Note
Torque Wrench Selection and Calibration for Part-145 Shops
Why three wrenches beat two, the most common torque error in maintenance, and what an auditor will ask about the one in your hand.
A torque wrench is the tool most likely to be looked at during an audit, and the tool most likely to be quietly wrong in daily use. Both for the same reason: accuracy is invisible.
The most common torque error in maintenance
Using the bottom of a wrench's range.
A click wrench is least accurate at the extremes of its range, and specifications are typically quoted for the upper portion of it. A wrench rated 60 to 340 Nm used at 65 Nm is being used exactly where it is worst, and a fastener torqued there may be well outside tolerance while the wrench clicked perfectly.
This happens because a shop owns fewer wrenches than the ranges it needs, and the big wrench is the one on the shelf. It is a purchasing problem that presents as a technique problem.
Three wrenches, not two
The practical answer is to cover low, mid and high with genuine overlap.
| Drive | Range | Covers |
|---|---|---|
| 1/4 in | 2 to 24 Nm | Instruments, avionics, connectors, small fasteners |
| 3/8 in | 20 to 100 Nm | Most airframe and engine work |
| 1/2 in | 60 to 340 Nm | Landing gear, wheels, high value fastenings |
Note the overlaps at 20 to 24 and 60 to 100. That is deliberate: a figure near a boundary can be set in the healthy part of one wrench rather than the extreme of another. Ours are 186, 214 and 292 EUR ex VAT, supplied calibrated. The full range is under torque wrenches.
Click, dial or electronic
Click for general work. It can be used one handed in an awkward position with no line of sight to a display, which describes most aircraft fastenings. It needs winding back after use, which is the discipline that gets forgotten.
Dial or beam where you need to watch torque build, or for checking rather than setting. No spring to relax, so it holds calibration well, and it needs to be read, which means you need to be able to see it.
Electronic where you want a record, or angle as well as torque. More capability, more cost, and a battery that will be flat when you need it.
What an auditor asks
Four things, in this order.
Is there a current certificate. In date, matching the serial number on the tool in your hand, from a laboratory accredited to ISO/IEC 17025 by an ILAC MRA signatory.
Who set the interval, and on what basis. The manufacturer recommendation where one exists, otherwise derived under ISO 10012. Note that torque wrench recommendations are often expressed in cycles as well as months, and a wrench on a production line reaches a cycle count long before the calendar does.
Is it in the register. One list, and it matches the crib in both directions.
What happens when one is found out of tolerance. This is the question that separates a documented system from a real one, and it is dealt with properly in our calibration guide.
The two habits that matter
Wind a click wrench back to the bottom of its range before it goes away. Never store it at setting. A spring held under load takes a set, and this is the single most common reason a wrench that has never been abused fails calibration.
Quarantine anything dropped or overloaded, immediately. A wrench used as a breaker bar is out of calibration whatever the sticker says. The person who did it is not always the person who reports it, which makes this a culture question more than a procedure one.
Where they live
A torque wrench in a drawer with other tools gets knocked, and the two habits above are much easier to keep when each wrench has an obvious home. A wrench missing from a cut pocket is also visible immediately, which matters because a wrench that has quietly left the crib is a wrench nobody is tracking against a recall date.
Foam for a torque wrench needs depth, normally 50 mm, and the pocket should not press on the scale. Our foam service lays out around the tools you hold, and we will not put marking on a scale or a drive on the etching side either.