AGK Systems - Hydraulic Systems Flashcards
What are actuators?
Turn hydraulic pressure into movement
Three types of linear Actuator
Single Acting
~hydraulic pressure in one direction against a spring
Double Acting Balanced
~Selectable equal hydraulic pressure in both directions but equal force
Double Acting (Unbalanced)
~Selectable equal hydraulic pressure in both directions but unequal force
What are Selector Valves?
Direct hydraulic fluid to various parts of the system
Amongst other things they can control the direction of their movement
Selector Valve Types
Closed Centre Selector
Open Centre Selector
Linear Valve Selector
Open Centre Hydraulic Systems
Only one actuator can be used at any one time
Small to mid sized a/c
Closed Centre Hydraulic Systems
Maintains constant supply of hydraulic fluid pressure using a Variable Volume Pump
Services supplied in parallel so can be operated simultaneously
Off loading valve allows pump to unload when services are not operated
~reduces engine power required
Multiple actuators can be moved at the same time
Light A/C systems
Uses self containing hydraulic power pack via reversible DC electric motor
Gear is held in retraced position by hydraulic lock
Pump comes on line when required
Thermal relief valve protects against over pressure
What is a Hydraulic Reservoir?
Stores hydraulic fluid
Allows for thermal expansion of hydraulic fluid
Accepts fluid retaining from low pressure side of hydraulic system
Holds excess fluid to compensate for leaks
They are pressurised to stop boiling at alt
Sight Glass to check the level
What are stack pipes?
Keep a level of fluid in them in case of an emergency
What is an accumulator?
Comprises pressurised gas and fluid in a cylinder separated by a piston or rubber diaphragm
Accumulator uses
Provide emergency braking system
~6 applications of brake
Used to damp out pressure pulses
Cater for thermal expansion
An Accumulator pressure gauge displays…
Pre charge nitrogen gas pressure the hydraulic system isn’t provided
Hydraulic System pressure when it is powered
What happens if you incorrectly pre charge an Accumulator?
harmful hammering
E.g. Hydraulic Leak
Accumulator Summary
Stores energy to keep hydraulic fluid under pressure
Provides limited supply of pressure in an emergency
Dampens out pressure fluctuations
Allows for thermal expansion
Cater for small internal leaks
Hydraulic Pumps
Provide fluid flow and therefore pressure, they do not provide pressure
Usually EDP’s
What are two widely used Hydraulic pumps?
Constant Delivery (fixed volume) Pump
~these are spur and rotary pumps often in light a/c
Constant Pressure (variable volume) Pump
~These piston pumps more common in larger a/c (around 3000psi)
Constant Delivery (fixed volume) Pump
Provides constant fluid supply
Typically spur gear
Flow rate dependant on speed of rotation
Gears normally rotate at constant speed
REQUIRES AN AUTOMATIC CUT OUT VALVE (ACOV)
Constant Pressure (variable volume) Pump
Output volume NOT dependent upon pump RPM
Volume CAN be altered
Self Regulates and only supplies fluid needed to maintain constant woking pressure
Engine Driven
On start up: pump swash plate is at maximum deflection
Pump is lubricated by hydraulic fluid
DOES NOT require ACOV
When is the Constant Pressure Pump at nil stroke?
When swash plate is perpendicular to input drive shaft
What is a case drain?
Keeps pump cool when not pumping as it may get very hot
What is a hydraulic motor?
Uses hydraulic pressure to generate rotary motion
Forces piston down in cylinder block
Useful for operating screw jacks to drive the flap and slat mechanisms
Speed of motor is dependant on fluid flow rate
Selector Valve can change direction of rotation
What does a Pressure Relief Valve do?
Protects system from over pressurisation
May act as a Thermal Relief Valve (TRV)
What is a Restrictor Valve?
Reduces fluid flow to slow the speed of operation of a service
A one way restrictor allows full flow in one direction and restricted flow in the other
E.g. slowing of landing gear extension
What is a Priority Valve?
Ensure sufficient fluid flow at all times to the primary controls and wheel brakes
It closes and isolates non essential circuits if system pressure drops below a set limit
What is a shuttle valve
Allows most appropriate pressure
Allows one service to be operated by two independent supplies
guarantees an adequate supply if either system fails
Under normal operating conditions the shuttle is to the left side
When the main supply fails the shuttle moves to the right and allows the emergency supply to take over
What is a hydraulic Fuse?
Upstream of components to shut off flow & prevent total fluid loss in the event of a major leak
Shuts off when there is a leak in the system by the difference in pressure - where the leak is
~Results in no fluid loss
What is a Shut Off Valve?
Similar to Hydraulic Fuses
They’re not self regulating
Operate by external controller
~E.g. control on FD
Large A/C Hydraulic System Schematic
What is a Hand pump?
Used for maintenance or emergency back up
Used on the ground to operate hydraulic doors, brakes, flaps and anything linked to that system
Hydraulic fluid is supplied from the bottom of the Reservoir
RAT
After landing, probably need a tug as it doesn’t provide any hydraulic power after landing due to the speed of the a/c
ONLY PROVIDES HYDRAULIC POWER FOR FLIGHT CONTROLS
Hydraulic Power Transfer Unit (PTU)
On modern a/c with more than one hydraulic system, back up power can be obtained via a PTU
Concept of PTU:
~hydraulic motor in one system
~drive hydraulic pump on another system
Air Turbine Motors (ATM)
High pressure bleed air from an engine powers an air turbine which in turn, powers an hydraulic pump
Used on older a/c designs
