Module 7 (Maintenance Practices) Sub Module 7.9 (Pipes and Hoses).pdf

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Module 7 7 - MAINTENANCE PRACTICES

Category – Category – A/B1

Sub Module 7.9 – 7.9 – Pipes and Hoses

MODULE 7  Sub Module 7.9 7.9

PIPES AND HOSES

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Module 7 - MAINTENANCE PRACTICES

Sub Module 7.9 – Pipes and Hoses

Contents PIPES AND HOSES .................................................................................. 1 PIPE BENDING ....................................................................................... 2 BELLING/FLARING AIRCRAFT PIPES ....................................................... 4 INSPECTION AND TESTING OF AIRCRAFT PIPES AND HOSES ................ 9 INSTALLATION AND CLAMPING OF PIPES ...........................................11

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Module 7 - MAINTENANCE PRACTICES

Category – A/B1

Sub Module 7.9 – Pipes and Hoses

PIPES AND HOSES Pipes and hoses can be called upon to carry a wide variety of different fluids within an aircraft, including fuel, hydraulic and engine oils, de-icing fluids, pitot and static air. The pressure within these pipes can vary from ambient to 300 M Pa (300 bar or 4000 psi). All pipes and hoses must be manufactured, installed and connected so that no leaks occur in service, because a leak in a very low-pressure pitot air tube can be just as dangerous as a leak in an extremely high- pressure hydraulic line. Rigid pipelines are, generally, made from stainless steel, Tungum (Trade name for a high-tensile, copper alloy) and aluminium alloy. Replacement pipelines are, usually, supplied by the manufacturer, ready for installation, with the pipe bent to the correct curvature and the pipe ends flared and provided with the appropriate end fittings.

Requests for the basic pipe material will require details of the: 

Metal specification (DTD, BS, AN etc.)



Outside diameter (OD)

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Gauge of the wall thickness (SWG)

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Length of pipe required.

Flexible hoses are obtained from the aircraft manufacturer using the aircraft’s Illustrated Parts Catalogue (IPC). It is possible that, in certain circumstances, a replacement hose can be manufactured in a workshop or hose bay. Approval to manufacture the replacement hose must be sought from the aircraft’s manufacturer.

In certain circumstances, it may be permissible to manufacture new pipelines from lengths of pipe. A new pipeline will be made, by cutting the basic pipe to the correct length, attaching the correct couplings and expanding the ends by the use of a flaring tool.

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Module 7 - MAINTENANCE PRACTICES

Category – A/B1

Sub Module 7.9 – Pipes and Hoses

PIPE BENDING To lessen the possibility of the pipe wall kinking when it is being bent, it may be filled with a special alloy, which can be removed after the bending operation. These alloys are known as ’fusible alloys’, some of which melt below 100  C and can, therefore, be melted out by immersion in boiling water.

The complete removal of the fusible alloy from the pipe is extremely important as its presence may lead to blockages or corrosion and, in steel tubes, which may be subsequently heattreated, the presence of any alloy would cause inter-crystalline cracking.

The pipe is oiled first, to prevent the alloy adhering to the tube wall. It is next plugged at one end, pre-heated and then filled with the melted alloy. Once cooled, the pipe can then be bent as required.

Simple Bending Jigs

 After bending, the pipe should be unloaded, by immersing it in boiling water until all the alloy has run out. The pipe must then be cleaned internally to ensure that any alloy adhering to the walls of the pipe is removed. This is accomplished by using a ‘pull through’ with the pipe immersed in boiling water or by using a steam cleaner.

 A simple bending jig (refer to Fig. 1) is supplied with a range of rollers and stops and the pipe is bent using the correct combination of components checking the new pipe against either a template or the old pipe.

