Avionics Assembly and Disassembly Techniques PDF

Avionics Assembly and Disassembly Techniques Aircraft Maintenance Practices There is normally an excellent description of how to remove and refit components from the aircraft in the Maintenance Manual, however some general aircraft maintenance practices information would be considered...

Avionics Assembly and Disassembly Techniques Aircraft Maintenance Practices There is normally an excellent description of how to remove and refit components from the aircraft in the Maintenance Manual, however some general aircraft maintenance practices information would be considered common knowledge. The Maintenance Manual information should always be used as the primary reference. Fitting Grommets Standard grommets are available in silicon, rubber, nylon, and TFE. The type of grommet to be installed must be suitable for the environmental conditions. The grommet material and upper temperature rating table indicates the temperature ratings of various grommet materials. Grommet Material Upper Temperature Limit (Degrees Celsius) TFE 260 Silicon, rubber, hot oil and coolant resistant 120 Nylon 85 If it is necessary to cut a nylon grommet in order to install it, make the cut at an angle of 45 degrees as shown in the cut grommet image. Cement the grommet in place with general purpose cement, with the cut at the top of the hole. Aviation Australia Cut angle for nylon grommet installation 2023-02-06 B-07c Maintenance Practices Page 81 of 255 CASA Part 66 - Training Materials Only  When installing caterpillar grommets, cut the grommet to the required length, making sure to cut square across the teeth as shown in Figure. Cement the grommet in place with general purpose cement, with the cut at the top of the hole. Aviation Australia Caterpillar Grommet Connections to Terminal Boards and Busbars Install terminal lugs on terminal boards in such a way that they are locked against movement in the direction of loosening. A maximum of four lugs or three lugs and one bus shall be connected to any one stud. Aviation Australia Terminal Board 2023-02-06 B-07c Maintenance Practices Page 82 of 255 CASA Part 66 - Training Materials Only  Plug and Connector Inspections Any join in a conductor be it a plug/socket, splice, terminal block or soldered joint is a place that causes a large percentage of all faults. The list of connector faults can include; contamination, corrosion, bent pins, pushed back pins/sockets etc. The possible faults this can cause make it cost effective to visually inspect any connector before making the connection. Aviation Australia Bent pin on a PCMCIA plug Aviation Australia Damage caused by a bent pin to the PCMCIA socket Once connected different connectors have different methods of combating connection issues due to vibration. Bayonet connectors have a positive lock i.e. an indent in the track the pin slides against. Threaded connectors may have a ratchet mechanism that stops them being undone, they must be replaced if this becomes unserviceable, other threaded connectors require lock wiring to stop them rattling loose. All connectors must have a positive locking device to stop it coming undone due to vibration. Bayonet connectors must be pushed in before they can be turned to undo them, some threaded connectors have a ratchet and sprung pawl which is sufficient to counter the vibration. Threaded connectors with no positive locking device must be wire locked in a similar manner to that for a bolt, i.e. pulling in the direction of tightening and tight. 2023-02-06 B-07c Maintenance Practices Page 83 of 255 CASA Part 66 - Training Materials Only  Wire locking connector Care must be taken here with terminology as some American texts refer to stainless steel wire locking as safety wiring. In our context safety wire is copper lock wire available in sizes from 15 - 32 thousandths of an inch (thou). This would be used for a connector without a positive locking device that has to be undone in an emergency. The reason for copper is that it will stop vibration or crew inadvertent disconnection, but in an emergency the wire can be broken, and the connector disconnected without resorting to tools. In military aircraft it may be used to secure guarded switches in the cockpit to stop inadvertent operation. Copper safety wire 2023-02-06 B-07c Maintenance Practices Page 84 of 255 CASA Part 66 - Training Materials Only  Electrostatic Sensitive Devices (ESD) Precautions ESD Safety precautions are covered more fully in Module 5. The following list is a general guide to the safe handling of ESD. Do NOT touch pins of any electronic component Do NOT touch sockets or pins within the plugs of any electronic equipment When removing ESD sensitive equipment from aircraft – aircraft must be grounded, and power removed Prior to disconnecting cables – personnel shall touch the equipment metal case to equalise any electrostatic potentials Immediately fit conductive covers/caps to disconnected plugs When installing electronic equipment in aircraft – touch plug outer shell to outer shell of equipment mating connector to aircraft earth equalise electrostatic potential. Unknows Avionics bay