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Tool Calibration - Art, Science OR magic?

You have a bunch of calibrated equipment, you are using the local calibration lab, all your tools have stickers showing compliance and you have a backup certificate in the file folder. You think you have everything in order to pass an FAA audit and you’re good to go, right? Maybe not. 

Having dealt with calibrated equipment in the form of its use, calibration requirements and dealing with audits over the years, there are some areas that need to be understood and addressed.

Know Your Equipment

What is the difference between a torque wrench purchased from Sears or from Snap-On? What is the difference between a truck scale and an aircraft scale? How about that pressure gauge you’re using to check your critical systems with? What about your track-and-balance kit? They are all calibrated pieces of equipment and they all require a 12-month annual calibration, but can they all be done by your local calibration lab? Does the lab meet your requirements and do they have the full ability to meet the specifications of your equipment? Does the equipment you want to have calibrated have the capability and/or proper range for the job? These are questions that need to be answered. 

First of all, there is a difference in the make and capability of equipment. The tool manufacturer’s accuracy statement, its ability to meet a requirement, its type, and range all determine if it is the right tool for the job. Is it really accurate for the airworthiness determination you are about to make?

Pressure Gauges

There was a problem with the transmission oil pressure system. It was reading at the low limit on the gauge in flight, and the low-pressure warning light would come on when hot and after landing. The technician grabbed a direct reading gauge and went to check out the system. (The system range was under 150 psi, low end red arc was 25 psi, normal range was 35 to 90 psi, and the check gauge used could read from zero to 300 psi, with five psi increments and was calibrated by a local calibration lab.) 

Let’s think about this situation. We are going to make a critical determination based on a calibrated piece of equipment. The direct reading gauge used here will directly have an effect on airworthiness determination, safety of the aircraft and its occupants, and/or tens of thousands of dollars of possible work and/or parts replacement.

The gauge is calibrated, but to what degree and/or accuracy? Did the documentation given with the gauge give actual pressure correlation to what pressure was applied, and what pressure was read? Or is an accuracy statement stating that it is one-, two- or three-percent accurate? What was the degree of accuracy to which the calibration lab was certified on the master instrumentation? Was an uncertainty statement given for the gauge and/or master given? Is there a first or second degree of traceability to National Institute of Standards and Technology (NIST) standards for the master? Did the calibration lab have the gauge manufacturer’s calibration instructions and acceptable accuracy limits on hand when the gauge was calibrated, and what are they?

These are good questions. Firstly, is the gauge in question the right tool for the job? Is a five psi reading division good enough to make a determination? If the accuracy of the gauge is stated as within limits, what limits were considered: the manufacturer of the gauge or that of the calibration lab? If it is a 300 psi gauge and the accuracy is two percent, you’re looking at a possible reading difference of six psi full span. If it is one percent, you’re looking at three psi full span. What is the lower limit? Does the calibration certificate issued make a low- and/or high-end statement or just a general accuracy statement? Just how accurate is this gauge in the 20 to 30 psi range and can you determine you are at, below or above the aircraft limits using this gauge? 

Some would say it’s calibrated, that we are good to go and we can make a determination. Cost is everything and we are right at 30 psi on our accurate gauge, so let’s increase pressure a little bit to get the light off and or replace the light switch. Let’s adjust the gauge in the cockpit using 30 psi as a reference point. Once the oil is cool and the summer heat is over, we should be OK and not be below 25 psi, right?

Are we are going to release the bird for another run? Are we going to make the adjustments based on the gauge used for calibrated equipment? Even though the transmission has only 400 hours until overhaul is due, we are at the low end but not below, and we should be OK, right?

This scenario plays out all over the world; high pressure, low pressure, high temp, low temp, and let’s not forget, torque (torque wrench), inches per second (IPS), weight (scales) and psi (pressure), as well as other areas that require the use of calibrated equipment. 

Know your job, know your equipment, know your calibration lab, and most importantly, know what the equipment manufacturer’s tolerance or accuracy is, and what is accuracy of the calibration lab. Choose the right tool for the job. Base your decision on comparable facts and known accuracy of test equipment. In the end, you’re the one signing off the log book!

Torque Wrenches

I have found that there are general, run-of-the-mill torque wrenches and there are some good, accurate ones. Accuracy varies by one to five percent and there is a difference between a cheap one and a high-quality click type or digital unit.  

The same holds true for the calibration labs conducting the work. Some only test and document; they do no adjustments or repairs. Some have second- or third-degree traceability and depend on other sources for confirmation of actual applied forces. Some use weights and some new-fangled digital torque transducers. 

