77 Questions
What is the primary function of a relief valve in a hydraulic system?
To prevent pressure buildup
What happens when fluid attempts to flow in the opposite direction in a ball check valve?
The pressure developed helps to hold the ball on its seat
What is the purpose of a thermal relief valve?
To relieve excessive pressure due to thermal expansion
Which type of valve is used to bypass pump delivery pressure to the reservoir?
Relief valve
What is the purpose of a system relief valve?
To prevent pressure buildup
What happens when the valve poppet is forced off its seat in a thermal relief valve?
Excessive pressurized fluid flows to the reservoir return line
What is the main difference between a system relief valve and a thermal relief valve?
Size
Why are relief valves commonly used in hydraulic systems?
To prevent pressure buildup
What is the purpose of a pressure regulator valve in a closed-centre hydraulic system?
To maintain the pressure within a specified range
What is the function of the check valve in the simple regulator system?
To prevent fluid from flowing back into the pump
What is special about the MS 28889 valve compared to the AN 6287-1 valve?
It has no valve core
What is the purpose of the roll pin on the MS 28889 valve?
To prevent the housing from being screwed too far into the body
What happens when the accumulator is full and pressure builds up in the system?
The piston and ball are in balance
What is the other name for the MS 28889 valve?
NATO Valve
What is one of the functions of the reservoir in a hydraulic system?
To provide a means of cooling the fluid when the pump is not under demand
What happens to the fluid in the reservoir when accumulators, actuating cylinders, and other units are not containing their maximum quantities of fluid?
The fluid is stored in the reservoir
What is the purpose of standpipes in hydraulic reservoirs?
To provide a reserve of fluid for emergency operation
Where is the emergency fluid drawn from when system fluid is lost in hydraulic systems?
From the bottom of the tank
What is one of the functions of the reservoir in terms of air in the system?
To allow a space for trapped air to settle out
What is the purpose of the power pump in the hydraulic system?
To force fluid through the system and eventually return to the reservoir
What is one of the functions of the reservoir in terms of system leakage?
To provide a reserve of fluid to allow for small leakage
What is a common feature of sophisticated hydraulic reservoir designs?
Filters, quantity indicators, and pressure relief systems
What is a major problem caused by foreign particles in the internal passageways?
System fluid or gas loss
What is a major advantage of quick disconnect couplings?
Ease and speed of connecting and disconnecting plumbing systems
What is the main feature of screw type fittings?
A set of teeth protruding from the rim of one half of the coupling
What is the purpose of the indicating pins on quick threading couplings?
To indicate when the coupling halves are properly connected
How do Inst-O-Matic hydraulic couplings operate?
By utilising a pullback sleeve which releases a set of retaining ball bearings
What is the function of the self-sealing valve located inside the quick disconnect fitting?
To eliminate system fluid or gas loss
How many types of quick disconnect fittings are mentioned in the text?
Three
What is the purpose of the indicator pin on an Inst-O-Matic coupling?
To indicate that the coupling is fully engaged
Where is the dial indicator mounted to provide reservoir fluid quantity indication?
On the end of the small diameter chamber
What is the purpose of the micro switch in the low fluid level detection system?
To cut pressure supplies to the secondary flight controls and general services
What type of pump is used in the engine driven pump (EDP)?
Self-idling swashplate type pump
What is the purpose of the non-return valve in the engine driven pump (EDP)?
To route the pressure output from the pump into the system
What are the three flexible hoses connected to the engine driven pump (EDP) for?
Suction, pressure, and case drain
Where is the accumulator located?
Centre fuselage
What is the purpose of the accumulator in the hydraulic system?
To regulate system pressure
How is the engine driven pump (EDP) secured to the gearbox?
With a V-band clamp
What is the main difference between hydraulic and pneumatic systems?
The type of fluid used
What is the branch of physics that hydraulic systems belong to?
Fluid power systems
What is the earliest known application of fluid power?
Moving objects using rafts or barges
What is the purpose of hydraulic systems in aircraft?