rack 2023-02-06 B-07c Maintenance Practices Page 85 of 255 CASA Part 66 - Training Materials Only  Avionics Component Installation Avionics equipment racks in the aircraft provide the Line Replaceable Unit (LRU) with rigid mounting to withstand G-forces and vibration, supply cooling and electrical connection to the rest of the aircraft. Avionics equipment mounting rack In modern transport category aircraft, the mounting restraint comes from tapered pins on the rack and alignment holes at the back of the LRU and a screw latch at the front. Avionics rack securing devices The connector block on the back of the rack normally has a set of key pins to ensure the wrong LRU is not connected in the wrong place. These pins fit into keyways in the back of the LRU. 2023-02-06 B-07c Maintenance Practices Page 86 of 255 CASA Part 66 - Training Materials Only  With these types of connection it is up to the engineer to physically push the LRU into the connector, this must be done with care, first it is necessary ensure any plug covers are removed then to inspect the pins in the connector to ensure nothing has been damaged, as with other connectors look for bent pins contamination and corrosion. Avionics equipment connectors Next settle the LRU onto the rack ensuring it is sitting flush on the base and centralised. Gently push the LRU back into the rack until the support pins and connector keys engage after this has been accomplished more firm pressure is required to fully engage the LRU. Lastly do up the knurled knob until the LRU is firmly held into the rack. The front hold down screws are then raised until the cup washer hooks over the catch on the front of the LRU and tightened finger tight, there is no requirement for tools to fit or remove these, some may have a ratchet under the knob requiring it to be pulled out in order to loosen There are other types of securing systems used to hold LRU’s at the front lower edge and are wound into the rack with a torque limited knob other may use part of the handle as a lever and latch system these types also requires the plug inspection before fitting. 2023-02-06 B-07c Maintenance Practices Page 87 of 255 CASA Part 66 - Training Materials Only  Cockpit LRU Fitment The majority of control panels in the cockpit are held in place with a Dzus fastener and rail system. To remove the fastener is rotated 90 degrees anticlockwise and a spring will push the clip out if forced further the clip will be damaged, to fit the fastener should be pushed down and rotated until locked again the lock is only 90 degrees from where it pushes down. When the LRU is locked in the fastener slot will be across the aircraft or horizontal in a vertical panel, when unlocked the slot will be along the rail or vertical. Cockpit LRU Dzus fastener Dzus fasteners and rail system Each aircraft has a different method of securing the cockpit displays into the instrument panels, below on the left the LCD display has a cam mechanism in the handle which will disengage it from the rear connector as well as being used to engage the connector and securing the display into the panel the handle is then secured by the 2 spring clips indicated. The LCD screen on the right is secured by 4 screws which also secure the handle in the stowed position. 2023-02-06 B-07c Maintenance Practices Page 88 of 255 CASA Part 66 - Training Materials Only  Location of glass cockpit locking devices Alternate location of glass cockpit locking devices Some light aircraft use a single captive Hex screw which when unscrewed will push the LRU out of its rack disconnecting as it does so. All that is required is a long Allen Key of the appropriate size, the crew may be central of offset to one side as shown, depending on where the electrical connector is situated. 2023-02-06 B-07c Maintenance Practices Page 89 of 255 CASA Part 66 - Training Materials Only  GA cockpit LRU securing device The majority of instruments may have a flange with a screw at each corner. Most of these instruments are mounted from the rear with the flange hidden from view it is important to realise this is not the only way to mount them. Conventional instrument mounting flange Some instruments are fitted into clamps an have no flanges of their own, this allows the instrument to be removed from the front of the panel. With clamped instruments there is no need to drop the panel for access to the rear or even loosen the clamp from the panel. To remove the instrument the clamping screws are loosened then the instrument can be withdrawn from the panel and electrical connectors disconnected. 