Toos fo the TradeBy Jim Sparks April 1999
Avisit to a well-equipped avionics shop can often be awe-inspiring. All of the various test equipment used to calibrate, repair, and certify a wide array of indicators and computers will leave most maintenance technicians a bit bewildered. The majority of this highly sophisticated, high dollar shop equipment is just that — shop equipment needed by bench technicians. Line maintenance and troubleshooting avionic equipment can be accomplished using equipment that is readily available, and in many cases, moderately priced.
When considering what tool is right for the job, a general knowledge of the system in question is essential. In all cases, equipment or aircraft manufacturer's documentation should first be consulted. It is also advantageous to scan these documents for "CAUTIONS" or "WARNINGS." A caution is implemented to advise a technician of a possibility of equipment damage occurring unless procedures are closely followed. A warning is used when there is a chance of bodily injury to personnel.
The majority of recent aircraft incorporates digital technology in flight deck displays, navigation, communication, and auto flight systems. An advantage here is Built In Test Equipment (BITE). Should a malfunction occur in a BITE-equipped system, a diagnostic code is often provided to the technician that indicates where the fault occurred. The technician then replaces the defective Line Replaceable Unit (LRU). In some cases, the fault may be caused by a wiring problem, so assessing what test equipment to use is essential in completing a timely and effective repair. Aircraft that utilize analog systems and instruments also require selecting the proper diagnostic tools.
Technicians involved in flight line troubleshooting have to be quick to decide if the problem is related to a specific piece of equipment or if it is a problem with the aircraft.
Some safety measures include not using a damaged piece of equipment. Always inspect electrical leads for damaged insulation or exposed metal as well as electrical continuity. Any damaged test lead should be replaced. Be sure the device is in good operating condition and always make sure the proper range and function is selected prior to use. It is always advisable to avoid working alone when troubleshooting or testing electrical circuits.
A regulated power supply is an important component. It can be used to determine the operational status of numerous components and can often be constructed with off the shelf components from most retail electronics dealers. Several considerations are important in the purchase or construction of a power supply.
• Will the systems being tested require AC or DC power?
• What are the power requirements needed?
• What is an appropriate voltage and amperage range?
• How sensitive are the components being tested to spikes?
• Do filters need to be installed to cancel electrical noise?
• What sort of circuit protection is needed (fuse or circuit breaker)?
A power supply can be something as simple as a battery or as complex as a device with digital displays and multiple variable outputs. The use of an external voltage source will often enable the technician to make an accurate determination of its proper operation.
A Decade or resistance/capacitance substitution box is another tool that can be quickly installed to determine the operating capabilities of most systems sensors and can frequently be used in conjunction with a power supply.
Oscilloscopes (O' Scopes) are extremely versatile tools for the aviation industry. They provide the technician with the means of relating the electrical signal to time — that is to say they provide two dimensions to the circuitry being tested. Unfortunately, the new digital models can be quite expensive ($2,500 to $5,000) depending on the features needed. Analog units are much less ($500 to $700), and will provide the flight line technician with significant capabilities. An O' Scope can be used to monitor voltage levels and check frequencies of electrical signals. Also, when a unit with "Dual Trace" capabilities is used, multiple circuits can be monitored and compared.
The television type screen or Cathode Ray Tube (CRT) on the face of this tool will enable a technician to view the electrical characteristics of the components in question. A scope draws a graph by moving an electron beam across the CRT, which results in a glow reflecting the characteristics of the electrical circuit being monitored. A grid of horizontal and vertical lines is etched on the display and serves as the reference for gauging circuit parameters. Anytime a disturbance occurs in a medium, the result is a wave. Waveforms are graphic representations of an electrical wave. Wave shapes will tell a great deal about the electrical signal. Anytime a change occurs in the vertical range, it means the amplitude or voltage has changed. Anytime there is a flat horizontal line, there is no change in voltage for that period of time. Sharp angles observed on a scope mean sudden change. Common controls for a scope display include focus and signal intensity. Focus is used to maintain an optimum trace on the grid and is accomplished by concentrating the trace beam to the maximum definition. An intensity adjustment will control the brightness of the trace beam. This is needed when using the scope in various ambient light conditions.
Trace Rotation is another adjustment needed prior to scope use. This is a means of calibration and is accomplished by placing the trace beam in the proper position relative to the grid. Some oscilloscope models also include a Beam Finder. This is a function that will allow the display range to change to accommodate the signal being monitored. Electrical noise is a frequent cause of weak radios or erratic flight deck displays. By properly installing an O' Scope, the source of the noise can be identified and corrected. These devices are very effective in troubleshooting inductive and capacitive systems. Selecting the proper test probe to use with the scope is determined by the function to be performed and the voltage levels to be encountered. Probes are classified as either voltage sensing or current sensing and proper termination of the cable ends is important to avoid unwanted signals finding their way into the probe cable.
