Hydrualic System-Functional Tests

After installation & reinstallation/Major maintenance of components it may be necessary to carry out adjustments to micro-switches, limiters, sensors...etc and to ensure the correct operation of warning devices and instrumentation, also operating systems will need to be tested to check for correct operation. This is generally referred to as functional testing. 

 All system functional tests will be specified in maintenance manuals and these must be always done in accordance with maintenance manuals(IAW MM) . Prior to any functional tests, it will be necessary to inspect the system(s) to be operated, this inspection should generally cover the followings,

• Quality and completeness of the maintenance task requiring a functional test.
• Checking of reservoir levels.
• Checking of accumulator pressures.
• Applying external gas pressure to reservoirs, if required.
• A thorough check of the area around services to be operated, to check for clearance,FOD, tools...etc.
• Correct connection of appropriate power sources (electrical and hydraulic).

It should be noted that the specific tests applied to particular aircraft and systems will be stated in the maintenance manual and all the steps must be done in accordance with the Manual only. 

     Functional tests are designed to test for correct sequence and speed of operation. Usually, if a major component replacement has taken place the system will be operated at low pressure/slow speed, to ensure correct operation before the full pressure/flow tests. During these tests all aspects of the system operation will be tested, this may include the timing of the operating sequence. The results of the functional test as Sluggish, slow, erratic operation or high temperature of fluid during operation gives indications of the faults in the system such as Aeration,Internal Leakages....etc. where it may cause the further inspection of the maintenance job.

Ram Air Turbine (RAT)

A ram air turbine (RAT) is a self contained turbine, or propeller, unit with a hydraulic pump attached, these are generally only used in emergency situations. The RAT is mounted on a shaft that provides swivel couplings for the pump supply line and pressure pipelines, when stowed it is held in position by a lock, aircraft skin contour is maintained by doors closing over the RAT stowage.RAT may be deployed into the airflow by selection of a switch on the flight deck, but is more usually deployed automatically when main system pressure drops below a predetermined value.

The RAT must be capable of operation over a broad speed range, maintaining normal hydraulic pressure, whether the aeroplane is at maximum speed and height or slowing for landing. To accomplish this the propeller is of a variable pitch design, controlled by a mechanical governor. An increase or decrease in RAT speed will change the centrifugal force on fly weights and blade pitch will be adjusted to maintain a constant RPM during operation .

A volumetric fuse or flow control valve is usually fitted to the RAT allowing the propeller to attain operating speed, before being loaded by the pump, this allows full system pressure to be achieved approximately 4-5 seconds after deployment into the air stream.

Image shows RAT deployed for maintenance.this RAT is stowed in the flap track fairing.the place of stowage can be different from aircraft to aircraft. generally it may be under the wings,under the belly, or at sideways of the aircraft.

Axial Piston/ Bent Axis Pump

This is a fixed volume multi-piston pump. The cylinder block and drive shaft rotate together, and because of the angle between the cylinder block and shaft axes, each piston moves into and out of its cylinder once each revolution.  The stationary valve block has two circumferential slots leading to the top of the cylinder block, which are connected to the fluid inlet and outlet ports, and are arranged so that the pistons draw fluid into the cylinders on the outward stroke, and expel fluid into the systems on the inward stroke.

In this pump the casing along with the cylinders are attached in an angle to the drive shaft and pistons.Due to this angle the volume of the chamber inside the cylinder and piston varies as it rotates. when the chamber volumes is increasing the Inlet port is opened to those pistons with increasing chamber. as the pistons start to go down inside the cylinder the fluid is pressed and pressured, as the piston moves to the lowest point inside the cylinder there its opened to the outlet port on the casing letting the trapped fluid to rush out creating a flow of fluid....

Gerotor Pump (Gear+Rotor )

The gerotor pump is a combination internal external gear pump. these pumps are having six tooth and Four tooth. The spur type drive gear is turned  by an accessory drive from the engine.  As it turns, it rotates the seven-tooth internal gear. 

In diagram the two marked teeth are meshed and the tooth of the spur gear almost completely fills the cavity in the rotor(Fig A).  As the drive gear rotates and pulls the driven gear round, the volume of the cavity increases until at (fig C) it is at its maximum.  During the rotation from (fig A) to (fig C) the expanding cavity is under the inlet port and fluid is drawn into the pump.

As the gears continue to rotate, the cavity formed by the marked teeth moves under the outlet port.  As the drive gear meshes with the cavity next to the marked cavity in the rotor, its volume decreases.  The fluid in this cavity is forced out of the pump through the outlet port.this process continues and provide a positive flow at the outlet.

Spur Gear Pump

The simple spur gear pump uses two meshing gears closely fitted into a housing.  One of the gears is driven by the engine accessory drive shaft, and this gear drives the other.  As the gears rotate into a direction , the space between the teeth on the inlet side becomes larger.  Fluid is sucked in at this stage, trapped between the teeth and the housing and carried around to the discharge side of the pump.

As the fluid comes to discharge the teeth of the two gears come into mesh, decreasing the volume, and force the fluid out the pump discharge.

A small amount of fluid is allowed to leak past the gears and around the shafts for lubrication and cooling(known as Case Lubrication of EDP).  This fluid drains into the hollow shafts of the gears and is picked up by the low pressure at the inlet side of the pump.  A weak relief valve holds the oil in the hollow shafts until it builds up to a pressure of about 3-6 psi.  This ‘Case’ pressure is maintained so that if the shaft or seal becomes scored, fluid will be forced out, rather than air being drawn into, the pump.  Air would otherwise displace the fluid needed for lubrication and the pump would be damaged due to less lubrication and overheating.