2023-02-06 B-07c Maintenance Practices Page 90 of 255 CASA Part 66 - Training Materials Only  Conventional instrument mounting clamp Instrument clamp - loosening and removing Be aware if the clamp is unscrewed from panel first them the screws from the clamp removed the clamp may fall behind the panel being difficult and time consuming to remove. In the picture above the horizontal arrows point to the mounting screws and vertical arrows point to the clamping screws 2023-02-06 B-07c Maintenance Practices Page 91 of 255 CASA Part 66 - Training Materials Only  Avionic Troubleshooting Techniques Defect Prevention Aviation maintenance consists of many separate tasks. For example, engineers are expected to perform routine preventative maintenance activities, which normally consist of inspecting, adjusting, lubricating, replacing, etc. They may or may not find anything broken during preventative maintenance. In fact, the intent of preventative maintenance is to keep things from breaking. Maintenance tasks include upgrade or overhaul existing systems or components even though these systems appear to be functioning properly. Such tasks are commonly done in response to regulatory requirements or to take advantage of technological advances. The basis for judging the efficiency and effectiveness of a maintenance organization, and of individual maintenance workers, is the ability to find and fix problems efficiently. Especially in today's competitive air carrier business environment, maintenance groups are judged on their ability to keep aircraft safely in the air not on the ramp or in the hangar. Built In Test Equipment (BITE) Early aircraft were relatively simple, and maintenance was straightforward. As aircraft and their components became more sophisticated, diagnosis and repair became more complex. As part of this evolution, automated test capabilities became part of the maintenance engineers "toolbox". Automated Test Equipment (ATE) describes a broad range of partially or fully automated test capabilities which require the LRU to be removed from the aircraft and sent to the avionics bay. There is no universally accepted definition of ATE, the equipment spans the range from extremely sophisticated and expensive test consoles to simple equipment that performs programmed checks on a single avionics module's output. All ATE is either set up to perform a series of tests without requiring human intervention or can be programmed to do so. BITE as with ATE, has evolved with the increased sophistication of aircraft subsystems. BITE is included in the design of a component, module, or subsystem. BITE also describes an extremely broad range of equipment, complexity and sophistication. The simplest BITE might allow a technician or flight crew member to perform a "go/no-go" self-test on a specific module. A more-sophisticated BITE system might allow a technician to perform tests with built-in switches and indicators on a module. The most-sophisticated BITE records performance information over a number of flight legs, diagnosing failures and displaying specific instructions to engineers on how to repair the system. 2023-02-06 B-07c Maintenance Practices Page 92 of 255 CASA Part 66 - Training Materials Only  Troubleshooting Flow Chart One type of maintenance Job Performance Aid (JPA) is called a "Flow Chart" in some manuals. A flow chart is a printed or computerised chart that directs the maintenance technician along a logical testing and diagnosis path for the particular system. After each test or observation, the flow chart branches to another test (or conclusion) based on the test results. An easy characterisation of a decision tree is a series of "if-then" statements, e.g. "If the voltage is below 'x,' then do this." Trouble shooting flowchart 2023-02-06 B-07c Maintenance Practices Page 93 of 255 CASA Part 66 - Training Materials Only  No Fault Found (NFF) Repairing or replacing avionics components is hardly the biggest cost in maintenance. Avionics do not wear out regularly like some other aircraft systems. They just break down, often unexpectedly. Unexpected maintenance is more expensive than scheduled repairs. And there is a frustrating maintenance problem that seems to afflict avionics components disproportionately the No Fault Found (NFF) unit. An avionics LRU may be removed because a pilot or mechanic finds something faulty in its performance. But when it gets back to the workshop, it tests as perfectly good, or is “No Fault Found.” These NFFs cost labour, time to remove, logistics costs to move often long distances expensive time to test on costly equipment and require burdensome increases in inventories. Far worse, NFFs may cause very expensive delays or flight cancellations, apparently for no good reason. NFFs are common in avionics and can be a major proportion of components sent to workshops for inspection. A numerically small problem is the poor behaviour of some LRU’s. Sometimes called “rogue” units, these are components sent to shops repeatedly and frequently always for the same reason, but they always perform perfectly on the test bench only to fail functional tests in the aircraft. But the majority of NFF’s are caused by poor troubleshooting techniques showing the importance of training. 