Avionics News October 2012 - 72

THEORY & PRACTICE

Continued from page 71

box. Perhaps this box is an autoflight computer. One could replace the air data system, control head, computer and even servos, but the new autoflight computer would still fail. The technician must know the proper questions to ask. During a recent training session, I was cautioned against using the box-swap method to troubleshoot a particular compass system. Although the right and left systems appeared identical, they were not. Switching out the couplers would cause a failure. This system knowledge is crucial.

System knowledge is important for pilots and owners, as well. Unless an aircraft is equipped with an avionics laboratory staffed with a technician, it’s likely the flight crew will literally need to do some troubleshooting on the fly at some point. Most of the time, technicians can see a problem during tests on the ground. Still, there are plenty of problems that only show while in flight or display different symptoms in flight. For these pesky in-flight only problems, the technician relies on a knowledgeable crew. For safety reasons, flight crews must have system knowledge. In some cases, the questions have life or death implications. For example, from an

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electrical standpoint, the flight crew must know what systems operate from which bus. Furthermore, they need to know where each bus gets its power. They must know when it’s appropriate to tie the busses and when it is not. The most crucial question and answer is just how long will the essential bus last, and exactly what is and what is not powered by the essential bus. During a complete generator failure, does the crew have the minimum 30-minute supply, or will the bus last longer? If I’m driving a jet, then I will want to know the answer. If I’m riding in the back, I’ll want my crew to know the answer. In some cases, I have had a very sharp student who uses the following rationalization: “It’s all quite logical, and I’ll figure it out when I need to.” This is dangerous thinking for all. Most systems are quite intuitive; thus, the majority of time, a sharp technician can figure things out. Still, there are plenty of examples where system design is not intuitive, at least not from the technician’s standpoint. An example would be the compass systems I mentioned earlier. For that matter, a design might not be intuitive, even from a pilot’s standpoint. As a pilot with virtually all my personal experience operating piston aircraft, I was quite surprised when I learned the emergency shutdown procedure for some jet aircraft is to pull the fire handle. As I learned the system, I understood why pulling the fire handle in this case did not discharge the extinguishers. If you are a student, hang in there, even when you think another block diagram will cause your head to explode. If you are in my class, then perhaps your first question should be whether or not I plan to use a standard deck of cards, a pinochle deck or Uno deck. q

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Avionics News October 2012

Table of Contents for the Digital Edition of Avionics News October 2012