Ever wonder why your tailboom attach points never crack but other operators have issues? Could it be that they are using inferior torque equipment and/or a non-audited calibration lab with tolerances not within the required limits you are looking for?  

How about those cracked transmission mounts? Some do and some don’t. Was it the helicopter’s OEM and manufacturing, or was it that you just under- or overtorque them? Hey, the wrench clicked at 50 so we must be good, right? 

I see it all over. Some things crack and some don’t. In general, no transmissions came off or out, no engines were lost out of the mounts and no rotor blades were flung off ... so we must be in the ball park, right? Nothing catastrophic, just cracks and wear we cannot explain — it’s got to be those pilots! As Yoda would say, “Cowboys they must be.” But is that really the cause?  

I would be interested to further my study of calibration labs and torque wrenches. A long time ago, I sent a few torque wrenches to a calibration lab and they all came back fine. Later I had to send them to a super-duper-expensive lab with all kinds of certifications (contractual requirement) and they were all scraped. Now, I’m not going to lose my Snap-On wrenches that cost me hundreds of dollars, so I sent them back to the first lab and they came back certified again.  

It is obvious that we have two totally different standards. I wish I had the time and money to send them back to Snap-On for a tear down and report on what was wrong and what had to be replaced. Also, I would have wanted to know if Lab A did repairs and adjustments or document just a ballpark accuracy of two percent, and did super-duper Lab B hold to a higher accuracy of one percent or less, or even an uncertainty? Did they attempt to repair or adjust, or just document that they were out of tolerance and that was it? It will be up to us to get them repaired and then sent back for certification.

So what if we used them on a main transmission mount to airframe structure? What if we torque down the upper gear box lid with them or mounted several rotor blades?

Food for Thought

Members of HeliMx’s Advisory Board came out of the woodwork with thoughts and commentaries on tool calibration. The following pieces of input come from two Advisory Board members.

First input: I have worked for Part 135 and 145 operators and the military in addition to part 91 operators. I always took it for granted that it was a requirement for once-a-year calibrations. When I arrived at my current employer, a part 91 operator, my then-DOM informed me there was no requirement for torque wrench calibration unless you were in a Part 135 or 145 operations. I was dumbfounded, so I started checking. FAR 43.13 states, “He shall use tools, equipment and test equipment and test apparatus necessary to assure completion of the work in accordance with accepted industry practices.”

OK. Now here is where I find it in a weird place — AC 43.13 1B Section 4, Avionics Test Equipment paragraph 12-53, Test Equipment Calibration: “Test equipment such as meters, torque wrenches, static and transponder test equipment should be checked at least once a year.” We could go back and forth on if Advisory Circular 43.13-1B is a recommendation or just acceptable methods.

Second input: Good stuff. The key word in the AC is the word “should.” It is interesting that when the AC talks about calibration standards, it says they “must” be traceable to one of three choices. So they say you “should” calibrate at least once a year, and if you do you “must” use a traceable standard. You are correct in that how to interpret this from an “advisory” circular is why this subject generates such wide interpretation of the rules. It seems that the bottom line is (to steal a phrase from one of my co-workers who is an A&P, DPE for helicopters, former attorney and all-around smart guy), that it all has to pass the “straight-faced test.” You need to be able to feel good about your tool calibration program and where you would be if the worst happened. I have seen many times where an FAA inspector will tell you that you have to do something, when there is no regulatory basis for what they are pushing. It is just what they think represents best practices.  

In Closing

I read an article a long time ago about calibration. It was about the FAA and a bucket. You see, some in the FAA have all these requirements but never can tell you where they are getting them from. As pointed out, AC 43 is somewhat non-binding and/or up to interpretation. It gets back to the point of the bucket. An inspector asked a question about a compressor wash that went something like, “The manual says three gallons a minute. How do you know you’re pushing three gallons a minute?” The DOM responded,”I measured three gallons in this bucket and timed the process.” (Most would leave it there and accept that statement about meeting the straight-faced test.)

However, the inspector wrote up the DOM for not having the bucket calibrated and the stop watch tested and because neither had a sticker nor certificate. The article ended with the DOM having a calibrated bucket in his office as a point.  

In closing, I’d like to address the straight-faced test and intent. Keep common sense and understanding of what we are doing in context to our maintenance practices, know what you’re doing and what it is based on, and know whether or not it is practical and can pass the test!

Please feel free to join the discussion. Tell us how you do it and we will gladly share it with our readers.