2023-02-06 B-07c Maintenance Practices Page 94 of 255 CASA Part 66 - Training Materials Only  Troubleshooting Terminology Heuristic Many strategies can be used to troubleshoot. There are written, step-by-step procedures, half-split algorithms, etc. One category of troubleshooting strategies consists of heuristics. Heuristics are simply rules-of-thumb useful for problem-solving. A typical heuristic might be to test the most unreliable component first, since it is known to fail often. If your torch does not work, try changing the battery first. Consistent Fault Set (CFS) Consistent Fault Set is one of the names given to the group of all possible failures that can reasonably explain a given set of trouble symptoms. The name comes from the fact that the group contains faults "consistent" with the symptoms. For example, when an automobile won't start, the CFS could contain an ignition unit failure, but would not contain a failed seat back adjustment control, even if issued a CFS list it does no more that give a starting point for fault finding equal to that of an engineer that knows how the system works. Shotgun Approach One method of troubleshooting is to replace various LRUs randomly until the symptoms of trouble disappear. This method is known as "Shot gunning" because a technician never really knows where he or she will find the failed part. Shot gunning is an extremely inefficient, expensive way to find a problem and a cause of NFF LRUs. Hypothesis A hypothesis is an assumption presumed to be true, but that must be proven or disproven by objective testing. In the course of troubleshooting, a maintenance technician needs to adopt one, or more, hypothesis as to what is causing the symptoms. Researchers have identified three relevant aspects of hypotheses. First, fault finding technicians tend to give more weight to information found early in the diagnostic process, i.e., we make up our minds quickly about what is causing a problem. Second, maintainers tend to adopt only a few hypotheses, even when a much broader range of hypotheses is consistent with the observed problem. Third, once a maintainer adopts a hypothesis, he or she tends to look for evidence supporting it while discounting evidence that refutes it. 2023-02-06 B-07c Maintenance Practices Page 95 of 255 CASA Part 66 - Training Materials Only  Psychic Blindness Psychic blindness describes a phenomenon discovered in the early 1940's and since shown to exist in different domains. Researchers have found that when people have spent time solving one particular type of troubleshooting fault, it is virtually impossible for them immediately change to diagnose a different type of problem. This phenomenon holds even when people are told that they will see a new and different type of malfunction, they try to find symptoms to point to the fault seen before. This mindset points the engineer to assume is it must be the same faulty unit as I had before. Tunnel Vision Tunnel vision describes viewing a situation as though through a tunnel, i.e., seeing in only one direction and being blocked from seeing information coming from conflicting sources. In the maintenance domain, tunnel vision is a well-known occupational hazard. Once a trouble shooter thinks he or she knows what is causing a problem, information that might disprove the hypothesis tends to be given less weight than information confirming it. Half-Split Test Theoretically, the most efficient test a maintenance technician can perform is the one that provides the most information. Early troubleshooting research involving electronic circuits found that the "best" test is the one eliminating roughly half the items from a set of possibly failed components. Such a test is called a "half-split" test. Test-Induced Failure When testing a system or component, there is some probability that the test will cause a failure. Because all systems and components will eventually fail, the need for testing should be balanced against the likelihood of a test induced failure. Test-induced failures are safety risks only when they remain undetected. For example: if a connector is disconnected to test for a voltage and when reconnected a pin is bent at best creating an open circuit in the same or another circuit that runs through the same connector. Or when testing a subsystem, that is functioning properly and turn it off which causes a voltage spike damaging the system. If there is a test-induced failure, the component will be left in a failed state and will not work the next time it is needed. One aspect of this is if the technician decides he requires a continuity test to prove a fault, disconnecting a connector may work as part of their fault finding but introduce a fault on another system. On this occasion the technician must ensure a functional test for all systems affected by the disconnection. 2023-02-06 B-07c Maintenance Practices Page 96 of 255 CASA Part 66 - Training Materials Only  Half Split Mindset The best model for troubleshooting is to see the system as a set of problems to be solved to get the final output of the system. The problem/solution mindset is so critical in really understanding how equipment really works, and hence how to troubleshoot it. The essential points are these: Understand the function being solved in the equipment. Understand how an error if one part affects the output from another part, so how the solution of each problem affects other problems solution’s, Understand this implementation of each solution. Having this kind of information in your head will help you pull in detail where needed to fill in the models of each system; just as you cannot keep all of the systems