Disassembly and re-assembly, in the terms of aircraft, can cover a range of activities from complete airframes down to component maintenance, with several steps in between. The reasons for dismantling and re-assembly may include:
Complete airframe disassembly for road/air shipment
Replacement of major components/modules
Replacement of minor components/modules
Disassembly & re-assembly of major components
Disassembly & re-assembly of minor components Complete Airframes
It may be necessary to dismantle a complete aircraft for the purpose of transportation by road or by air. This could be for recovery from an accident site, remote from the airfield or for movement of the aircraft when it is totally non-airworthy, due perhaps to severe corrosion or an unknown maintenance history.
Because many larger, modern aircraft are manufactured at several different locations, the completed modules are assembled in the final build hall of the primary manufacturer.
The joining points are often known as ‘transportation joints’, and, in extreme instances, can be the points where the aircraft may be dismantled again to allow transportation (refer to Fig.
20).
Typical Manufacturer’s Joining Points Fig. 20
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The instructions for the dismantling operation will be found either in the aircraft’s Maintenance Manual or in a special dismantling procedure, issued by the manufacturer. During the dismantling operation, precautions must be taken to prevent injury and damage.
General precautions would include such items as:
The aircraft should, if at all possible, be dismantled within a hangar. If this is not possible, then level and firm ground will suffice
Sufficient clearance in the hangar must be available, both to clear the airframe when on jacks, and to allow heavy lifting cranes enough room to manoeuvre over the aircraft
All precautions, in accordance with the manufacturer’s instructions, must be taken prior to the aircraft entering the hangar; such as de-fuelling and the removal of devices such as emergency oxygen canisters
When the aircraft is jacked-up, all trestles must also be placed in position. This allows the aircraft to be climbed upon and, later, ensures that it will not overbalance when a major part (such as a wing), is removed.
Whilst the manufacturer’s instructions will give the details for a specific aircraft, the following sequence of dismantling gives an idea of the correct order of disassembly:
Main planes
Tail unit
Undercarriage units
Centre section
Fuselage.
Re-assembly is in the reverse order of disassembly, and all parts must be cleaned, protected and serviceable, prior to their installation.
Replacement of Major Components/Modules
This type of operation will normally be completed at a large maintenance base, where all the required equipment is available. An example could be the replacement of a wing that has suffered major damage.
Other types of similar work might be the replacement of damaged wing tips, empennage surfaces and nose cones. If the aircraft is at an ‘outstation’ when the damage occurs, confirmation should be sought as to whether the aircraft can be flown back to base for repair, or repaired where it is.
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Replacement of Minor Components/Modules
Some components have to be repaired by replacement. Repairs to flying control surfaces, for example, are often done in a repair bay. The component may be replaced on the aircraft by a serviceable item, or reinstalled once the repair in the bay has been completed.
Other components, which are replaced first and repaired later, might include some structural items such as doors of all types, and most fairings and cowlings. Most of these tasks are simple removal and replacement operations that are covered in the AMM.
Disassembly and Re-assembly of Major Components
Most of the work done, during this phase of maintenance, is scheduled in with normal aircraft maintenance. The components may not only be removed and reinstalled at different times during the maintenance, but work will also be done on the items whilst they are removed. They may also be removed to allow access to other parts of the airframe during the maintenance.
Items such as engines, propellers, landing gears and wheels require some form of maintenance. This may include a simple condition check, or a full overhaul of its component parts, allowing checks on internal component parts for wear, damage and corrosion.
The full procedure for this type of work will be carried out in accordance with the CMM. This book will give all the operations required to dismantle the component and will advise what to look for whilst the item is undergoing maintenance. It will also state the re-assembly method, including the fitting of new parts such as seals, gaskets, oil and other consumables that have to be replaced, during overhaul.
Disassembly and Re-assembly of Minor Components
A typical passenger aircraft can contain hundreds of small components that work together as parts of a larger system. This can include a wide range of hydraulic and pneumatic components that can be mechanical, electromechanical or electrical in operation.
Other components might include those installed into fuel, air conditioning, pressurisation, electrical and electronic systems.
These components have their own CMM to allow maintenance and trouble-shooting to be done. Some components are only removed once they fail (On-Condition), while others receive regular maintenance.
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Instruments, electric and electronic components can be dismantled and serviced by the aircraft operator. It normally requires the use of a dedicated overhaul facility, which can provide the correct environmental conditions and is equipped with the special test equipment required to carry out maintenance and repair.
Operators of smaller aircraft, or those who operate only a few aircraft, will usually send components requiring repair or maintenance to a ‘third party’ maintenance organisation. This company will have the special facilities, equipment and personnel, to complete the required work on components from a number of different customers.
Basic Disassembly and Re-assembly Techniques
All of the previously mentioned procedures require the use of the correct techniques over a wide range of working practices.
These techniques will ensure that the components are removed, dismantled, re-assembled and re-installed in accordance with both the relevant manuals and using the correct ‘standard practices’.
