Hyd System Maintenance n Pnematics

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AM-1113 HYDRAULICS AND PNEUMATICS

11.10 Indication and Warning System

11.11 Hydraulic System Maintenance

11.12 High Pressure Pneumatic System

11.13 Engine Air Bleed Pneumatic System

INDICATION AND WARNING SYSTEM

Instruments and Indicators

Provide monitoring and indication of fluid quantity, pressure and temperature.

Hydraulic indicators and controls will be grouped together on one panel on the flight deck

Warnings of an urgent or emergency nature will be repeated on the master warning panel.

Quantity Indicators

Transparent window or tube on the reservoir

Pressurised reservoirs will have a float switch or rheostat device with level indication on a gauge

2 gauges, one on the flight deck and another on the reservoir itself or a ground servicing panel

A low level warning will usually be given by a light on the flight deck hydraulic panel and the master warning panel.

Temperature Indicators Temperature sensors are fitted to the reservoirs

Overheat indication on the flight deck hydraulic panel

Some systems may provide temperature gauges.

Sensors may be of the bi-metallic strip, resistive bulb or thermistor type, depending on the type of indication required.

Similar sensing devices will be fitted to the casing of electric motors used to drive hydraulic pumps, to give an indication of motor overheat.

Pressure Indicators

Pressure gauges fitted into the flight deck hyd panel

Direct reading gauges (bourdon tube type ,pressure relay valves

Electrically signalled systems use a pressure transmitter, which is a bourdon tube connected to a variable resister, and a voltage proportional to pressure is sent to a moving coil type gauge.

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Pressure Switches

Used to illuminate warning lamps

To indicate low system pressure

Also used to indicate that a particular pump is providing pressure above a certain pre-determined value.

Other Indicators

Wheel brake accumulator pressure.

Engine driven pump isolation cock position.

Reservoir pressurisation system status.

Power transfer unit operating status.

Methods of Displaying Information

Analogue dial displays for quantities/pressures

Warning lights to alert flight crews of emergencies

Glass cockpit

Flight Deck Analogue Hydraulic Panel

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Airbus A380 cockpit EICAS System Schematic

EICAS Maintenance Panel/Display

HYDRAULIC SYSTEM MAINTENANCE

Contamination

Hydraulic system maintenance requires clinical cleanliness

Contamination may lead to failure of components, excessive wear or a reduction in system efficiency

Types of contamination

Particulate contaminants Liquid or soluble contaminants

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Contamination

Causes and sources of contamination

Foreign matter introduced during servicing. Solid particles caused by natural wear of moving

components. Incorrect fluids used during system filling. Overheating of system fluid leading to chemical changes. Incorrect handling of ground maintenance and servicing

equipment.

Particulate Contamination

Normal wear of moving parts will result in the production of fine metallic particles

Usually be removed by system filters, whilst magnetic chip detectors may be fitted to give indication of system wear/damage

The biggest source of these particles is obviously the pumps

If pumps are replaced it may be necessary to also replace filters or filter elements


In order to minimise the risk of system contamination the following general guidelines should be followed:

Always blank off connections of any removed pipelines and components.

Always ensure clean storage and working areas for hydraulic components.

Ensure any fluids used for lubrication of seals prior to fitting are clean.

Always use the correct equipment for topping up reservoirs and systems.


General guidelines . . . .

Part used cans of fluid should be disposed of in accordance with local regulations.

Apply the same procedures to ground servicing equipment, as to the aeroplane system.

Always investigate the cause of filter blockage indication.

Soluble or Liquid Contamination

Liquid contamination of hydraulic fluids are more difficult to detect

Contaminated fluid can cause degradation of seals, valve erosion and corrosion of metals

Incorrect hydraulic fluid may cause deterioration of seals

Degraded static seals will eventually allow fluid leakage, possibly in several locations

Soluble or Liquid Contamination . . .

Leakage across dynamic seals will be purely internal

Causes slow or sluggish operation of actuators

Increased flow rates may cause overheating of the fluid.