in your head at once, you also cannot effectively troubleshoot without a reservoir of more detailed knowledge about each system, or the ready ability to absorb more information about each system as needed. Having a problem/solution mindset also helps keep you focused in troubleshooting. You have built models of each system, and you have learned to think in terms of problems and solutions. What about technique? This is, the step that everyone wants to jump to first, but I would strongly suggest that the technique of troubleshooting goes hand in hand with the models and the mindset of troubleshooting. If you do not have the models and the mindset, it is going to be more difficult to accept the result. In terms of technique, half split, this technique consists of these basic steps. 1. Trace out the entire path of the signal. This is the first place where your knowledge of the system you are troubleshooting comes into play; if you cannot trace the path of a signal (a flow, or data, or even the way data is passed between layers in a network), then you cannot troubleshoot effectively. 2. Find a halfway point in the path of the signal. This halfway point would ideally be in a place where you can easily measure the signal, but at the same time effectively splits the entire signal path in half. 3. One mistake made in troubleshooting is to start with the easiest place to measure, or the points in the flow they understand the best. If you don’t understand some part of the data path, then you need to learn it, rather than avoiding it. System knowledge is crucial to troubleshooting. If you have inaccurate models of the system in your head, you are going to fail to troubleshoot a problem. 4. Measure the signal at this halfway point. If the signal is correct, move closer to the end of the signal path. If the signal is incorrect, move closer to the source of the signal. 2023-02-06 B-07c Maintenance Practices Page 97 of 255 CASA Part 66 - Training Materials Only  The Half Split method is time proven across many different fields, from electronics to mechanics, from electrical to fluid dynamics. It might seem like a good idea just to “jump to what I know,” but this is a mistake. Troubleshooting Example An engineer was called out with another technician to repair a radar system. The transmitter just was not producing power. There was a resistor that blew in the “right area” all the time, so they checked the resistor, and sure enough, it appeared to be short circuited. They ordered another resistor, shut things down, and went home. The next day, the part arrived and was installed by someone else. The resistor promptly showed a short again, and the radar system failed to work. What went wrong? They checked what was simple to check, what was a common problem, and walked away thinking they had found the problem. It took another day’s worth of troubleshooting to find the real problem a component that was in parallel with the original resistor, had shorted out. The resister showed a short because it was in parallel with another component that was short circuited. Lesson learned: do not take short cuts, do not assume the part you can easily test is the part that is broken, and do not assume you have found the problem the first time you find something that does not look right. Make certain you try to falsify your theory, instead of just trying to prove it correct. 2023-02-06 B-07c Maintenance Practices Page 98 of 255 CASA Part 66 - Training Materials Only  Troubleshooting Keypoints 1. Build accurate models of the system and all subsystems as possible. This is probably where most failures to effectively troubleshoot problems occur, and the step that takes the longest to complete. In fact, no-one ever really completes this step, as there is always more to learn about every system, and more accurate ways to model any given system. 2. Have a problem/solution mindset. This is probably the second most common failure point in the troubleshooting process. 3. Know the system you must fault find on sufficiently well to recognise failures. Training/ studying the system is important, you must be able to differentiate between the system working optimally and when it is not working correctly. 4. Read the technical manuals to enable you to understand Wiring diagrams, Schematic Diagrams, Maintenance Manuals description of operation etc. 5. Have the skill and knowledge to operate the test equipment and understand the results of tests. 6. Troubleshooting must be carried out in a logical manner with no shotgunning or trial and error methods. 7. Use all the human senses. See the senses used when troubleshooting systems table for further information. 