AFS Topic 1 Module 1 DIFF

Hydraulic Systems Overview

• Hydraulic systems use fluid to transmit power and motion.
• Fluid flows from the reservoir to the power pump, where it is forced through the system and eventually returns to the reservoir.

Reservoir Functions

• Ensure a good supply of fluid to the pump
• Cater for ram displacement
• Allow for thermal expansion
• Provide a reserve of fluid to allow for small leakage
• Provide a means of filling and checking the system
• Allow a space for trapped air to settle out (in certain systems only)
• Provide a means of cooling the fluid when the pump is not under demand
• Provide an emergency supply of fluid to a hand pump (in certain systems only)

Accumulators

• The primary purpose of an accumulator is to store hydraulic fluid under pressure.
• Accumulators store fluid that is not being used in the system, allowing for a reserve of fluid in case of an emergency.

Filters

• Hydraulic filters are required to filter out any particles that may enter the hydraulic system.
• Filters can be installed in the pressure and/or return lines.
• The most commonly employed filter type for aircraft hydraulic systems is the micronic filter.
• A by-pass valve incorporated in the filter head will allow unfiltered fluid to flow over the top of the element in the event of filter element blockage.
• Some filters have a manual indicator or a warning light to show when the filter is blocked.

Actuators

• Double-acting balanced actuators have the same surface area on both sides of the piston.
• An example of a double-acting balanced actuator is a servo actuator in a flight control system.
• Rotary actuators use a piston with a rack of teeth cut in its shaft, and the teeth mesh with those in a pinion gear that rotates as the piston moves left or right.
• An example of the incorporation of a rotary actuator is when there is a requirement to retract a light aircraft's landing gear.

Heat Exchangers

• A heat exchanger is a component that transfers heat from one body to another.
• Heat exchangers are used to cool the hydraulic fluid in the hydraulic system.
• Heat exchangers are usually located in the return side of the hydraulic system.
• Heat exchangers are available in various forms, including air cooled and fuel cooled heat exchangers.

Hydraulic System Layout and Operation

• Hydraulic power is used to operate the ailerons, spoilers, rudder, elevators, flaps, and slats, landing gear, nose wheel steering, and wheel brakes.
• The GENFLY Trainer Hydraulic System has two independently pressurized systems, logically termed No.1 and No.2.
• Fluid is stored in individual reservoirs and is pressurized by engine-driven pumps (EDPs).
• Pressure relief valves are used in both systems to prevent over-pressurization, and filters are used to prevent contamination.
• No. 1 system supplies the primary flying controls, secondary flying controls, and general services.
• No. 2 system supplies a tandem supply to the primary and secondary flying controls supplied by No. 1 system, except the spoilers.

Reservoirs

• There are two basic types of reservoirs: in-line and integral, which can be further classified as pressurized or non-pressurized.
• Reservoirs are not designed to be completely filled, but have an air space above the fluid level to allow for expansion of the fluid when heated during system operation.
• Reservoirs provide a means of checking the fluid level and replenishing the fluid.
• The quantity indicating method can be a dipstick on the filler cap or a remote indicating system to display the quantity on the aircraft's flight deck.
• Replenishment is normally accomplished by adding fluid directly to the reservoir through a filler opening.

Types of Reservoirs

• Non-pressurized reservoirs are designed for aircraft that do not fly at high altitudes or engage in violent maneuvers, such as helicopters.
• Non-pressurized reservoirs are usually located at the highest point in the hydraulic system, providing a head of fluid pressure to the hydraulic pumps at all times.
• Pressurized reservoirs are used in aircraft that fly at high altitudes or engage in violent maneuvers.

Valve Types

• There are several types of valves used in hydraulic systems, including:
  • Flow control valves
  • Check valves
  • Relief valves
  • Selector valves
  • Pressure reducing valves
  • Sequencing valves
  • Restrictor valves
  • Hydraulic fuses
  • Shuttle valves

Flow Control Valves

• Flow control valves are used to control the flow of fluid within the system.
• They can be fully opened to allow full flow or partially closed to restrict flow.

Check Valves

• Check valves are non-return valves that allow fluids to flow in one direction only.
• They are used to prevent backflow in the system.

Pressure Reducing Valves

• Pressure reducing valves are used to meter fluid to a sub-system.
• They are equipped with a relief valve that opens to relieve excess pressure in the sub-system.

Sequence Valves

• Sequence valves are used to time the operation of certain hydraulic sub-systems in a proper sequence.
• They are used in applications such as landing gear door sub-systems.

Quick Disconnect Couplings

• Quick disconnect couplings are used to connect and disconnect plumbing systems quickly and easily.
• There are three types of quick disconnect fittings: screw type, quick threading, and Inst-O-Matic.

Engine Driven Pump (EDP)

• The engine driven pump is located in the rear fuselage and is driven by the No.1 engine gearbox.
• It is secured to the gearbox by a V-band clamp and connected to the hydraulic system by three flexible hoses with self-sealing quick release couplings.
• The pump is the self-idling swashplate type, which automatically idles when the required operating pressure is developed.

Accumulator

• The system accumulator is located in the centre fuselage.
• It consists of a cylinder and a freely floating separator piston.