Simple Bending Jig Fig. 1 ISO 9001:2008 Certified PTC/CM/B1.1 Basic/M7/03 7.9 - 2

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Module 7 - MAINTENANCE PRACTICES

Category – A/B1

Sub Module 7.9 – Pipes and Hoses

Hand pipe-bending machines Hand pipe-bending machines are available for pipe sizes up to 12 mm (< ½ in) and for sizes of 12 mm to 25 mm (½ in to 1 in).  A typical hand pipe-bending machine (refer to Fig. 2) would have a matching former and guide for each pipe size in the range, giving a bend radius of approximately four times the pipe diameter.  A pressure indicator allows adjustment, so that when bending thin wall pipes, there is no risk of ‘wrinkling’ or ‘flattening’. The roller ensures that the load of bending is transmitted axially to the pipe, via the guide, which ensures that no sliding and so no damage, takes place between the guide and pipe.  Accurate bends can be made either from a drawing or a template, by following simple instructions when marking out the bend. This is especially true when the bends have to be a specific dimension from the end of the pipe or a series of bends have to be made to produce a complex shaped pipe assembly.

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Pressure Indicator

 Adjustable Stop

 Adjustable Screw

Former Roller

Bending Arm

Pipe Guide

Pull this way to Bend Pipe

Hand Pipe-Bending Machine Fig. 2

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Module 7 - MAINTENANCE PRACTICES

Sub Module 7.9 – Pipes and Hoses

BELLING/FLARING AIRCRAFT PIPES Flaring can be achieved only when the end of the pipe has been accurately squared off and cleaned out. Once a flare has been formed correctly, it should remain completely fluid tight at all normal pressures. Flaring tool Pipe flaring tools; come in a variety of sizes, with a range of pipe sizes that can be flared by each particular tool. A typical flaring tool (refer to Fig. 3), is used to flare tubes in the range 12 mm to 25 mm ((½ in to 1 in). Sets of half-bushes or dies cover the range of tube sizes for each machine. The flaring tool is usually mounted in a hand vice or some other rigid mounting. Once the half-bushes have been installed, the union-nut and collar are placed onto the tube and the tube is then clamped into the bushes, with the tube end flush with the end of the dies or half bushes. The threaded sleeve is slowly fed into the end of the tube whilst simultaneously turning the expander cone via the rotation handle. This spreads the end of the tube until it contacts the inner face of the bushes. A correctly finished flare should leave prescribed amount of the tube projecting from the collar. The finished flared end with the union nut and collar can be connected to a variety of other end fittings. These can include other pipes, and both internal and external adapters fitted to a number of different components.

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Module 7 - MAINTENANCE PRACTICES

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Sub Module 7.9 – Pipes and Hoses

Half Bushes Latch Fitting Threaded Sleeve Rotation Handle

Pipe Latch Fitting Securing Screw

Expander Cone Rotation Handle Expander Cone Threaded Sleeve

Pipe Flaring Tool Fig. 3 ISO 9001:2008 Certified PTC/CM/B1.1 Basic/M7/03 7.9 - 5

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Module 7 - MAINTENANCE PRACTICES

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Sub Module 7.9 – Pipes and Hoses

Standard flared pipe couplings Various types of standard flared pipe couplings (refer to Fig. 4), are available in aircraft fluid systems. These couplings have different angles and whilst they may look similar, they are not interchangeable. The AGS system uses a 32   flare whilst the  AN system uses flares of 74   included angle. Care must be taken to ensure that the correct couplings are fitted when manufacturing these pipes. Collar

Externally coned adapter

Internally coned adapter

Flared pipe

Union nut Pipe to external cone adapter

Spherical-ended adapter nipple

Sleeve

Spherical-ended adapter nipple Pipe to internal cone adapter

Pipe to pipe

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Standard Flared Pipe Couplings Fig. 4 For Training Purpose Only Rev. 00 Mar 2014

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Module 7 - MAINTENANCE PRACTICES

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Sub Module 7.9 – Pipes and Hoses

Flareless couplings The flaring operation leaves the tube end in a stressed condition which, as the flare takes a large amount of the vibration loadings, can result in fatigue failure.

Hooked Sleeve

Under-ightened pre-set To overcome this situation, the ‘flareless coupling’ was introduced. The flareless coupling, achieves its sealing properties by the deformation of a sleeve, built into the coupling (refer to Fig. 5). The end of this sleeve has a sharp, hooked shape, which is known as a ‘pilot’. It is the action of this sharp hook, cutting into the pipe, which provides the required sealing properties.