Traces of solvent can cause damage to sys components

Chlorinated solvents, such as Trichloroethane, Trichloroethylene, Chlorothene and Freon, when combined with water will form Hydrochloric acid, which will eat away metals, particularly light alloys.

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Contamination of the synthetic ester based fluids can result in a agent that causes accelerated corrosion rates

Additionally overheating of these fluids can lead to the formation of acids than can attack materials within the system.


Soluble or liquid contamination can be minimised:

Ensure correct fluids are used at all times.

Always ensure clean storage and working areas for hyd comp.

Thoroughly investigate hydraulic system overheating, including sampling of fluids.

Ensure that after solvent cleaning, all traces of solvent are removed.

Sample system fluids in accordance with manufacturer instructions.

Fluid Sampling

Samples of system fluid can be used to determine:

Particulate contamination Acidity Specific gravity Viscosity Water content

The procedure and periodicity specified by manufacturer

Fluid Sampling

It is usually necessary to circulate fluids and to ensure that they are at working temperature prior to taking a sample

Ensured by extracting a sample within 20 minutes of system shut down

If the systems have not been operated then a ground run will be required.

Fluid Sampling

Some sample testing techniques may be used on site

The Millipore Patch test

Others will require laboratory facilities not usually present in a maintenance facilities

The Millipore Patch test

Consists of: Two containers, mounted one over the

other

Top container provides a holder for a filter pad

Lower container is attached to a small vacuum pump

Channel between the 2 vessels is opened and closed by a tap

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The Millipore Patch test Draining There will be occasions when it will be necessary to remove all

fluid from a particular system

General procedure:

Depressurising system accumulators and reservoirs Removing reservoir caps, if of the vented type Open drain cocks to release all fluid into a suitable container

Be aware of the quantity of fluid contained in the system and ensure sufficient empty containers available

All drained fluid must be disposed of in accordance with local regulations

Flushing

Flushing of hydraulic systems may be required

Whenever contamination of fluid is found or suspected May also be necessary to flush a system after extensive

work or component replacement.

General procedure:

Draining the system. Replacing filters or filter elements. Refilling the system to a specified level. Operate the systems for a specified time.

Flushing

General procedure ...Cond

Draining out all fluid. Replacing filters or filter elements. Refilling with clean fluid to the specified level. Running the system for a specified time and sampling for

the presence of Contamination.

Filling Hydraulic System

Ensure that the correct equipment and fluid is used

All ground servicing and test equipment must be treated with the same procedures applied to aeroplane systems

Should be marked with the fluid that they contain

Procedure contained in the maintenance manuals is strictly observed or an overflow could occur during system operation

Filling following extensive work or after flushing, there will be significant quantities of air trapped

Remove all traces of air to ensure efficient system operation by a process known as venting

Operating systems usually using a ground hydraulic test trolley to move fluid through the system and force the air back to the reservoir, from where it can be vented to atmosphere.

Venting Hydraulic System

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Small amounts of air trapped in systems is removed by a process known as bleeding

It will be necessary to provide a small pressure/flow to the system when performing any bleeding task

Ensure reservoir is filled with fluid to the correct level and is maintained at this level during the entire procedure

Commence at the highest and/or most remote point from the reservoir, bleeding the smallest volume of the actuator first.

Bleeding Hydraulic System Component Replacement

Ensure zero system/accumulator pressure

N2 pressure in accumulator

Work involving the reservoir return system depressurise reservoir

Safe the system by removal of fuses or by tripping CB

Use approved blanks and pressure proof

Opportunity inspection

Replacement components must be checked for correct part number and storage life

Visually inspect for signs of degradation

Drain and flush through with clean system fluid

When reconnecting pipelines fit squarely, initially hand tight, bleed and torque load to specified value

Component Replacement . . .

Hoses require care when fitting to ensure that they are not kinked or twisted during the tightening process

Fitting lines on hose assemblies may be used for this purpose

After fitting they should be inspected for correct routing and to ensure that they will no foul on structure or any moving components

Component Replacement . . .

