In my last column, I covered the why and how for calibrating common shop tools such as torque wrenches during my visit to Essco Calibration Laboratory in Chelmsford, Massachusetts. However, when it came time to calibrate my cable tensiometer, the process I witnessed blew my mind.
In the field, mechanics routinely use cable tensiometers to check tensions on aircraft control cables. The process is simple: Clamp the tensiometer on the cable, take a reading, compare it to the calibration chart with the tool (referencing cable size), and you get the resulting tension. All of this is done while the cable is installed in the aircraft and under tension.
When the Essco technician calibrated my tensiometer, he did something completely different. He mounted the tensiometer on a loose cable on the calibration machine, then he tightened the cable until the calibrated machine read the proper tension. He did this dozens of times for various tensions and cable sizes until he could properly fill out the new calibration chart for my tensiometer.
If you didn’t catch on yet to why my mind was blown while witnessing this process, let me explain. As mentioned earlier, when we use a cable tensiometer in the field, we clamp it onto a cable already under tension and read off the resulting number. This means that the number we measure accounts for the strain that the measuring tool itself places on the cable (it deflects the cable in order to measure the tension). However, when the tool is calibrated, Essco technicians clamp the tool onto the cable first, then pull the cable until the cable is under a known load (with the cable tensiometer attached). This is a completely different process that results in a completely different tension.
To prove the point, I asked the technicians to run a test. We used Essco’s calibration machine to place a cable in a fixed position at a fixed tension (15 pounds), then we attached my tensiometer. The calibration machine now read 20 pounds. Next, we did the process in reverse (the way that they calibrate). We attached the tensiometer, then brought the tension up to 15 pounds on the calibration machine. Then we held the cable in place and removed the tool. The tension measured dropped to 10 pounds. That’s a 30-percent error in measurement!
As an industry standard, cable tensiometers are calibrated to measure tensions of a “free cable.” Imagine attaching a cable to the ceiling and suspending a 10-pound weight at the bottom. You clip the tensiometer on the cable and you measure 10 pounds because you still have a 10-pound weight hanging down. However, the weight actually moved up a little bit to account for the deflection that the tool placed on the cable. This is how our tools are calibrated.
Unfortunately, that is not how the cables in our aircraft work. For the most part, aircraft control cables are closed systems with both ends of the cable fixed, placing the cable under a designed tension. Place your cable tensiometer on the cable and you raise the tension of the cable while measuring it, sometimes by a lot.
So, why aren’t aircraft falling out of the sky with up to 30-percent error rates in their cable tensions? I contacted the engineering team at a large general aviation manufacturer for the answer (they didn’t want to go on record with the company’s name). The answer is simple: They know that the numbers our gauges are reading aren’t accurate in an “absolute sense.” They design cable tensions so our gauges will “read” 15 pounds when they actually want the cable to be under 10 pounds of tension. They know we’re not getting an accurate reading, but they’re OK with it because they’ve already accounted for it. In addition, most control cables are not used anywhere near their load limits. They have been set to ensure that there isn’t too much slack or tension on the pulleys. They design it so that everything moves smoothly and safely, then clamp on a tensiometer, see what it says, and put that number into the maintenance manual. The “true tension” that the cable is under? Well, that might be anyone’s guess.
With that mystery solved, I can sleep well at night knowing that my aircraft’s bolts are tight, its linkages are within spec, and its cables are under the exact tension that the designers wanted…whatever that might truly be. Special thanks to the wonderful folks at Essco for opening their doors to my visit during these challenging times. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.