To generate power for landing gear and flight control systems
What is the principle that hydraulic systems operate on?
Pascal's Law
What is the main advantage of hydraulic systems?
High power-to-weight ratio
What is the term for the use of pressurised liquid to generate, control and transmit power?
Hydraulics
What is one of the ways accumulators act as a cushion or shock absorber in hydraulic systems?
By absorbing pressure fluctuations in the system
What is a characteristic of piston type accumulators?
They are commonly used in aircraft hydraulic systems
What is a component of a diaphragm accumulator?
A synthetic rubber diaphragm
What is a function of accumulators in hydraulic systems?
To store fluid under pressure for emergency operation
What are the two main types of accumulators used on aircraft?
Piston and diaphragm
What is a component of a piston accumulator?
A floating piston
What is a characteristic of diaphragm accumulators?
They are constructed with a metal sphere
What do accumulators supplement in hydraulic systems under peak load conditions?
The hydraulic pump output
What is the primary characteristic of a fixed delivery pump?
Delivering a fixed amount of fluid at a given number of revolutions per minute
What is the function of the universal link in a fixed delivery pump?
To connect the drive shaft to the cylinder block
What is the purpose of the slots in the valve plate?
To provide fluid feed to the pistons on the inlet side
How do the pistons move in a fixed delivery pump?
In and out of the cylinders alternately
What is the angle of rotation between the drive shaft and the cylinder block?
At an angle
What is the component that connects the pistons to the drive shaft?
Ball sockets
What is the purpose of the cylinder block?
To house the pistons
What is the result of the pistons moving outward from the cylinder block?
The volume of the cylinder spaces increases
What happens when the axis of the cylinder block is parallel to the axis of the drive shaft in a variable delivery pump?
No fluid is delivered
How does the swash plate type variable delivery piston pump operate?
By altering the piston stroke using a rotating swash plate
What is the purpose of a variable delivery pump?
To vary the volume of fluid being delivered at a given number of revolutions per minute
What is the correct procedure to deflate an accumulator using an AN 6287-1 valve?
Loosen the swivel nut one turn and depress the valve stem
What is the effect of changing the alignment of the rotational axis of the cylinder block in a variable delivery pump?
The piston stroke is altered
What is the main difference between the AN 6287-1 valve and the MS 28889 valve?
The MS 28889 valve has no valve core and a larger swivel nut
What is the purpose of the roll pin on the MS 28889 valve?
To prevent the housing from being screwed too far into the body
What is the function of the pistons in a multiple-piston variable delivery pump?
To draw fluid through the valving surface slot and force it through the outlet slot
What happens when the accumulator is full and pressure builds up in the system?
The pressure pushes up on the piston and down on the ball
What is the difference between a multiple-piston variable delivery pump and a swash plate type variable delivery pump?
The method of altering the piston stroke
What is another name for the MS 28889 valve?
NATO Valve
What is the result of the pistons moving within their respective cylinders in a variable delivery pump?
Fluid is delivered through the outlet slot
What is the purpose of the rotating swash plate in a swash plate type variable delivery pump?
To alter the piston stroke
What is the purpose of the pressure regulator valve in a closed-centre hydraulic system?
To unload the pump when no unit is being actuated
What is the function of the check valve in the simple regulator system?
To allow fluid to flow in one direction only
What is the purpose of tightening the swivel nut on the AN 6287-1 valve?
To provide a metal-to-metal seal
Study Notes
Hydraulic System Components
- Fluid flows from the reservoir to the power pump, where it is forced through the system and eventually returns to the reservoir.
- The reservoir ensures a good supply of fluid to the pump, caters for ram displacement, allows for thermal expansion, provides a reserve of fluid for small leakage, and provides a means of filling and checking the system.
Reservoir Functions
- Stores unused fluid when accumulators, actuating cylinders, and other units are not containing their maximum quantities of fluid.
- Provides a reserve of fluid to meet all requirements when the maximum amount of fluid is being used in the system.
- Equipped with standpipes in systems that require a reserve of fluid for emergency operation of landing gear, flaps, etc.