Correctly tightened pre-set

The individual parts of the coupling are assembled and the nut is simply screwed down on to its union until finger tight, then turned one further turn with a spanner. This action bows the sleeve and causes it to bite into the tube at its forward end.

Over-tightened pre-set

When the nut is slackened, the sleeve remains permanently bowed and attached to the pipe. This pre-setting can be done either with the service union or with a special hardened steel union that is only used for pre-setting. Flareless Pipe Coupling Pre-set Fig. 5

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Module 7 - MAINTENANCE PRACTICES

Sub Module 7.9 – Pipes and Hoses

 After pre-setting, the pipe should be inspected to ensure the sleeve is correctly bowed (It is permissible for the sleeve to rotate on the pipe). In service, the nut should be tightened until a distinct resistance is felt, then tightened further, - the amount depending on the tube size and material. Under no circumstances should the nut be tightened further to stop any leaks, this action will permanently damage the tube end and sleeve.

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Module 7 - MAINTENANCE PRACTICES

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INSPECTION AND TESTING OF AIRCRAFT PIPES AND HOSES Before any inspections can be done, it must be ensured that the components are scrupulously clean and that all critical areas are visible if the inspection is done while the component is in its normal, installed location (in situ). Rigid pipes should be inspected for signs of:   Chafing





Corrosion – both externally and internally where possible



Cracking of flared ends where appropriate



Deformation and Dents



Deterioration in condition of end fittings and their threads.

Sub Module 7.9 – Pipes and Hoses 

Date of manufacture – to ensure that it is within its prescribed life, and that it will remain so until the next inspection



Deterioration in condition of end fittings and their threads

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Flattening, kinking or twisting.

The relevant maintenance manual will state the intervals of inspections and the criteria which must be met before rigid pipes or hose assemblies may be considered fit for further service. Bore testing of pipes

Hose assemblies should be inspected for such defects as: 

Blistering – Both externally and internally where possible



Burn damage or discolouration



Chafing, circumferential cracking or crazing of the outer cover

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Pipes should be tested to ensure that the bore is clear and dimensionally correct after forming. One method of satisfying this requirement is to pass a steel ball, with a diameter of 80% of the internal diameter of the pipe, through the pipe in both directions. When the design or size of the pipe and end fittings, makes this test impractical or when a more searching test is required, the drawing will normally require a flow test to be performed. Hydraulic pressure testing of pipes Hydraulic pressure testing consists of firstly carrying out a flow test. This means a full bore flow by pumping fluid through the pipe and checking the flow at the open end. If this check is satisfactory, the open end should be suitably blanked.

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Once the flow test has been carried out, the oil pressure should then be built up to that prescribed on the drawing, usually 1½ times the maximum working pressure. The duration of the test must give the pipe a chance to show any leaks or other problems. Pneumatic and oxygen pressure testing of pipes These pipes are usually given an initial hydraulic pressure test, using water as the test medium, followed by a compressed air test that is limited to maximum system pressure. Using highpressure air during the test is very dangerous and the pipe(s) under test should be placed behind a protective screen and/or submerged in water.

Sub Module 7.9 – Pipes and Hoses

Testing flexible hoses Once the manufactured hose has been checked for satisfactory physical condition, the hose must be flow and pressure tested. The flow test will verify whether the hose inner lining is secure and not acting as a form of non-return valve. This is achieved by passing the fluid through the hose assembly both ways to confirm that there is an equal and free flow. Where a replacement hose has been manufactured in a local hose bay, a bore test may be done, in the same manner as that with rigid pipes, by use of a ball bearing being rolled in both directions through the hose. In this instance, however, the diameter of the ball should be 90% of the internal diameter of the hose’s end fittings.

Cleaning after test  After a pipe has been tested, it should normally be flushed out using a suitable solvent, dried out using a jet of clean, dry air and blanked off, using the approved blanks. Pipes that will be used in high-pressure air and gaseous or liquid oxygen systems must be scrupulously clean and free from any possible contamination by oil or grease. It is normal to recommend that pipes for use in these systems are flushed with Trichloroethane or some other suitable solvent, blown through with double filtered air and blanked-off, with the approved blanks immediately afterwards.

