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Saturday, July 1, 2006

Tool Calibration

James Careless

Every mechanic knows that his or her tools need to be properly calibrated, but what some don't always understand is precisely why?

The reason--the precise importance of calibration, to be exact-- is that improperly calibrated tools and measurement devices don't do what you expect them to do. For instance, a click-type torque wrench whose spring is `out of spec' can end up delivering less torque per turn; resulting in nuts that aren't on as tight as the manufacturer requires. Similarly, a tire gauge that isn't reading properly may result in increased aircraft tire wear due to under-inflation.

Besides compromising aircraft performance, component life, and sometimes even safety, improperly-calibrated tools result in work that is less than optimum for customers. This, in turn, can lead to MROs and repair shops losing clients; not something anyone can afford in the current competitive climate. So what should you be doing to stay on spec with your equipment? To find out, Aviation Maintenance turned to the experts.

Keeping Basic Tools within Specs

When it comes to keeping tools and gauges performing within their specifications, FlightPath Services (www.flightpathservices.com) of Teterboro, New Jersey, are the experts. This is because this FAA-approved repair shop has the calibration equipment on site to precisely measure the accuracy of hand tools, tire gauges, and hydraulic gauges; among others. Take click-type torque wrenches, for instance. The point at which the wrench `clicks' in release torquing pressure while the handle is moving backwards is determined by the condition of the spring inside the wrench head. "This spring controls a cam, which is what you hear click when the torque wrench is turned," says FlightPath Services' chief inspector Doug Svoboda. "The spring creates resistance against the direction the wrench is being turned in, thus applying torque. The problem is that the weaker the spring gets, the sooner it allows the cam to click; resulting in less torque being applied to the bolt."

Clearly, a click-type wrench with a worn spring is going to result in a mechanic tightening bolts and nuts less than they should.conds tightening each one; thus adding time to the job. To detect when a click-type wrench is at this point, FlightPath Services puts it on a `Torque Analyzer'. "This device reads the amount of torque delivered by the wrench when the cam clicks," says Svoboda. "It can tell us precisely how far a wrench is out of spec, and whether it could continue to be used, or should be replaced."

Tire pressure gauges and hydraulic gauges are two more common tools that are also prone to becoming out of spec. "Inside each of these devices is a flexible that expands under pressure, driving small gears that, in turn, move the needle across the gauge's measurement face," Svoboda tells Aviation Maintenance.

To find out whether these gauges are reading accurately, FlightPath Services hooks them up to sources of known calibrated pressure. For instance, to check out hydraulics gauges, "we use a `deadweight test' that applies force to a hydraulic cylinder, which in turn is connected to the gauge being tested," he says. "We can compare the gauge's readings with our known values, to determine how far off it is."

So what can mechanics and technicians do to keep their tools within calibration spec as long as possible? "First, be sure to buy good tools right off the start," Svoboda replies; "knock-off tools that you might get at an automotive service store won't stay true as long. Second, store each of your tools in protective cases. The more you let them get knocked around, the sooner they will go out of calibration. Finally, when it comes to gauges, the Number One source of problems is overpressure: People push too much air or fluid into the unit, and it blows the assembly. To prevent this from happening, decide what pressure level you are expecting to find, then select a gauge where this level will end up in the middle of its range."

Ensuring That Electronics Tell the Truth

Increasingly, we live in a world of digital electronics; a world built on the certainty of countless `0s' and `1's blasting through aviation avionics and subsystems. However, this certainty is an illusion, because every single one of these digital components is made of materials that react to temperature, humidity, and voltage. As they react over time, the transistors, capacitors, and wires begin to evolve away from their original compositions. In sufficiently extreme conditions, they can go substantially `out of spec', without anyone noticing.

Take all those aircraft which are being stored in Arizona," says Jack Somppi, a product manager with Fluke Corporation's precision measurement group (www.fluke.com). "Airlines like this area because it's dry, which is easy on their aircrafts' exteriors. However, the heat out there is hard on their electronics, with temperatures getting up to 100 degrees Fahrenheit or more. In these conditions, electronics can act a bit differently than they do as 32 degrees. Moreover, not only does heat shorten component life - in fact, it's the enemy - but sitting in the desert can result in dust accumulating on contacts and other exposed connections."

As a `metrologist'--defined by Merriam-Webster as someone involved in "the science of weights and measures or of measurement", and by Somppi as someone "who doesn't give a damn about the weather; that's a meteorologist"--Jack Somppi lives in a world of `calibration guys'. "These are people who are pretty specific characters," he says, "whose world is comprised of parts per billion, and whose goal is to reduce uncertainty."

Into this world comes the wide range of test and calibration equipment made by Fluke; from handheld thermal imagers and infrared thermometers to bench multi-tool calibrators and multi-meters. Their collective task is to help technicians troubleshoot parts and components; especially those especially daunting `intermittent faults' that tend to occur in flight and nowhere else.

According to Somppi, electronics calibration problems stem from a variety of causes. "I've already touched upon temperature, but humidity can also be a real killer," he says. "Take the runway lights at Minneapolis International Airport. Over time, moisture got into the underground cables, leading to unexplained intermittent failures. The only way to fix it without a total rebuild was to do an insulation resistant test on the wires, which allowed them to pinpoint the breaks in the wires."

Then there's overvoltage: "Putting too much power through an piece of electronics can shorten its life at best, and end it at worst," Somppi says. And don't forget physical factors such as vibration and shock. How many times has the media reported that the cause of a catastrophic airplane crash was due to two seemingly-inconsequential parts rubbing each other raw; allowing power to arc between them?

One thing is certain: With today's aircraft becoming more and more dependent on electronic systems, proper calibration and monitoring of such systems is becoming increasingly important for mechanics and technicians. In doing these tasks, care must be taken to ensure that the information being provided by aircraft diagnostic programs is accurate, says Somppi. As well, "you must make sure that the diagnostic readings being measured by your own equipment is accurate," he tells Aviation Maintenance. "That's why we at Fluke put such emphasis on `testing the testers', to ensure that our clients are getting accurate readings at all time. We also recommend that customer test equipment be brought into our facilities on a regular basis, so that we can make sure it is operating within its own specs."

Lessons To Remember

If there's a moral to be drawn from the tales above, it is that a technician's work is only as good as their tools. If their tools are worn and out of calibration, then the work they do with them runs the risk of being substandard and, in some instances, just plain wrong.

In the case of hand tools, prevention is the simplest way to extend accuracy. This means storing tools in ways that keep them from being knocked around. One trick passed on by Doug Svoboda is to keep torque wrenches at their lowest possible settings when not in use, in order to minimize wear on their springs.

In the case of electronics, life isn't so simple. Digital tools are made of physical materials that exist in the real world of heat, humidity, and electricity. As such, even the best of tools can go off-spec over time, which is why regular calibration tests are a necessity. In turn, calibrators themselves needed to be verified from time to time, to ensure that they aren't making matters worse instead of better. In general, the better you treat your tools--no matter how simple or complex; mechanical or electronic--the longer they are likely to stay calibrated, and thus able to help you do your job the way you want to do it. Still, "the more you use something, the more it will change," says Jack Somppi. "Even in the best of conditions. And also, even with instrumentation that is just sitting on the shelf, it will change with time, so checking is necessary even with little to no use."

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