AVIONICS R. Fred Polak | Editor AVIONICS TROUBLESHOOTING PART ONE
SCIENCE, ART AND TRIBAL KNOWLEDGE
Avionics systems come in all shapes, sizes and func- tions. Older systems tend to be hardware intensive,
whereas newer systems usually have less hardware but are loaded with software. In either case, the function of the system does not usually change too much, but the size, weight and power consumption of the newer system tends be much less than the older system it replaces. This is great from an aircraft installation point of view, as saving weight and power consumption is always a plus. It is quite another story from a maintenance perspective. I have taught hundreds of avionics classes over the years, and one question that I am asked over and over is this: “What is the secret to troubleshooting an avionics system correctly the first time?” There really is no secret. Let’s discuss the steps/procedures one should follow to help minimize wasting time, manpower and money in trouble- shooting avionics systems.
System types The first step in any troubleshooting procedure is to iden- tify the system believed to be at fault. In the case of avionic systems, this is important to the extent that the cause-and- effect syndrome comes into play. System B is showing a fault because it relies on an input from System A. System B is showing the effect, but System A is the real culprit. Trust this axiom: “Whichever system is understood the least will be blamed the most.” For most aircraft that have one, that would be the autopilot. Three basic types of systems are in use in our industry
and we troubleshoot them differently. The oldest systems out there are what I refer to as federated systems. These systems have several line replaceable units (LRUs) and each one has a particular task. There might be controllers,
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HelicopterMaintenanceMagazine.com October | November 2013
a computer, displays and in some cases, analog-to-digital converters and switching units. Lots of boxes and wires, but because each unit had a particular function, it was not too difficult to troubleshoot. The second type of system that has evolved is called an
integrated system. Imagine the federated system mentioned above — and through the marvels of very large scale integra- tion (VLSI) and surface mount technology (SMT), you had the ability to take an LRU and reduce it in size to that of a circuit card assembly (CCA.) A system that had three or four LRUs might now have only one. Each of the older LRUs has become a CCA in a new LRU. Less weight, space, wiring ... the list goes on. On the negative side, with all that integration, it becomes more problematic to troubleshoot, as replacing the new LRU will cost more since it houses more functions. Lastly, we come to the distributive system — the third type of system and “the new kid on the block,” if you will. This system uses a lot of commercial off-the-shelf (COTS) devices and specializes in very secular software to do spe- cific functions. As an example, think of this system as using four Windows PCs connected together along with four displays, all running the same operating system. One PC is running Microsoft Word, another is running Excel, another running PowerPoint and the last running Outlook. Now, replace these programs with an air data function, an EFIS function, an autopilot function and a flight director func- tion. No more separate computers, displays and in some instances, even sensors. All has been integrated into a very efficient package where hardware has become generic and minimal, while software has taken over. Again, from an aircraft perspective, less weight, less space, fewer wires, less power — this is a good thing. However, troubleshoot- ing has become a whole other issue. The days of going out on the ramp with a TIC box are long gone. Just about all
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