8. Recognising that a system is operating below optimum level or that a fault exists may not be as simple to recognise as one would think. Low transmitter power, decreased sensitivity to a sensor input, slight audio distortion, are a few of the hard to recognise from an extensive list of hard to recognise faults. Aviation Australia Senses used when troubleshooting systems 2023-02-06 B-07c Maintenance Practices Page 99 of 255 CASA Part 66 - Training Materials Only  The fault that has been reported when the user notices what they discern as a problem may not be technically accurate. Be able to decipher non-technical terminology and discern a way of replicating the situation in which the fault was said to have occurred. Diving straight into trouble shooting on just the report of a fault is a doubtful process, it could result in spending hours of company time chasing a non-existent fault. This may involve extensive setup to simulate the conditions as well as testing all possibilities of “finger trouble” (operator error). Before extensive testing is performed confirm that there is an actual fault to find. Once the fault is confirmed there is a tendency for the inexperienced technician to conclude the cause of the fault at this stage, instead the diagrams and manuals should be consulted to gain further information excluding areas of the equipment and narrowing the area of the system requiring more testing. Aviation Australia Complex system schematic Having a sound knowledge of complex systems before troubleshooting them can quickly allow the elimination of large chunks of a system. 2023-02-06 B-07c Maintenance Practices Page 100 of 255 CASA Part 66 - Training Materials Only  Troubleshooting Example The operator reports that the output from L is incorrect. Before starting to test the system we could ask if M and K outputs are correct? If the answer is M and K are serviceable then we can eliminate all common parts of the L and K/M outputs, C, D, E, G and I. The fault must be in any of the components A, B, F, H or L. Assuming that the operators debrief is 100% correct, may lead you down a false trail. If he is 100% correct he has eliminated half of the components which could mean half of the test and half of the equipment required to test each component for fault finding. Aviation Australia Faulty system component elimination Perform a functional test to confirm the fault. This confirms or disputes the operators report, a simulation of the conditions reported by the operator must be carried out eliminating operator error as the cause of the supposed fault. Review the symptoms and see if there are any clues that lead to a possible cause. If this does not hold any promise perform the first half split. Test the output of F going to H as this is the centre between the inputs A and B and the output L. If output from F is serviceable then the fault is after so H or L. If the output from F is unserviceable then the fault must be A, B or F so skip the next step. 2023-02-06 B-07c Maintenance Practices Page 101 of 255 CASA Part 66 - Training Materials Only  Aviation Australia Faulty system elimination by test Test the output of H to decide if component H or L is at fault. If output F was unserviceable before jumping in and testing unnecessarily have a look at the result for the output of F to check if an input from component A or B could be the source of the fault. In this example say that the result is such that A would be unlikely to have caused the output from F, then first test B output first to confirm B is unserviceable. If B is serviceable then we must run the output from A test, as we said A would be unlikely to cause the output from F but not impossible. Aviation Australia Faulty system elimination by test If A output is serviceable then the fault is unit F. 2023-02-06 B-07c Maintenance Practices Page 102 of 255 CASA Part 66 - Training Materials Only  Re-racking LRU's Re-racking an LRU is a valid method of repairing a fault. Modern LRU’s (black boxes) have a great many connections in the rear connector/connectors and anywhere there is a connection there is always the possibility of a poor connection if the LRU is removed and refitted the connection between the pin and socket is re-made offering the possibility of the fault being repaired and saving the cost of a LRU being returned for repair without being faulty. Once the faulty part is identified it must be replaced, so paperwork is brought up to date and then stores can give the spare part, fitted tested and if the troubleshooting was correct a serviceable system. Confirming a Faulty Component Another method of confirming the fault is found is by using a spare already on the aircraft, i.e. if the aircraft has a duplicate system for redundancy the part from the duplicate system can be moved into the faulty system and testing to confirm the system is now serviceable. The replacement part can now be acquired from stores fitted to the duplicate system which must also be tested. This method is suitable for non-computerised parts such as sensors, but computerised parts of modern aircraft may require software change before it can work in the new position, this could make this method less attractive. The engineer must also be aware that if a faulty LRU is moved from one system to another it may possibly cause damage to the originally serviceable second system. All the steps carried out must be documented and every connector disconnected investigated so that every system that has a wire in the connector is also tested to identify and test induced faults. 2023-02-06 B-07c Maintenance Practices Page 103 of 255 CASA Part 66 - Training Materials Only