Using a device of this sophistication will require some preparation. In fact, prior instruction for a first time Oscilloscope user is encouraged. There are introductory manuals available through most electronics suppliers or scope manufacturers as well as basic electronic courses through many community colleges.
The ability to make electrical circuit measurements of voltage, current, and resistance will help in understanding proper circuit operation and identifying trouble spots. A Volt Ohm Meter (VOM) or Multimeter will always earn its keep in the hands of the well-versed electrical technician. Two types of meters are being used to make electrical measurements.
The first has a mechanically operated pointer that deflects along a calibrated scale to provide the user a specific value when viewed and is called an analog meter. A digital meter has no sweep arm; instead a digital read out proves a group of numbers to display circuit values. Each one of these has special benefits. The meter movement in an analog device works as a small electrical motor. To run the motor, an electrical potential and current flow are required. This means the meter is an additional load on the circuit being tested. A digital meter will have much less of an effect on the system as the current draw is insignificant.
Regardless of meter type, circuit measurements for voltage or resistance are taken with the meter in parallel to the component being tested and in series with the component for amperage tests. When in doubt of circuit's characteristics, it is always best to start with the meter set to the highest range available and then work down. Many newer Digital Volt OhmMeters (DVM) have a feature called "Auto Range," which automatically adjusts the measuring circuits to the correct voltage, current, or resistance range of the circuit being tested. In most models, this starts on the lowest range and works its way up. In certain situations, depending on circuit tendencies, the auto range may cause shifting to occur. This can cause what are frequently interpreted as circuit malfunctions if the range is not noted during each measurement.
In some meters, an audio tone will be heard if the signal being measured is out of the selected range. Another common feature on a DVM is a "HOLD" selector. Using hold will enable a technician working in a tight environment to take a measurement, then remove the test leads, and then read the result.
A Diode Test is available on many multi-testers and will allow the full potential of the meter's internal battery to initiate a current flow through a semi conductor device in a forward bias condition. This feature will allow a reasonably accurate means of checking the conducting as well as blocking functions of a diode. However, the best way to check a diode or most other electrical components is under actual electrical loads. Therefore, since the diode is a current-controlled device, the best way to check its operation is with an amp meter installed in series. In many cases, when meter leads are touched together and DVM is set to measure continuity or resistance, an audible tone will be heard. This is a real benefit when the technician is in a cramped area and is concentrating on the pins of an electrical connector. The resulting tone will advise the technician of a complete circuit or a short depending on the installation of the meter.
Multi meters are available in many shapes and sizes and many technicians will use several varieties depending on task at hand. Small, hand held "Pen Meters" make working in a junction box a one-handed operation. When selecting a multimeter, consideration should be given to the capabilities needed and the usual working conditions. The price range is as extensive as the list of available features and starts around $10 and goes up to around $3,000.
An appropriate set of test leads is needed to compliment any meter. In most cases, a meter will be supplied with a standard probe set. Many technicians find that special applications require special leads. These include probes that will pierce insulation, have special grabbers to cling to circuit cards, or even a standard alligator clip. Various accessories are also available for a basic meter such as temperature conversion modules and current sensing clamps. These devices can turn a basic DVM in to a world class-troubleshooting tool without a prohibitive cost.
Clamp Meters are valuable devices and are becoming more commonplace. There are units for both AC and DC electrical circuits. An AC clamp works on a transformer principle and when it is brought in close proximity of a wire carrying AC current, the meter can provide the user with an accurate indication of amperage. This is accomplished without disconnecting any wires. A DC Amp clamp is also available and works on a Hall Effect — that is, it senses the electromagnetic field surrounding a conductor. These are very useful tools as current flow in a circuit is a sure means to determine the health of the system in question. The prices on these components range from $70 to $500.
Another device that has been popular with telephone repairmen is a tone generator and inductive probe. By connecting the tone generator to a wire and aircraft ground or wire pair, a signal can be introduced. Then, by placing the inductive probe within about 12 inches of the routed wire (even in a bundle), an accurate determination can be made as to location of a short or open circuit.
What a concept — knowing where the wire fault is before removing all of the aircraft interior and all for a price of $50 to $300.
With all the talk of high tech, high dollar troubleshooting equipment, one of the best diagnostic tools for electrical systems is a magnetic compass or a magnetic stud finder. Most relays, contactors, and motorized electrical devices produce magnetic fields, so by placing a compass or stud finder by an inductive component and energizing it, there will be no doubt if there is current flow. All this capability for $1 to $2.
It is important to note that any equipment used for return to service must be calibrated. That means a calibration sticker must be readily available and should reflect that the device is capable of doing its intended job. Not only is this true for electrical testers, but also for ratcheting crimping tools.
With thoughtful application and proper maintenance, test equipment gives limitless capability for troubleshooting and returning aircraft to service in a timely fashion.