Aircraft Maintenance: An inside look at tool calibration

However, before these tools can be used on an aircraft, it’s important to make sure that the tools themselves are up to the task; something known as “tool calibration.” While you might assume that the trusty torque wrench that’s been in your tool box for the past 10 years is plenty accurate, there’s no way to know unless it’s been checked. While there are no rules governing the tools you use on your car, the FAA has created guidance about what tools you can use on your airplane.

§ 43.13 Performance rules (general).

(a) Each person performing maintenance, alteration, or preventive maintenance on an aircraft, engine, propeller, or appliance shall use the methods, techniques, and practices prescribed in the current manufacturer's maintenance manual or Instructions for Continued Airworthiness prepared by its manufacturer, or other methods, techniques, and practices acceptable to the Administrator, except as noted in § 43.16. He shall use the tools, equipment, and test apparatus necessary to assure completion of the work in accordance with accepted industry practices.

AC 43.13-1B, 7-40. TORQUES

The importance of correct torque application cannot be overemphasized. Undertorque can result in unnecessary wear of nuts and bolts, as well as the parts they secure. Overtorque can cause failure of a bolt or nut from overstressing the threaded areas. Uneven or additional loads that are applied to the assembly may result in wear or premature failure. The following are a few simple, but important procedures, that should be followed to ensure that correct torque is applied. …

1. Calibrate the torque wrench at least once a year, or immediately after it has been abused or dropped, to ensure continued accuracy.

Although the regulation only requires “accepted industry practices” be used, the advisory circular explains that the FAA interprets that to mean that all torque wrenches, force gauges, and other tools should be calibrated annually.

When it was time to get my own tools calibrated this year, I decided to tag along for the calibration process at Essco Calibration Laboratory in Chelmsford, Massachusetts. Essco President Mike Walsh, Executive Vice President Troy Thomas, and Account Manager Tom Masterson were kind enough to let me witness Essco testing my torque wrenches, cable tensiometer, and force gauge. The process and the tools they used were fascinating.

Our first stop was to calibrate my click-style torque wrenches. Essco is an ISO/IEC 17025:2005 accredited facility. In laymen’s terms, that means that other calibration shops send their master tools to Essco to get calibrated. They are the “top of the food chain” when it comes to this kind of work, and the test equipment showed it. What was most interesting was how these click-style torque wrenches are adjusted when they’re found to be out of calibration. The torque mechanism itself isn’t adjusted at all.  It’s the reading on the handle that gets altered. Basically, the wrench setting on the handle is turned until it “clicks” at a known torque value measured by the calibration machine. Then, the outer wrench handle with the gauge markings on it is loosened and rotated to match the correct number. As long as the wrench is linear in its error, you can simply change the number it tells you to match where it clicks and you’re done. They test each wrench at several settings to ensure that it meets the required accuracy for the tool.

You may wonder how expensive torque wrenches compare to the inexpensive ones found at your local “big-box” hardware store, and I brought along a few examples to see how they performed. Quality does matter. Out of five click-style torque wrenches I had Essco test, two brand-new low-cost wrenches couldn’t be calibrated at all. When they were set to be accurate at the low end of the scale, the high end was far out of range. They simply weren’t linear enough to adjust into tolerance. I didn’t find a specific brand to be better or worse—it was just hit or miss with the inexpensive wrenches, whereas higher quality wrenches were predictable and could be successfully adjusted.

Next, Essco checked the calibration on my Amtek digital force gauge that I use to measure push/pull forces on landing gear mechanisms. They mounted it in a fixture and applied calibrated forces in both directions, eventually pronouncing it “A-OK” without adjustment.

The last stop was to test my cable tensiometer, which turned out to be the part of the process that caused my head to explode. I’ll leave you in suspense on that one until next month. Suffice to say, there’s a mystery to be solved about how aircraft designers measure cable tensions versus how mechanics do it in the field. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.