As the pump output pressure increases, there is a tendency for the case to distort and allow increased leakage.  To prevent this, some pumps have high-pressure oil from the discharge side of the pump fed through a check valve into a cavity behind the bushing flanges.  The bushings are forced tight against the sides of the gears, decreasing the side clearance and minimising leakage, also compensating for bushing wear.

Hydrualic Pumps

To supply a flow of fluid to the actuator, a pump must be provided.  It is important to realize that the pump does not deliver pressure.  Hydraulic pressure is created only when an attempt is made to compress a fluid.  Fluid pumped through an open-ended pipe will have no pressure, but, if the pipe is connected to an actuator, the resistance to the flow will create pressure.  The rate at which a single hand pump can deliver fluid is very slow.  In the aircraft hydraulic system an engine driven pump is therefore installed in addition to the hand pump.  The purpose of the hand pump would then be for emergency use and for ground testing of the system.

Hand pumps

These are usually of double acting type, delivering fluid on each stroke.  As the piston moves upward in the cylinder, fluid is drawn in through the inlet non return valve (NRV) into the cylinder.  At the same time fluid above the piston is discharged through the outlet NRV.  As the piston moves downward, the inlet NRV closes and the transfer NRV opens, allowing fluid to flow through the piston.  Since the volume above the piston is smaller than below the piston, part of this fluid is discharged through the outlet NRV.

HYD Reservoirs

The reservoir’s purpose is to store, receive and supply hydraulic fluid. Some years ago reservoirs simply consisted of a tank with several connections, a filler and sight glass assembly and possibly for some form of filter or strainer for initial filtration of fluid as it being filled up.  Indeed, there are still many of this type of reservoir in common use on small or older aircraft,these type of reservoirs would usually incorporate a stack(stand) pipe to ensure a reserve of fluid for use in an emergency.

However these type always had few of drawbacks during its operation as a reservoirs. these are required to fulfill following conditions in order to work effectively.

• The necessity to maintain a head of pressure, under all conditions.so that the tank has to be placed at higher point than the EDP (Engine Driven Pump)
•  The possibility of the oil becoming aerated during manoeuvrings.
• Cavitation in the supply pipeline to the pump during banking.
• A lowering of hydraulic fluid boiling point at high altitude.

Basic Hydraulic System

In the basic hydraulic system, as shown below the basic components that all hydraulic systems may consist of, 

Actuator

Sometimes referred to as a hydraulic jack, this provides the output force required to operate various aircraft services.  Consists of a piston,on which the hydraulic pressure acts, which is secured to a rod, also known as a ram, which imparts the force developed by the piston.

Selector 

This component provides control of the system, and may be manually or electrically operated from the cockpit.  The terms ‘pressure’ and ‘return’ may only be applied to pipelines up to this component, the pipelines from the selector to the actuator may be referred to by their purpose ex: UP/DOWN,OPEN/CLOSE,RETRACT/EXTEND

Pump

The pump provides a flow of fluid to the system.  its should be mentioned pump provides flow, not pressure, as it is the resistance to that flow that causes a pressure rise,without making a resistance to the flow the pressure cannot be created.  In the illustration a simple hand pump is shown, many aircraft systems use engine or electrical motor driven pumps to provide greater flow rates and so speed up system operation.

Reservoir

The reservoir provides a means of holding surplus fluid, its purpose is to receive, store and supply fluid to and from the system.  It maintains a surplus, so that in the event of a leak the system will still have sufficient fluid to operate correctly.

It stores fluid that is temporarily not required by the system, as can be seen the actuator does not have equal volumes on each side of the piston.  When retracted (piston rod fully into the body) the actuator will hold less fluid than when fully extended (piston rod fully out of the body) and so this fluid must be stored when not in use.

The reservoir will also supply fluid to the pump to maintain a flow within the system and receives return fluid from the system.

Pipelines

The system pipelines are used to direct fluid to the appropriate components.  They may be made from various materials depending upon the pressure at that point in the system and the operating environment.  Within the system they are normally referred to by the nature of their oil flow ie. RETURN LINE or PRESSURE LINE.

Types of Hydraulic Fluid

The following are the types of fluid commonly used in aircraft operating today, some of them remain in use in aging aircraft and so may become obsolete as aircraft types are withdraw from service.

Due to the differences in composition, hydraulic fluids may not be mixed, even those of the same base type.  During the design stage, aircraft manufacturers will select a fluid appropriate to the system or aircraft and specify this in maintenance manuals, this is then the only permissible fluid to be used for that aircraft.  Should cross contamination with other fluids occur, such as may be caused by filling with the incorrect fluid type, then the whole system must be drained and flushed out prior to system operation.  The manufacturer will usually give advice in the aircraft maintenance manuals regarding what action is required with regard to rubber seals etc. should the system become contaminated in this way.

DTD 900/4081	A natural Castor based oil, golden yellow to brown in colour, it must be used with natural rubber seals.  It is flammable, strips paint and attacks synthetic rubber.  It is toxic in a fine spray mist.
Aeroshell 41/Mil-H-5606/ Def Stan 91-48	A mineral based oil, red in colour, must be used with synthetic rubber seals.  It is flammable and attacks natural rubber.  Its density and lubricating properties vary with temperature.
Skydrol 500B/Mil-H-8446	A synthetic, phosphate-ester based oil, purple in colour, its major advantage is that it is fire resistant.  It will strip paint and attacks both natural and synthetic rubber.  Slightly heavier than water, it does however, have a very broad range of operating temperatures.  Seals for use with synthetic oils are made from Butyl, Ethylene, Propylene, and Teflon.  It will absorb moisture from the atmosphere if exposed to it.
Skydrol 500B-4	An improved type, which has the same properties but provides more resistance to wear and erosion of orifices and valve lands.
Skydrol LD-4	A special low-density version, it provides a weight saving of approximately 5% on a volume basis, compared to other Skydrol types.