Hose Installation and Routing

Seal Replacement

Whenever seals are removed from components they are to be discarded and new items fitted

New seals/gaskets obtained by use of part number only and check that storage life has not been exceeded

Extreme care is required when handling sealing items

Ensure correct orientation when using V or U section seals Soak seals in the system fluid for a few minutes, to lubricate and aid fitting, and will also check for seal/fluid compatibility.

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New seal should always be fitted when assembling components

Seals normally soaked in the system fluid for a specified time (24 hrs) to ensure pliability

When installing O-rings, extreme care must be used to prevent the ring being nicked or damaged by either the sharp edges of the threads or by the tool

The proper tool should always be used when fitting seal to avoid damage

Seal Installation

Methods of Seal Fitting/Removal Methods of Seal Fitting/Removal

SEAL MATERIALS

Blue dot or stripe: Air or MIL-H-5606 hydraulic fluid

Red dot or stripe: Fuel

Yellow dot: Synthetic engine oil

White stripe: Petroleum-base engine oil or lubricant

Green dash: Skydrol hydraulic fluid

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Maximum storage life for seals from the date of cure, (4 years).

The maximum life for rubber parts is usually 6 years from the date of cure.

The service life of the seals is usually a maximum of 2 years

If the component life is less than two years it is customary to change the seals at the lesser period.

STORAGE OF SEALS

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Acceptable storage conditions for rubber parts would be a temperature of between 10C to 21C and a relative humidity of 65 %.

Items should be packed in suitable airtight containers.

These protect the seals from excessive light and atmospheric oxygen, both of which have a detrimental effect on rubber.

STORAGE OF SEALS Storage of Components

Hydraulic components are normally packed in sealed container or plastic bags

Desiccant (silica gel) used to ensure a moisture free atmosphere to prevent the formation of corrosion.

Stored containing system fluid as this will maintain seals in a wetted condition and prevent degradation

All connections must be blanked.

Storage of Components

Storage life of components and assemblies is determined by the life of non-metallic parts and depends upon the materials those parts are made of.

Stored items to have the date of packing and storage life marked on them

Inspect for signs of degradation before use

Fluid contained in them should be flushed out using clean system fluid.

Functional Testing

After installation/reinstallation of components it may be necessary to carry out adjustments to micro-switches 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.

Functional tests will be specified in maintenance manuals and these must be strictly adhered to.

Preparation for functional tests

Quality and completeness of the maintenance task

Checking of reservoir levels

Checking of accumulator base pressures

Applying external gas pressure to reservoirs, if required

Thorough check of area around services and clearance

Correct connection of appropriate power sources

Leak Tests

Applied to systems after replacement of components to ascertain that the system is free from leaks

Using low pressure, gradually increasing to full pressure and flow to check external leakage

For internal leaks, run the system to full pressure and switch off the pressure supply

Pressure drop rate is then timed and compared to values specified in MM, also referred to as a leak rate test

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Flow Rate Test

To ascertain that hydraulic pumps are producing acceptable flow rates

Occasionally for testing whole system flow rates

Some aeroplanes have flow meters installed as part of their diagnostic instrumentation

Ground test equipment (flow meters) fitted prior to testing

Hydraulic test trolleys incorporate flow meters in their instrumentation and therefore the task is made much easier.

Operational Tests/ Functional Tests

Designed to test for correct sequence and speed of operation

Following major component replacement, the system will be operated at low pressure/slow speed, to ensure correct operation prior to full pressure/flow tests

Sluggish, slow or erratic operation may indicate aeration of fluid or excessive internal leakage

if fluid temperature indication is given on flight deck, this may also be used to indicate excessive internal leakage.

General System Maintenance

General lubrication tasks will be required on all operating system pivot points and linkages.

Inspect filter blockage indicators, chip detectors regularly and fluid sampling at specified intervals.

