Understanding Durometer


You’ve heard of odometers, speedometers, altometers (lots of –ometers). But what, pray tell, is durometer? Hint:  It’s not an instrument you slap on the inside of your cockpit.


Figure 1 – Where to find durometer or “Hardness Rating” on an O-ring product page on SkyGeek.com (CLICK TO ENLARGE)

Basically, durometer refers to both the measuring device and the measurement of a material’s hardness. In fact, “duro” is Latin for “hard/tough.”  The “-ometer” refers to the fact that it is based on a scale; it is a measurement of some kind. Durometers indicate a material’s resistance to permanent indentation. In the aviation industry it often corresponds to products with an elastomer, polymer, or rubber composition. This is especially true when dealing with O-rings.

You will find specifications on the product pages of many O-rings we sell. Durometer is synonymous with “Hardness Rating.” (See Figure 1)

So why is this important? When applying an O-ring to a part of your aircraft that requires one, its material composition will give you an indication of its function and durability. Some assemblies call for harder elastomers, others softer. It’s the durometer that will indicate if the O-ring is the right fit.

For a brief overview of durometer, you can watch this informative Youtube clip. The content is great at giving a casual viewer a basic understanding of the topic. However, be warned: the speaker in the video is weird and his mannerisms may or may not be amusing.


Shore Durometer Chart courtesy of the incredibly useful blog site from Alan Garratt. (CLICK TO ENLARGE)

Another aspect of durometer you may be wondering about is the use of the word Shore. Not only does Shore refer to the creator of the durometer, Albert Shore (1876-1936), it also is used in classifying the types of durometer scales.

There are many types but the two most common are Type A and Type D. According to Alan Garratt, “There is a simple logic laid behind the sequence of the early Shore scale letters and how they evolved to measure harder and harder materials. Later the need to measure materials softer than the range of Shore ‘A’ accuracy proved more difficult to fit into this progressive sequence of lettering.”

Garratt’s blog, Shore Durometer History,  is an excellent resource on the topic and is shore sure to provide those technically-inclined geeks out there with information on the development of this system. In particular I recommend reading the section “Evolution of the Shore Scales.” The bulleted points briefly explain why the classification is the way it is. Even better – there is a convenient chart of the various scales to compare and contrast (as seen above).

As mentioned, Type A is considered perhaps the most common of the Shore durometer scales. But within that type is a range. Numbers on this range go from softer to harder as they increase. Still, that may be too vague to understand so we retrieved this comparative list from Mykin Inc. The list provides real-world examples associated next to the numbers to give you a better idea.




Rubber Band


Pencil Eraser


Car Tire Tread


Running Shoe Sole


Leather Belt


Shopping Cart Wheel


Why Safety Wire?

Here’s a familiar scenario: You’re on a budget and money can only be allotted toward tools and equipment that get your aircraft up and flying. What do you purchase?

Obviously every pilot and plane owner will have different requirements based on their own unique maintenance needs at any given moment.

“Safety first.” Isn’t that the expression that should take precedence and determine what supplies are truly required?


Page 7-19 of the FAA’s Advisory Circular 43.13-1B on “Safetying.” (CLICK TO ENLARGE)

With all that cloud-surfing at high speeds you are bound to encounter areas on your aircraft where fasteners loosen. Complications arising from vibrational forces are matters that should not be dismissed so easily. Routine maintenance and repair should include the use of safety wire.

In fact, according to the FAA’s 14 CFR Part 43 the use of safety wire is one example of a preventative maintenance item and is included as an item in a propeller check.

So what is safety wire and why should you use it? We have defined safety wire and its use in a previous blog post.

However it bears repeating that safety wire is a means of preventing vibration from compromising applicable fasteners from loosening in the event that they fail during operations.

Aviation is not natural. When you transport a person through the air you have to compensate for natural forces that resist. That which is unnatural is usually dangerous. And so that’s why the aviation industry ensures there are systems in place to buffer and backup. In other words, one component fails there is another in place and another. Safety wire works on the same principle. It’s peace of mind.

Besides peace of mind, there are other benefits to using safety wire. First it can act as an aid during inspections. If safety wire is applied and it is out of place or broken, this indicates that vibrational forces have acted on those fasteners and thus may need to be repaired or replaced before next flight. But how would you know this? Safety wire, in its twisted configuration, is highly visible and displays an assurance that fasteners held to scrutiny are secure.

