During routine maintenance it was determined that the thrust reverser installed on the No. 2 engine was not being monitored as it should have been by the digital flight data recorder (DFDR) however, all other aspects of operation appeared to be normal.
Investigation of the system revealed the thrust reverser positions were monitored by a group of dual contact switches. One set supplied information to the control and indication circuit, while the second set would report to the full authority digital engine control (FADEC). Aircraft manufacturer’s documentation was very elusive when it came to identifying the data path to get this information to the DFDR.
When presented with the above dilemma what would be a good approach to troubleshooting the problem? Diagnostic methods and techniques are frequently left up to the discretion of the technician assuming responsibility for resolving the problem.
The fact that a digital flight data recorder and a digital engine control are possible culprits, a logical troubleshooting tool would be a digital volt ohm meter (DVOM), right?
To put everything in proper perspective, all computerized systems can be broken down into three groups; input or sensors, data processing, and outputs or loads.
Everybody knows that digital systems involve 1s and 0s. So when I connect my DVOM to a digital circuit how do I know if I am looking at a 1, perhaps a 2, or even a 3?
Avionics suites installed in the aircraft of today have the ability to monitor many inputs, interpret data, and provide appropriate responses to the crew and other aircraft devices or systems. With this sophistication come enhancements for maintenance technicians. Most digital devices include some form of self-diagnostics.
Unfortunately it is the expectation of many that these built-in test equipment (BITE) circuits are all-knowing and will always point to the faulty device. The real purpose of diagnostic systems is to provide troubleshooting data and it is the interpretation of this information coupled with other diagnostic techniques that will most frequently isolate the true problem.
Many look at electricity as an energy source but fail to consider the application as a communication tool. Three general classifications are used to classify how electrical signals are interpreted: analog, digital, and discrete.
The analog group includes the variables such as voltage, amperage, and frequency. Almost everyone has heard the phrase “Information out is only as good as information in,” and with many of the computer systems utilized on modern aircraft much of the input data comes from sensors. So what does a sensor look like? It could be the switch that monitors the extension of a main landing gear to determine the ground or flight configuration, it could also be the potentiometer linked to the wing trailing edge flaps that reports position.
There is also the rotary variable differential transducer (RVDT) that sends power lever position to the electronic engine control (EEC) by using variations on an alternating current signal. Even the tachometer generator that supplies wheel velocities to the anti-skid system is a sensor and depending on aircraft type will generate either a frequency or voltage proportional to wheel speed. Troubleshooting analog circuits can most often be accomplished using conventional techniques and a multimeter.
Digital communications also take on many forms. There are the industrywide standards such as Aeronautical Radio Inc. (ARINC) 429 and even proprietary languages utilized by avionics systems manufacturers. Circuits that use digital data have provisions to allow them to interact with digital buses. These paths for digital data are often thought of as wires and it is true that they are in fact made up of at least two wires twisted together and surrounded by a shield. The electrical properties of buses are specifically defined and are impedance critical. That means abnormalities with the shield, high resistance at connections, improper clamping, or even an excessive cable bend may interrupt or at least decrease the efficiency of the data transmission.
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