Reservoir fluid levels and accumulator base pressures

Inspect pipelines and components for safety of mounting, corrosion, fretting damage, leaks and clearance

PNEUMATIC SYSTEMS

Pneumatic Systems

Advantages as compared to a hydraulic system

Weight saving Air is freely available No return lines are necessary Pipelines can usually be of a smaller diameter.

Systems utilise pneumatic power

Most system on smaller aeroplanes utilise pneumatic power

LG retraction and lowering systems Windscreen wiper operation Wheel brakes

Larger aeroplanes will usually utilise it for:

Pressurisation of cabins Pressurisation of components Anti and de-icing operation of air turbines

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Typical High Pressure System Compressors

Multi stage compressors

4 stages being the most common in most aeroplanes

The volume of each successive stage is smaller than the preceding one

Compression of the air takes place, raising its pressure and temperature

Air passes between each compression stage is cooled by integral coolers.

Four Stage Air Compressor : Operation Four Stage Air Compressor : Operation

Four Stage Air Compressor : Operation Pressure Regulator

Controls the maximum pressure allowed to build up in the system

Off loads the compressor(s) when power is not required

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Pressure Regulator Oil and Water Trap

Remove any oil or water present in the air supplied by compressor

Separate oil and water by spinning using radial drillings and cooling by expansion

Needs draining at regular intervals

A baffle prevent separated oil and water entering the system during manoeuvring of the aeroplane

Dehydrator

Air leaving the oil and water trap may still contain some moisture:

freeze at altitude and block pipelines and valves

may also cause corrosion

Desiccant: activated alumina

The desiccant need renewal.

Filter at outlet

Relief Valve Prevent system pressure from

exceeding the normal maximum operating pressure that the system is designed to use.

System pressure could rise due to a fault with the pressure regulator or an increase in temperature of air trapped in parts of the system.

The operating pressure for this valve is usually adjustable by means of increasing or decreasing spring pressure

Air Filter Remove solid particulate contaminants

and any residual oil vapour and prevent them passing into the system.

Filters are usually of the paper gauze, glass wool, felt or fine metal mesh type

Drain plug at the base of the filter bowl

If moisture is found filter housing should be thoroughly dried and the filter medium replaced

If oil is present the compressor and oil and water trap should be examined for faults.

Air storage cylinders

Storage cylinders, often referred to as storage bottles

Hold compressed air not currently required by the operating systems.

When systems are operated the cylinders will provide the initial impetus to the system, and the compressor(s) will come on line to replenish the cylinder and take over system operation

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The cylinders are made of steel and are usually of wire wound type to provide extra strength to the cylinder walls

The cylinders are usually mounted in upright position and stack pipes are provided for the supply and gauge points.

A drain plug is provided for removal of contaminants

All pressure cylinders, both airborne and non-airborne types, require testing at regular intervals

Test dates will usually be stamped on the cylinder neck May also be painted on the body.

Air storage cylinders

Some services will operate at pressure lower than that available from the compressor and air storage cylinders

These systems will be supplied via a pressure reducing valve.

Pressure Reducing Valve

Allow pressure to the PRIMARY or ESSENTIAL operating systems at all times, closing off pressure to the SECONDARY or NON-ESSENTIAL systems should power system pressure fall below a pre-determined level.

Pressure Maintaining Valve Isolation Valve

Manually operated valves designed for use by maintenance personnel

Fitted downstream of storage cylinders

Wire locked in the open position

Pneumatic Operating Systems: Undercarriage SystemElectrically Operated Landing Gear Selector

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Damped Actuator

Shuttle Valve

The shuttle valve allows operation of the system by either normal or emergency pressure sources

Pneumatic Brake Control Valve

Pneumatic System Maintenance

Pneumatic systems utilise high pressures and there is a potential for SERIOUS INJURY to occur if personnel do not follow laid down procedures.

Reference should always be made to the AMM for details of procedure and safety.

