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In my last column, I covered why and how to calibrate common shop tools like torque wrenches during my visit to the Essco calibration lab in Chelmsford, Massachusetts. However, when it came time to calibrate my cable strain gauge, I was blown away by the process I witnessed.
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In the field, mechanics routinely use cable strain gauges to check the tension on aircraft control cables. The procedure is simple: Attach the strain gauge to the cable, read it, compare it to the calibration table using the tool (with reference cable size) and get the resulting tension. All this happens when the cable is installed in the aircraft and live.
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When the Essco technician calibrated my voltage meter, he did something completely different. He placed a strain gauge on the loose cable on the calibration machine, then tightened the cable until the calibrated machine showed the correct tension. He did this dozens of times for different voltages and cable sizes until he could correctly fill out the new calibration chart for my strain gauge.
If you still don't understand why I witnessed this process, let me explain. As mentioned before, when we use a cable strain gauge in the field, we attach it to a cable that is already live and read the resulting number. This means that the number we are measuring corresponds to the stress that the meter itself puts on the cable (it rejects the cable to measure voltage). However, once the tool is calibrated, Essco technicians first attach the tool to the cable and then pull the cable until the cable is under a known load (with a cable strain gauge attached). It's a completely different process that results in a completely different voltage.
To prove it, I asked the technicians to run a test. We used Essco's calibration machine to set the cable to a fixed position with a fixed tension (15 lbs) and then attached my strain gauge. The calibration machine now reads 20 lbs. We then performed the procedure in reverse order (the way they were calibrated). We attached the strain gauge and then increased the tension to 15 pounds on the calibration machine. We then held the cable in place and removed the tool. Measured tension dropped to 10 pounds. That's a measurement error of 30 percent!
Essco did nothing wrong. The technicians followed the industry standard for calibration procedures, but it took me weeks of research to get to the bottom of this mystery. Here's what I found:
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As an industry standard, cable strain gauges are calibrated to measure "free cable" tension. Imagine attaching a cable to the ceiling and hanging a 10-kilogram weight at the bottom. Attach a strain gauge to the cable and measure 10 pounds as the 10-pound weight is still hanging. But the weight actually increased a bit to account for the stress the tool put on the cable. This is how our instruments are calibrated.
Unfortunately, cables don't work that way on our plane. Generally, aircraft control cables are closed systems with both ends of the cable fixed, with the cable placed under design tension. Place a cable strain gauge on the cable and you will increase the tension in the cable as you measure, sometimes by a lot.
So why don't airplanes fall out of the sky with up to a 30 percent error rate in cable tension? I asked the engineering team of a major general aviation manufacturer (they don't want to name the company) for a response. The answer is simple: they know that the numbers read by our meters are not correct in the "absolute sense". They suggest cable tension so our gauges will "read" 15 pounds when they really want the cable to be under 10 pounds. They know we're not getting accurate data, but they're okay with that because they've already factored it in. Additionally, most control cables are not used anywhere near their load limits. They are positioned to ensure that there is not too much play or tension on the pulleys. They design it so that everything moves smoothly and safely, then they clamp the strain gauge, see what it says, and write that number down in the maintenance manual. What is the "real voltage" of the cable? Well, that's anyone's guess.
With the mystery solved, I can sleep well at night knowing that my plane's bolts are tight, its connections are up to spec, and the cables are at exactly the tension the designers intended...whatever that really is. Special thanks to the wonderful people at Essco for opening their doors to my visit during these challenging times. Until next time, I hope you and your families are safe and sound and I wish you blue skies.
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Jeff Simon is an A&P mechanic, IA, pilot and aircraft owner. He has spent the last 22 years promoting owner-assisted aircraft maintenance and created the first alternator clutch check tool available on ApproachAviation.com. Jeff is also the creator of SocialFlight, a free mobile app and website that maps over 20,000 aviation events, hundred dollar burger destinations and also offers educational aviation videos. Free apps are available for iOS and Android devices, and users can also visit www.SocialFlight.com.
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