Ball Check Valve
- Installed with a spring to hold the ball or cone on its seat whenever there is no flow.
- When fluid attempts to flow in the opposite direction, the pressure developed helps to hold the ball or cone on its seat and creates a tighter seal.
Relief Valves
- Pressure-operated valves that bypass pump delivery pressure to the reservoir.
- Used to prevent pressure from building to a point where it might blow seals, burst or damage components within the hydraulic system.
- Types of relief valves include system relief valves, thermal relief valves, and simple ball type relief valves.
Pressure Regulator Valves
- Required in closed-centre hydraulic systems to maintain pressure within a specified range and to keep the pump unloaded when no unit is being actuated.
- The most simple pressure regulator is the balanced type, which operates by maintaining a balance of forces between the pump pressure and the accumulator pressure.
Quick Disconnect Couplings
- Development is directly related to the advent of compact high-performance aircraft and aerospace ground equipment.
- Advantages include ease and speed of connecting and disconnecting plumbing systems, and elimination of system fluid or gas loss.
- Types of quick disconnect fittings include screw type, quick threading, and Inst-O-Matic.
Engine Driven Pump (EDP)
- Located in the rear fuselage and driven by the No.1 engine gearbox.
- Connected to the hydraulic system by three flexible hoses fitted with self-sealing quick release couplings.
- The three hoses are: suction (supply from the reservoir), pressure (to the user services), and return / case drain (to the return line back to the reservoir).
Accumulator
- Located in the centre fuselage and consists of a cylinder and a freely floating separator piston.
- Stores energy and provides a reserve of fluid for the system.
Hydraulic Systems Introduction
- Hydraulic systems use pressurized liquid to generate, control, and transmit power.
- They are used in various applications in aircraft, including landing gear and flight control systems.
- Hydraulic systems belong to the branch of physics known as fluid power systems, which work by moving fluid.
- The fluid used in hydraulic systems is incompressible.
Hydraulic Principles Terminology
- Pascal's Law: not defined in this text, but it is a fundamental principle in hydraulics.
Mechanical Advantage
- Not defined in this text, but it refers to the ability of a system to multiply force or speed.
Hydraulic System Components
- Fixed delivery pumps:
- Deliver a fixed amount of fluid at a given number of revolutions per minute.
- Consist of a drive shaft, cylinder block, pistons, and a valving surface.
- Operate by rotating the cylinder block, causing pistons to move in and out of the cylinders.
- Variable delivery pumps:
- Designed to vary the volume of fluid being delivered at a given number of revolutions per minute.
- Can be multiple-piston or swash plate type.
- Multiple-piston type: alters the stroke of the pistons by changing the angle of the rotational axis of the cylinder block.
- Swash plate type: alters the stroke of the pistons by using a fixed and rotating swash plate.
Hydraulic Systems
- Layout: not described in this text.
- Operation: hydraulic systems use pressurized fluid to transmit power.
- Characteristics: not described in this text.
- Electrical system interface: not described in this text.
Accumulators
- Act as a cushion or shock absorber by absorbing pressure fluctuations in the system.
- Store enough fluid under pressure for emergency operation of certain actuating units.
- Supplement the hydraulic pump output under peak load conditions.
- Types: piston and diaphragm.
- Piston accumulator:
- Constructed of a metal cylinder, floating piston, air or nitrogen charging valve, and hydraulic fluid port.
- Requires less space than an equivalent diaphragm accumulator.
- Diaphragm accumulator:
- Constructed of a metal sphere, synthetic rubber diaphragm, air or nitrogen charging valve, and hydraulic fluid port.
Pressure Regulator Valves
- Required in closed-centre hydraulic systems to maintain pressure within a specified range and keep the pump unloaded when no unit is being actuated.
- Simple pressure regulator: balanced type, which operates by maintaining a balance of forces between the pressure in the system and the force on the piston.
Learn about the flow of fluid in aviation systems, including reservoirs, power pumps, and filters. Understand how foreign matter is separated from the fluid.
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