Pneumatic System MaintenanceRemoval of Components Keep clean and free from dirt and moisture

Blanking open ends

Isolate storage cylinders from the system by isolation valve or completely discharge

Aircraft that have a pneumatic undercarriage retraction and lowering system, should have ground locks fitted to all LG

The landing gear lever must be labelled DO NOT OPERATE

Place chocks if the aircraft has pneumatically operated wheel

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Installation of Components

Before installing a new or reconditioned item it should be inspected for damage and deterioration that may have occurred during transit or storage.

The item part number and modification state should be checked against that required for the aeroplane.


Installation of Components . . .

All moving parts should be checked for freedom of movement and in the correct sense

Seals and minor spare parts should always be renewed as a matter of course.

After installation the system should be thoroughly tested in accordance with the AMM, and all connections etc. should be locked.


Charging the System

Pneumatic systems are fitted with one or more charging valves which also act as non-return valves

Oil and water traps and a dehydrator, to ensure that the air is clean and dry

All supply hoses should be capped when not in use

Hoses should be blown through with compressed air to purge them prior to connection to the system.


Checking For Leaks Leakage may be caused by:

Deterioration of seals Loosening of pipeline connectors Scoring of cylinder walls Worn valve seats


Checking For Leaks Large external leaks can often be heard

Application of a neutral (non-corrosive) soap solution will make small leaks visible by the formation of bubbles

All traces of this solution must be removed after testing

Very small leaks may be detected only by a drop in system pressure over a long period of time (eg. 12 hours).

ENGINE AIR BLEED PNEUMATIC SYSTEM

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System Operation

Gas turbine engines used on modern aeroplanes produce large quantities of compressed air

This air may be drawn off and directed to pneumatic systems

Air extracted from gas turbine engines is usually referred to as charge air

Used for cabin air conditioning and pressurisation, anti-icing, drinking water pressurisation and hydraulic reservoir pressurisation.

System Operation

Sources of Charge Air:

Engine compressors Auxiliary Power Units (APUs) Ground power units and trolleys

The system is designed to control both the pressure and temperature of the charge air

Filter fitted before supply to user systems.

Pressure Regulator and Shut-off ValvePressure actuated electrically controlled valve has 4 functions:

Opens or closes:

In response to the control switch on the bleed air control panel in the crew compartment

When temperature and pressure limits are exceeded Also close when the engine fire handle is operated to arm the

extinguisher circuit. Prevents reverse flow when duct pressure exceeds engine supply

pressure.

Modulates to limit pressure in the pneumatic manifold to approximately 45 lb/in.

Limits temperature to 230C.

Pre-cooler

This component, also known as a heat exchanger, cools bleed air to 200C to 230C.

It is a single pass cross flow type and uses cold air from the engine fan to cool bleed air

The amount of cooling air from the fan, is controlled by the pre-cooler control valve.

Pre-cooler Control Valve

This valve modulates in response to a temperature sensor located downstream of the cooler.

The sensor consists of an oil filled sensing element which operates a push rod, when bleed air normal temperature is reached the oil expands and a ball is unseated by the push rod.

This allows air in the pre-cooler control valve servo line to vent to atmosphere, allowing the valve to open increasing the flow of cooling air from the fan.

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Temperature Limit Thermostat

The temperature limit thermostat consists of a temperature sensor containing a fluid surrounding a bellows assembly and an output rod, which actuates a ball valve.

If the temperature in the duct reaches 230C an atmospheric port is opened, reducing the control air pressure at the bleed air regulator.

This causes the regulator and shut-off valve to move towards the closed position, reducing the amount of bleed air passing through the pre-cooler. With less hot bleed air flowing the pre-cooler will cool it more efficiently.

Bleed Air Over Temperature Switch The engine bleed air over temperature indication consists of

an over temperature switch on the ducting downstream of the pre-cooler.

This switch is connected to a warning light on the bleed air control panel.

The switch will illuminate the light and close the pressure regulator and shut-off valve if the duct temperature reaches 255C.