Safety wire is also relatively inexpensive and the tools and accessories are not hard to find. Also these tools are not as rigidly limited in use, meaning it’s not like you need a specific screwdriver or bit to properly fit a screw head. Safety wire pliers, twisters, and tabs have a wide, almost universal field of use.

Still, safety wire has a few downsides that are worth noting. Casual plane owners may find it takes time to properly install; in this case it might be easier to seek assistance from a qualified mechanic or technician.

Another problem may occur when cutting excess wire. Small pieces may cause injury to either person or plane so it is important to properly clean the area of excess bits of wire scrap after completion of installation as well as to wear adequate safety gloves and eyewear during application. After all safety first, right?

For more information on the implementation of safety wire, aka “safetying,” please refer to, Pages 7-19 to 7-26 Section 7, Chapter 7 of the FAA’s Advisory Circular AC 43.13-1B.

SpaceTEC’s Overview on Screwdrivers and Screw Heads

Greetings, avgeeks!

I received some encouraging feedback from my last post about SpaceTEC so I figured I would serve up another helpful short clip. Instead of safety wires, this brief guide covers the basics about screwdrivers:

Of all hand tools, screwdrivers are among the top in terms of popularity and usefulness. And yet they are not complex devices that require a PhD. As SpaceTEC states as a running footer in the above clip, screwdrivers offer simplicity when it comes to applied mechanics; they are made for the sole purpose of loosening or tightening screws. Nothing hard to understand there.

Search for a screwdriver and your head might get all twisted up since there are a variety of types—everything from slotted to jewelers. So how in the heck do you choose what is right for your application or assemblage?

According to SpaceTEC, screwdrivers are “classified by shape, type of blade, and blade length.” They recommend selecting the largest blade (or bit) that will fill the screw head. This makes sense. Why? If you’ve ever tried to fit too large of a bit into the head, you know that it obviously won’t work. Neither will trying to place a bit too small into the screw head. They won’t fill in the hollow correctly. They also won’t effectively turn the screw. Instead, a bit that is too small or too large for a socket will warp and misshape the screw head so that the right blade won’t even work. Oh, and it can even damage the bit itself. Talk about a frustrating and costly blunder.

The two most well-known screwdrivers are the straight/common/ flathead (which fit with slotted head screws) and Phillips head (which fit in heads that form perfect crosses). There’s also offset screwdrivers that have a Z-shape and are composed of two right angles that are designed for use in hard-to-reach areas. Offset screwdrivers can have tips that are either common or Phillips or both.

To select an appropriate screwdriver the main issue revolves around the screw head. Therefore it is really the screw that determines how to select the right driver. SpaceTEC lists nine screw heads, which are briefly identified below:

Slotted – The most basic type. Can’t get any simpler than this design and in fact it is probably the oldest and cheapest to make. A straight vertical or horizontal (depending on your position) line down the middle of the head.

Phillips – Perhaps the most famous. Try and not find this type of screw head. It’s certainly a household screw. Its cruciform shape allows for better torque.

Pozidriv – Nope, that is not a misspelling; the Pozidriv (which may or may not be derived from the term “positive drive”) is another cruciform type head. However, it almost looks like it is a “bloated” cross, i.e. kind of widened in the mid-section. An advantage over its predecessor: it has straight flanks as opposed to the round flanks of Phillips. This helps prevent what is referred to as “cam out,” i.e. that accidently and incredibly frustrating time when the screwdriver slips out of the screw head.

Torx – Also known by its less appealing, technical name “hexalobular.” This recess is designed to resemble a star with six points. This allows for greater torque while resisting the tendency to cam out. That results in less fatigue on your hand but also prolongs the life of the bit on your screwdriver.

Hex – Short for hexagon, it has a six-sided polygonal shape. It is specially designed for use with Allen wrenches (or “keys”). The advantage here comes with size: Allen keys can fit and rotate in smaller areas where other screwdrivers can’t.

Robertson – A good way to know this screw head’s advantage is by remembering this: “Robertson retains.” It has a square-shaped socket (indentation or recess). Its tapered design offers reduced cam out and product damage while speeding up production. This type of head is used prominently in Canada.

