The Construction, Operation and Maintenance of Aircraft Hydraulic Systems Flashcards
What is the pressure most aircraft hydraulics operate at?
3000 PSI (Pounds Per Square Inch)
What effects static fluid pressure?
Static Fluid Pressure is directly proportional to the height of the column and, It no way depends on the shape of the container or the amount of liquid the container holds, only height of column effects pressure,
What is the relationship between area, distance, and volume in a hydraulic system?
Force= Area⨉Pressure (F=AP)
What is the relationship between Area, Distance and Volume?
Volume= Area⨉Distance (V=AD)
What are the two mechanical advantages in a hydraulic system?
- Ease at which force can be transmitted remotely over
large distances - Large gain in mechanical advantage made possible by varying
the size of actuator pistons
How is mechanical advantage achieved in a hydraulic system?
It is achieved by having an output piston that is larger then the input piston
What is viscosity and what effects viscosity?
It is the internal resistance to flow and as the temperature decreases, viscosity increases. Viscosity and Temperature are inversely proportional to each other.
What is flash point in hydraulic systems?
The temperature at which a liquid gives off vapour in sufficient quantity to ignite (Flash) momentarily when a flame is applied, high flash point is good for hydraulic systems
What is fire point in hydraulic systems?
Temperature at which a substance gives off vapour in sufficient quantity to ignite and continue to burn when exposed to a spark or flame, high fire point is good for hydraulic systems
What are the three types of hydraulic fluid?
Vegetable Based Fluid~ are flammable, were used in early days if aviation and are dyed blue for identification
Mineral Based Fluid~ used in General Aviation (GA) and Military Aft, has good lubricating properties and prevents formation of corrosion, is dyed red for identification and is flammable
Synthetic Fluid~ Used in most commercial aft, non-petroleum based are extremely fire resistant and dyed purple for identification, Example would be SKYDROL
What to do if aircraft hydraulic system serviced with wrong hydraulic fluid?
Immediately be flushed and drained from system and seals should be replaced IAW manufacturers specifications
Explain the Operation of a Powered Hydraulic System?
A powered hydraulic systems consists of Reservoir( Hydraulic Fluid Stored and Flow of it), EDP (Engine Driven Pump) where the hydraulic fluid is only pumped and flows thorough the system only when the engine is on.
It is permanently coupled to the engine so the flow of hydraulic fluid does not stop meaning its continuous. To control the fluid and stop it from being over pressurised, we need to have a Pump Control Valve (PCV) which is opened when pressure is not needed meaning fluid flowing from bottom of reservoir to PCV and then to reservoir again in cycle avoiding actuating any system,
The system also consists of a Double Acting Selector Valve (DASV) which offers hydraulic drive in both directions and also holds the fluid in the actuator by maintaining equal pressure on both sides of Piston.
For the system to actuate the Pump Control Valve (PCV) is closed and the Double Acting Selector Valve (DASV) is put in pilots desired input position which allows fluid to pass through the actuator into the Double Acting Cylinder pushing the piston to actuate the system such as the landing gear. The fluid coming to the opposite side of piston is flowing to the Reservoir as the system does this in a cycle.
Once the piston has completed its travel, fluid pressure rises enough to cause the release of the relief valve which allows the fluid to go back to the Reservoir and maintain the pressure avoiding it to reach the bursting pressure which can potentially damage the Hydraulic Pump,
The Pilot then returns the DASV (Double Acting Selector Valve) to its neutral position blocking of fluid and pressure from reaching the Double Acting Cylinder and Piston, The PCV (Pump Control Valve) is open again to relive the Engine of the Fluid Load and allows for the circulation of fluid to flow through the PCV (Pump Control Valve) and reservoir and goes on in this cycle as the EDP (Engine Driven Pump) keeps supplying hydraulic fluid.
This Powered Hydraulic System can operate both the landing gear and flaps and even more actuators if necessary. Note: “ If the pilot wants to actuate another system he must select that system and close the PCV (Pump Control Valve) to stop fluid from using that path and put the Selector Valve in correct position to allow fluid and pressure to actuate that system”, Each time a system is actuated, the pilot must to the same thing and once system is actuated then PCV must be opened again once DASV( Double Actuated Selector Valve) reaches neutral position allowing the release of load on engine and continue with the EDP to Pump Control Valve to Reservoir Cycle.
What is a Relief Valve?
Fluid can only move the piston of an actuator with a certain rise of pressure, Once the fluid reaches this pressure it moves the piston from one end of its travel to the other, once the piston has reached the end of its travel the rising pressure that was used to move the piston almost immediately rises as there us no way for the fluid to go, For this reason we need a RELIEF VALVE to take the loads, If there is no Relief Valve then it can cause damage to the Hydraulic Pump.
Explain the Operation of an Unloading Valve and its Description ?
It has the same function as the pump control valve except that once the unloading valve detects the drop in maximum pressure along actuator cylinder. It will open and take the load, the spring will automatically open the unloading valve causing it to take that load. “An Unloading Valve is simply just a pressure control device that is used to dump excess fluid to tank a little or no pressure” Integrating an unloading valve into a hydraulic system means that we can get rid of the Pump Control Valve and also means that every time the pilot wants to actuate a hydraulic system, he doesn’t need to operate the valve every time pressure is required by the system.
Explain the Operation of an Accumulator?
The Hydraulic Accumulator takes a specific amount of fluid that is under pressure from the hydraulic system and stores it until it is required within the system. The accumulator takes the fluid when pressure within the hydraulic system is increased and gas will be compressed. Once the pressure is reduced the compressed gas will expand forcing the stored fluid to the hydraulic system in emergencies if the power fails or the a working closing stroke is carried out with the help of the energy present in the accumulator. “The Accumulators secondary function is to prevent the pressure from changing so rapidly that it could damage the system providing a shock absorbing action”.
