• When the toe pedals are depressed, the master cylinders supply servo pressure to the brake control valves of both systems.
• Pressure from No. 1 system is metered through the Normal Brakes brake control valve, anti-skid control valve, and priority change over valve to the wheel brake cylinders.
• The pressure applied to the brakes is proportional to the toe pedal depression.

Power Brake Bleeding

• It is important to follow the applicable maintenance manual in detail when power brakes are bled.
• The bleeding procedure is not complete until both the main and emergency systems are free of air.

Fluid Quantity and Type

• The hydraulic system of a modern aircraft may use phosphate-ester-base synthetic fluid (Skydrol), mineral-base MIL-H-5606, or synthetic MIL-H-83282A fluid.

Aircraft Steering

• Steering systems are used to control the direction of movement of an aircraft while taxiing.
• Most aircraft are equipped with some form of steering.

Mechanical Steering

• Mechanical steering systems are found on small aircraft.
• The system uses bungee rods connected to the rudder pedals to control the direction of movement.

Rudder Steering

• Some small aircraft have alternate steering methods that utilize cables, springs, and levers associated with the rudder system or the brakes to effect steering.
• Rudder deflection causes the tail wheel to steer as the aircraft rolls forward.

Pneumatic Steering

• The system layout was previously covered in Topic 1, except there are no return lines and are vented overboard at the steering metering valve.
• An electrically operated shutoff valve is normally incorporated in the system to isolate steering when gear up is selected or steering is selected off.

Brake Steering

• Brakes can be used for steering on tail wheel and tricycle (front wheel) configured aircraft.
• This type of steering is also called differential braking.
• Depressing the left brake pedal causes the left brake to be applied, which causes the tail wheel to deflect to the left as the aircraft's tail deflects to the right, resulting in a turn to the left.

Extension and Retraction Methods

• Most aircraft undercarriage assemblies use a combination of hydraulic and electrical power for extension and retraction.
• Emergency extension can be achieved by back-up hydraulic systems, pneumatics, mechanical crank systems, or gravity systems.

Electrical Undercarriage Systems

• The system enables the landing gear to be extended and retracted by one motor.
• The 28VDC supply to the motor is controlled by the landing gear control lever, landing gear up and down relays, and up-lock and down-lock position micro-switches.
• Each undercarriage leg has its own set of up-lock and down-lock micro-switches.
• All three up-lock micro-switches are connected in parallel to each other, as are the down-lock micro-switches.
• A ground safety micro-switch is used to prevent accidental retraction of the landing gear while the aircraft is on the ground.

Retraction

• The ground safety micro-switch closes when the aircraft is airborne and the shock absorber struts fully extend.
• When the pilot moves the landing gear control lever to the ‘up’ position, 28VDC is supplied to the coil of the landing gear ‘up’ relay.
• The motor turns, driving the gearbox to retract the landing gear.
• As each landing gear leg begins to move, the contacts of its down-lock micro-switch automatically close.
• Once each landing gear leg reaches the fully retracted position, the respective up-lock micro-switch is actuated and its contacts open.
• When all three up-lock micro-switches open, the landing gear ‘up’ relay is deactivated and the DC voltage supply to the ‘up’ winding of the motor is interrupted.

Emergency Landing Gear Release Handle

• Located in the cockpit, this handle is used to provide a restricted flow during down selection and a free flow during retraction when the down line is connected to return.
• Purpose is to control the rates of retraction and extension, and compensate for the effects of gravity.

Emergency Up Override Lock Toggle Switch

• Mounted on a black and yellow striped area to the right of the landing gear selector lever.
• Allows the landing gear to be retracted when on jacks for ground maintenance purposes, as the airspeed will be below 150 knots.

Depressurising Relay

• Energized to make contacts in the up or down power line if all of the landing gear actuators and doors are not in the selected position when the Landing Gear Selector is initially placed into a new selection.
• De-energized once all of the landing gear actuators and doors have reached the new selected position.
• Allows power to the landing gear selector valve solenoids, which port flow to the up or down lines as directed by the landing gear selector, and places the landing gear selector valve in the neutral position when de-energized.

Landing Gear Down Selection

• The depressurising relay is energized to allow power to the landing gear selector valve solenoids, which port flow to the down lines.
• The landing gear doors open, assisted by spring struts, and fluid is expelled from the up side of the jacks.

Emergency Down System

• Used in the event of failure of No.1 hydraulic system to lower the landing gear.
• The system mechanically unlocks the three doors and releases the nose gear uplock, and opens a valve to allow compressed nitrogen to pressurize the main gear jacks and lower the main gear.
• Initiated by turning and pulling the EMERGY LANDING GEAR RELEASE handle in the cockpit.

Gear Retracting

• When the nose door is fully open, it mechanically operates the nose gear sequence valve.
• The nose gear sequence valve directs pressure through the gear up inhibiting valve, which unlocks the NLG downlock and ports fluid to the nose gear actuator to retract the nose gear.
• The MLG door jacks reach the fully open position, its individual ‘door fully open’ switch is closed, and an electrical signal changes the ports on the gear sequence valve to allow fluid to flow to the retraction side of the MLG retraction jack.