An AMM and CMM dictate the correct type and size of locking wire or split pin to be used during overhaul or maintenance of a component. These publications also stipulate exact detail of items such as the lock wire angle of approach and the correct positioning of a split pin. AMM chapters 20 and 70 list the standard practices that should be used during overhaul.
Other locking devices include items such as single tab washers, shake-proof washers, circlips and locking rings. Some can only be used once only whilst others are re-used provided they are still serviceable.
The replacement of spring washers is ‘advised’ during overhaul and repair, especially on engines and pumps. Other devices used for locking or holding fasteners in position, such as multi-tab washers and locking plates can normally be reused.
Stiff nuts with fibre or nylon inserts can be checked to ascertain if a certain degree of stiffness is still available. If the nut can be run along a thread by hand it should be replaced. They should not be used in high temperature areas.
In all matters relating to aircraft, the manufacturer has the final say on which fasteners can be reused and which must be replaced.
Because friction is essential to keep the fasteners secure, sometimes it is necessary to do a ‘torque check’ on the bolt/nut combination, in order to confirm their continuing serviceability.
This is especially true of all metal fasteners that can normally be re-used.
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The majority of nuts, bolts and set-screws, on an aircraft, are subject to a standard torque value. This depends on their material, finish, lubrication, thread type and size, although the manufacturer’s torque value will be the correct one to use.
The correct torque loadings are normally applied using a torque wrench that has been previously calibrated to the correct value.
In some special instances, pre-load indicating (PLI) washers may be specified.
When assembling any component or major airframe part, the manufacturer will specify whether the torque value is ‘lubricated’
or ‘dry’. Lubricated values are measured with the threads and all mating surfaces lightly lubricated with oil, sealant or anti-seize compound as appropriate.
When assembling some components, it may be vital that certain alignments, dimensions or profiles are achieved. During initial production, most of the airframe and many of the components are assembled in a jig.
A jig is device that allows the manufacture, repair or rigging of components to a high dimensional accuracy. This guarantees consistency over a number of components. The jig holds all of the items securely, so that, when assembled, the whole component is exactly the shape that the designer has stipulated.
Jigs are used to build fuselage and wing sections in the factory.
They are also used to ensure that small actuators are pre-set to the exact length, to assist in ‘rigging’ the controls containing the
actuator.
Small Part and Component Identification
When disassembling or removing any component, it is vital that all small items such as bolts, screws, nuts, washers and shims are clearly identified.
This involves not only identifying the items by part number, but also recording their correct location and which aircraft they have been removed from as, in some hangars, more than one aircraft may be in a state of disassembly at any one time.
Some items may simply be attached to the major assembly using many small ‘tie on’ bags with identification labels. If a number of different sized fasteners are removed from a component such as a windscreen, they can be located in a locally-made holding jig which keeps the different parts in their same relative position to the original item. This should allow all the screws to be returned to their original locations when the screen is reinstalled.
Any part which is removed must have its identity and location retained until it is reinstalled.
Discarding of Parts
A number of items, when they are removed from their original position, have to be discarded. The ‘once only’ policy is a combination of the manufacturer’s recommendations together with normal engineering practice.
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Items that are usually discarded at removal are filters, sealing rings, desiccants, fuels and oils of all types.
There are many other items that have a given ‘life’. This may be counted in flying hours, calendar time or operating cycles, which will mean that items have to be replaced throughout the life of the aircraft.
If aircraft, or major components of them, have been disassembled, it may be policy to replace components with
‘zero life’ items prior to re-assembly. This will allow the aircraft to fly for considerable time before any parts become due for replacement.
Freeing Seized Components
When dismantling any part of an aircraft, it is not unusual for the technician to encounter a seized fastener. Depending upon its location, the AMM may recommend a range of actions to assist in the removal of the item(s).
These actions may involve use of penetrating oil, which works its way down through the seized threads, providing both an anti-corrosion action and lubrication for the threads. Other actions may involve the application of heat or cold to a specific part, so that their relative diameters change, thus lowering the friction between the parts.
Use of Correct Tools
It is normal for technicians to own a comprehensive tool kit, containing tools recommended for the work planned and which are of the highest quality. In a number of situations, it may be possible for a technician to use an incorrect tool that may appear to be the correct one for the task. It is most important that only the correct tools are used for each and every maintenance task.
When, for example, using a cross point screwdriver, it is possible to find at least six different types of screw bits, each of which only fits its own respective screw head slot.
The removal of nuts from bolts is normally accomplished using a socket and wrench set and these sockets can be of the twelve-point or six- point type. When spanners need to be used, preference should be given to a ring spanner rather than an open-ended spanner. Adjustable spanners or ‘mole grips’
should never be used on aircraft.