The pressure regulator and shut-off valve may be re-opened by operating a reset switch, adjacent to the over temperature warning light, once the duct temperature has cooled to a pre-determined level.

High Stage Valve

A pneumatically actuated and controlled valve fitted into the high-pressure bleed duct.

At reduced engine power settings, i.e. during cruise, descent etc. the high stage valve will automatically open to allow the high stage compressor to meet the demand of the various user systems.

It is controlled by the high-pressure bleed regulator.

High Pressure Bleed Regulator

The high-pressure bleed regulator senses a drop in the pressure in the high-pressure compressor, associated with operating at reduced power.

It opens the high stage valve to allow the high-pressure compressor to supply the pneumatic system.

A non-return or check valve is fitted into the low/intermediate bleed pipeline to prevent this higher-pressure flowing back into the engine.

Pressure Relief Valve

A pressure relief valve is fitted into the engine bleed air system, between the low/intermediate pressure check valve and the pressure regulator and shut-off valve, to prevent main duct pressure exceeding 100 lb/in.

Pressure Transmitter A pressure transmitter is fitted into each engine bleed air

system to send electrical signals to a duct pressure indicator on the bleed air control panel at the flight deck.

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WING ANTI-ICING VALVE

This valve is controlled from the flight deck pneumatic control panel, opening of this valve allows hot, high-pressure air to flow to the wing leading edge to prevent the formation of ice.

Isolation Valve

The isolation valve is of the butterfly type, operated by a 115v ac motor

It is either fully open or closed, and is controlled from the flight deck pneumatic control panel.

It is normal to provide a means of manual operation for this valve.

The control switch has 3 positions; Close, Open and Auto, in the close position the valve is closed and isolates the left and right systems from each other.

Isolation Valve

The open position fully opens the valve allowing cross feeding of air from one side to the other, and the Auto position closes the isolation cock if all engine bleed air and air conditioning packs are operating.

However, the isolation valve will open automatically if either engine bleed air or air conditioning pack switch is off.

Air Cleaner and Purge Valve

Pneumatic Ground Service Connection A pneumatic ground service connector and check valve are

provided for the connection of a ground service trolley.

The pressurised air supplied through this connection may be used for engine starting and ground testing of the pneumatic system and its user systems.

The maximum pressure supplied through this connection is usually in the order of 40 to 45 lb/in.

The check valve prevents loss of air through the connection during normal operation of the aeroplane bleed air system

Secondary seal may be provided by a blanking cap fitted to the connection to prevent ingress of dirt and moisture.

Potable Water Pressurisation

Potable (drinking) water systems consist of a stainless steel or fibreglass tank, providing water to galley and toilets.

The tanks are usually pressurised to 10 - 15lb/in using engine bleed air, the air passes through a pressure reducing valve and then to the tank.

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Auxiliary Power Unit Air Supply

The Auxiliary Power Unit (APU) normally provides high pressure air when the aeroplane is on the ground with its main engines not running, and will provide adequate air pressure to supply the aeroplanes systems in this configuration.

APU operation in flight may provide a back up in case of engine failure or main engine bleed air system malfunction.

Auxiliary Power Unit Bleed Valve

The APU bleed valve is fitted downstream of the APU in the air ducting, when the APU is operating above 95% RPM the bleed valve can be opened to supply air to the system.

The APU bleed valve control switch is located on the flight deck pneumatic control panel.

The APU bleed valve will be closed automatically if the APU fire-warning switch is operated to arm the fire extinguisher.

APU Pressure Relief Valve

A pressure relief valve fitted between the APU bleed valve and check valve prevents duct pressure exceeding its maximum value.

The check valve will prevent system pressure being lost through the APU during normal system operation.

Controls And Indications Bleed air switches (main engines and APU).

Isolation valve switch.

Overheat system test button.

Engine bleed valve trip reset.

A Pressure gauge or indicator for each system.