Tri-wing – As the name indicates, this screw head has a triangular recess with three wings extending from the vertices of the triangle. This socket was particularly designed for use in aeronautics but has since been extensively used for electronics equipment in other industries.

Torq-set – Another cruciform. Don’t adjust your glasses, this socket looks similar to a Phillips head but something seems off. Actually, that’s it—while it has a cross-shape, the “arms” are offset as if the lines were broken up. Thus, a Phillips screwdriver will not fit. This screw head is used in aviation.

Spanner – This screw head consists of two round holes that look like a pair of eyes. This design’s main purpose is to prevent tampering.

If you’re an avgeek, chances are you already know about screwdrivers and screws. Still, brains are like machines. Just as machines often need to be well-oiled, frontal lobes need lube in the form of learning and re-learning. Reinforcement is, after all, a major component in keeping things structurally sound and operational.

SpaceTEC’s Take on Safety Wire

Is it the red or the blue wire? Careful, if you cut the wrong one your ashy remains will be found after the dust settles from a mushroom cloud explosion…

Greetings, geeks! I haven’t posted in a while so figured I would. I’ve had safety wire on my mind lately so I decided to search the Interwebs for anything that would serve as a quick guide or at least a refresher. What I found was a nice short video from SpaceTEC:

“Safety wiring is considered a redundant means of securing components to prevent them from becoming loose, should the primary retention capability fail during operation.” That is what the first screen of the video says. ‘Redundant’ used in this context does not mean something negative; SpaceTEC is not talking about overusing words in a five-page paper in English class. Here, redundant simply refers to the purpose of safety wiring; it acts as an additional and precautionary measure so that parts, most often hardware, remain intact. When it comes to securing fasteners (nuts, bolt, screws, etc.) and preventing vibrational forces from loosening parts, safety wire is a reliable and inexpensive means that leads to peace of mind.

The next screen from the above video states: “Items shall be safety wired in such a configuration that the safety wire shall be put in tension when the parts tend to loosen.” The screen displays two images—illustrations of a safety wire installed on bolt-heads and safety wire used on Castle nuts. This serves as a nice visual aid to give you an idea of the appearance of the configuration. Such a configuration allows for the safety wire to act as an antagonist to the part, meaning as the nut loosens, the wire tenses up. It is similar to how muscles function: as one muscle expands or extends, a corresponding muscle contracts. Imagine if both muscles contracted at the same time? Snap! Well, if a safety wire loosens while a part loosens, it defeats the whole purpose of the configuration.

The third screen retains the two images from the previous screen. But now Aircraft Circular AC 43.13-1B is mentioned. “AC 43.13-1B covers all the aspects of general safety wire practices. There are three common sizes: 0.020, 0.032, 0.041. New safety wire shall be used for each application.” Check out Pages 19-25 of the Aircraft Circular AC43.13-1B where the FAA provides guidelines for “safetying.”

For the fourth and final screen of the video, safety wire pliers are briefly touched upon, particularly how they should be used to apply the wire: “Safety wire should be twisted six to eight turns per inch. The pigtail S/B 1/4 to 1/2-inch (three to six twists).” A picture illustrates this point.

For those who don’t know, SpaceTEC —located in Cape Canaveral, Florida— is the National Science Foundation’s (NSF) Resource Center. Its primary mission is to serve as an advocate for employing aerospace technicians. The organization achieves this by providing an academic outlet for such individuals. This leads to a well-trained workforce for commercial, civil, and defense space activities relating to the aerospace and aviation communities.

According to SpaceTEC: “Its certification programs offer performance-based examinations that result in industry-driven nationally recognized credentials that reflect the competencies employers demand. The certification program is offered through a nation-wide consortium of community and technical colleges, universities, business and industry organizations, and government agencies.”

The good news is that SpaceTEC recently received a grant renewal from the NSF; this was accomplished through the NSF’s Advanced Technical Education (ATE) program. This will certainly help with further developing the certificate program, which consists of five key areas – Applied Mechanics, Basic Electricity, Industrial Safety, Materials & Processes, and Tests & Measurements.

Thanks to SpaceTEC for offering a quick reference for the applied mechanics of safety wire.