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The manufacturer of the aircraft often specifies special tools, when standard tools are unable to complete the task. Unless approved to do so, the technician should never substitute conventional tools for the special tools that are called for in the manual. Damage to the part being worked upon will almost always result from the use of incorrect tools.
‘Murphy’s Law’
This ‘law’ states that:
‘If a part or component can be installed incorrectly, someone, somewhere will install it that way.’
There are numerous solutions in the fight against this problem.
For example, when pair of pipes or hoses are to be joined, there is the risk of the two pairs of couplings being ‘cross connected’.
This could result in serious damage if the pipes carried fuel and hydraulic oil.
To prevent this happening, pipes and couplings usually have different diameters. Alternatively, the two sets of couplings would be located at different places, so the pipes could not be wrongly connected under any circumstances.
The same logic is applied to control cables that, of course, must also never be cross-connected. In this instance, the turnbuckles are located at slightly different locations at each cable break, again making it impossible to connect the wrong pair of cables together.
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TROUBLESHOOTING
Troubleshooting is a form of problem solving, often applied to repair failed products or processes. It is a logical, systematic search for the source of a problem so that it can be solved, and so the product or process can be made operational again.
Troubleshooting is needed to develop and maintain complex systems where the symptoms of a problem can have many possible causes. Troubleshooting is used in many fields such as engineering, system, administration, electronics, automotive repair, and diagnostic medicine. Troubleshooting requires identification of the malfunction(s) or symptoms within a system.
Then, experience is commonly used to generate possible causes of the symptoms. Determining the most likely cause is a process of elimination - eliminating potential causes of a problem. Finally, troubleshooting requires confirmation that the solution restores the product or process to its working state.
In general, troubleshooting is the identification of, or diagnosis of "trouble" in the management flow of a corporation or a system caused by a failure of some kind. The problem is initially described as symptoms of malfunction, and troubleshooting is the process of determining and remedying the causes of these symptoms.
A system can be described in terms of its expected, desired or intended behavior (usually, for artificial systems, its purpose).
Events or inputs to the system are expected to generate specific results or outputs. (For example selecting the "print" option from various computer applications is intended to result in a hardcopy emerging from some specific device). Any unexpected or undesirable behavior is a symptom.
Troubleshooting is the process of isolating the specific cause or causes of the symptom. Frequently the symptom is a failure of the product or process to produce any results. (Nothing was printed, for example).
The methods of forensic engineering are especially useful in tracing problems in products or processes, and a wide range of analytical techniques are available to determine the cause or causes of specific failures. Corrective action can then be taken to prevent further failures of a similar kind. Preventative action is possible using failure mode and effects analysis (FMEA) and fault tree analysis (FTA) before full scale production, and these methods can also be used for failure analysis.
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Flight line aircraft maintainers have an excellent record of identifying and correcting causes of system and subsystem failures and keeping highly complex, aging aircraft flying.
These technicians continue to troubleshoot an aircraft until the problem is isolated; sometimes aided and sometimes hindered by current maintenance and logistics processes and procedures or even the maintenance environment itself. It takes little analysis to determine that the success or failure of the troubleshooting process hinges on the technician.
The AF has placed great emphasis on enterprise process improvements and cost reduction programs. However, it has placed less emphasis on the processes and systems supporting the technician in making effective troubleshooting decisions at the task level. Thus, while the current troubleshooting environment is functional, it is often inefficient. The AMIT program's purpose was to identify the factors (especially human factors) that contribute to inefficiency in troubleshooting intensive portions of the maintenance process, and devise proven (i.e., field tested) means to improve efficiency. If those means are demonstrated to be statistically significant, their introduction into the AF maintenance environment should have a meaningful, positive impact on aircraft availability.
Terminology
For this effort, the AMIT Team defined troubleshooting as "a process that includes all activities related to determining the correct repair action." This included all activities from problem notification through repair verification; understanding the current discrepancy, technical data research, historical maintenance data research, determining aircraft condition, and communication.
Efficiency and inefficiency are inverse indicators of a technician's troubleshooting proficiency, where proficiency is a vectored quantity (i.e., a mathematical representation of a physical phenomenon where direction is as important as the value; "velocity" and "force" are two such physical phenomena).
In other words, proficiency is a combination of the time it takes (troubleshooting time) to make the right decision (troubleshooting accuracy).
The AMIT program did not measure AF technicians' troubleshooting proficiency quotients or even attempt to develop a method for doing so. Had that been done, however, an efficiency distribution with an upper bound of some practicable state of proficiency and a lower bound of an acceptable state of sufficiency would result as shown in Figure 1 (Gott, S. P. 1998).
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ISO 9001:2008 Certified For Training Purpose Only
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7.18 - 50 Mar 2014
A statistical analysis of this theoretical efficiency distribution would yield a dividing line. Those troubleshooters with proficiency quotients to the right of the line would be considered
A statistical analysis of this theoretical efficiency distribution would yield a dividing line. Those troubleshooters with proficiency quotients to the right of the line would be considered