Warning/caution captions for both systems for:

(i) Wing-body overheat. (ii) Conditioning pack trip off. (iii) Engine bleed trip off.

Pneumatic System Ducting

The ducting used in pneumatic systems is usually made of stainless steel or titanium, to withstand both temperature and pressure.

Ducting is made up from many sections for both ease of manufacture and economy in replacement. It is made from thin walled materials joined together, usually with V band type clamps.

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Pneumatic System Ducting

Thermal expansion can be accommodated by manufacturing the ducts slightly shorter than required, tightening the clamp will draw the ducts together, providing a gas tight seal whether the ducting is hot or cold.

Another method often used is to provide expansion couplings, usually in the form of a thin metal bellows, which will allow expansion and contraction without the possibility of ducts distorting.

Throughout their length the ducts are secured to the local structure by cleats, clamps or tie rods.

Duct Maintenance and Repair

Thin walled ducts may be damaged as a result of:

Improper removal or installation. Mishandling or incorrect tightening of clamps. Abnormal operating conditions

Duct Maintenance and Repair

General Maintenance Smooth dents are normally permissible providing they do

not restrict the flow of air through the duct.

Shallow scratches and gouges are permissible if they are not more than 10% of duct wall thickness and, providing the bottom of the scratch is smooth.

Any defect or damage within approximately (6mm) of a weld renders the duct unserviceable and it should be replaced.

Duct Fitting And Handling Precautions

Great care must be exercised when fitting or handling ducts as any hot air leak can cause a fire or spurious fire warning.

General precautions:

Correct Torque loading Always replace seals on re-assembly. Ensure the duct ends mate easily with other system

components and are concentric to each other before fitting clamps.


General precautions:

Duct retaining rods or clamps must not load or put any bending stress on the ducting or joints.

Ensure clamps are positioned so as not to foul any moving components running close to the ducting.

Where lagging is fitted, it should be inspected to ensure it is in a serviceable condition.

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General precautions: At normal temperatures Skydrol is compatible with Titanium.

At temperatures above 130C the fluid becomes acidic and attacks Titanium, causing it to corrode and become brittle.

If this condition has occurred, the contamination will be evident by the appearance of a bright glossy brown or a dull black residue, and evidence of this condition must be treated in accordance with the manufacturers instructions.

Leak And Fire Detection

Because of the high temperatures involved leaks from pneumatic system ducting and components may cause fires

It is important to correctly install and test system ducting to prevent hot gas leakage

Hot leak and fire detection incorporated into ducting areas.

Automatic Leak Detection

Automatic leak detection systems comprise of heat sensitive devices fitted around ducting runs, to alert the flight crew of a leak or rupture of the ducting.

One such system is a heat sensitive element that is attached to duct supports and follows the duct throughout its length, except in engine nacelles and bays where overheat and fire detection is already provided.


Heat sensitive devices consists of an electrical conductor which is encapsulated in an earthed metal tube

Between the conductor and the earth is a material whose electrical resistance is inversely proportional to temperature:

i.e. the higher the temperature the lower the resistance of the material.


Under normal conditions the resistance of the material prevents the flow of electricity between the conductor and the earth, as temperature increases the resistance of the material decreases and allows a current to flow.

This will illuminate a warning caption on the flight deck instrument panel

A test facility is incorporated into the system for use by maintenance and flight crews.

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Spot Detectors

In some aeroplane installations spot detectors are also incorporated

Detectors operate on the principle of the Bi-metallic strip.

Fitted at significant locations within the ducting system, and will alert operators to the presence of hot air leaks in these areas.

Leak Detector Maintenance Great care is required to avoid damage to hot leak and

fire detection sensors, when carrying out maintenance activities in their vicinity.

Any bending or overstressing of detector elements may render them inoperable, or worse still cause spurious fire warnings whilst the aeroplane is airborne.

Whenever replacement of elements or spot detectors is required it is vitally important that the correct type of sensor is used, since different locations may require sensors operating at different temperatures.

Hyd System Maintenance n Pnematics

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