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The hose should then be ‘proof-tested’ by capping one end of the hose and applying the test pressure, usually twice the working pressure, to it for between one and five minutes.

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Sub Module 7.9 – Pipes and Hoses

INSTALLATION AND CLAMPING OF PIPES Prior to installation, the pipe should be checked to establish that it is of the correct type and that there is evidence of prior inspection and testing. This may involve checking the inspector’s stamp and part number. Once the pipe has been checked for signs of damage, dirt or corrosion, and found serviceable, it must then be immediately installed. When transporting lengths of pipe, especially long lengths, great care must be taken not to kink or otherwise damage the pipe prior to installation. Once in position, the pipes should be loosely placed into position in the supporting clamps, and adjusted so that the connections align correctly. The connections can then be tightened up, the clamps fastened and any bonding leads attached. Pipe supports Multiple pipe supports are often used to save space and these can be made from a variety of materials, such as fibre blocks, aluminium alloy, moulded rubber or nylon. The clamp halves are usually joined together and attached to the aircraft structure by bolts. It is important that the edges of the semi-circular recesses are not sharp and are of the correct size for the pipe in use.

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In some instances, packing will be installed between the pipe and the clamping material. This will usually be to reduce vibration or to insulate the pipe and clamp material, if they are likely to suffer from electrolytic corrosion. Individual pipe clamping is usually achieved using ‘P’ clips. These are light alloy loops with a rubber sleeve, which wrap around the pipe and are held by a single bolt to the aircraft structure. To avoid the risk of fretting occurring between the pipe and various parts of the aircraft, minimum dimensions must be observed between these components, which can be found in the AMM. The CAAIPs list these dimensions as 6 mm (0.25 in) from fixed structure, 18 mm (0.7 in) from control rods and 25 mm (1 in) from control cables, but the AMM must always take precedent. Connection of pipes When connecting pipes with brazed, flared or flareless couplings, there are a number of points to be considered. 

Union nuts must be free to rotate and can be slid back from the end of the pipe without fouling.



All loose items such as nipples and washers, are of the correct type and correctly located.



All pipe ends align correctly without any undue pressure on the pipe. (Pipes should never be forced into position, neither should they ever be pulled-up into position by their union nuts).

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Module 7 - MAINTENANCE PRACTICES

Sub Module 7.9 – Pipes and Hoses

Maintenance of pipes and hoses The correct methods of installing pipes and hoses (refer to Fig. 6) must be followed if damage (and possibly disaster) is not to result. Pipes attached to the airframe structure, are often shielded and will not usually be liable to accidental damage. Other pipes may be located in exposed positions, where they may be susceptible to damage or corrosion. Pipes located in wheel bays or attached to an undercarriage leg could easily be damaged by stones and mud or corroded by thrown-up water. Some pipes may be badly sited and may be subject to abuse from carelessly performed and unrelated servicing activities. Chafing can occur in many places, such as clamps and clips, so care must be shown to eliminate or at least reduce the chances of this happening. Cracking of pipes can occur when pulsations are present and/or the pipe has sharp bends. This risk must also be considered when inspecting pipe runs. Liquid leaks can be found by the presence of fluid, or at least dampness, on the pipe or clamps. Gaseous leaks must be searched for using one of the proprietary leak-detecting fluids. The relevant AMM will give details on how a particular hose is installed in the aircraft, but, in general, a hose should be at least 3% longer than the maximum distance between end fittings. Consideration should also be given to the orientation of a hose and, once correctly installed, the witness lines, marked on the hose, should be straight.

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Module 7 - MAINTENANCE PRACTICES

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Sub Module 7.9 – Pipes and Hoses

Structure

Hose to tight

Hose twisted and under tension

Hose correct tension Correct and Incorrect Methods of Hose Installation. Fig. 6

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Sub Module 7.9 – Pipes and Hoses

Pipe identification tape Once a pipe has been fitted to the aircraft, it should have system identification tape attached to enable engineers to identify which system each pipe belongs to. The tape comes in rolls of about 25 mm wide and uses colours, symbols and letters to differentiate between different pipes. A small length of the tape is wound around the pipe at convenient points.

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