Anyone who is interested in aviation has noticed that most commercially produced aircraft have a very similar architecture. Although powered flight has been through many extraordinary changes in the past century, many of the basic structural characteristics remain unchanged.
Over the years, many aircraft concepts have been tested, aircraft speeds have increased, wing size and shape have been modified, the power plant has evolved, and the range has increased significantly. With the evolution of technology and all of the modern manufacturing methods available today why are airplanes riveted instead of welded?
Although welding has been around since the 1800’s and is an effective way to bond materials together, rivets have several advantages in the aircraft industry such as the ease of inspection, maintenance, and repeatability. The typical thickness of aircraft skin also makes it less appropriate for welds.
While rivet technology might seem simple, it has a lot of unique benefits that make it a great fit for aircraft design. Furthermore, they are advantageous when it comes to maintenance and safety considerations as we’ll cover in the rest of this article.
The Change of Aircraft Skins
Aircraft have taken on several shapes and sizes over the years but for the most part all of them have a basic structure that includes all of the same components: A fuselage, wings, stabilizers, flight control surfaces, and landing gear. Even though the basic components remain essentially the same, the material that covers those components has included fabric, metal, and composites.
Not only have the skin types changed, there has been and continue to be many different aircraft structures underneath the skins. A very typical aircraft structure is categorized as being semi-monocoque. This type of design splits the aircraft stresses up between the skin and the internal structure of the aircraft. The internal structure consists of bulkheads, stringers, longerons, spars, and ribs. Although the external loads are shared, the skins (where most rivets are used) have to be robust.
With the use of different materials the method of bonding them together can range significantly. Aluminum remains the most prevalent aircraft skin today, and riveting has proved the most overall effective means of bonding this type of metal (at least in aviation).
Types of Bonding
A rivet is a mechanical fastener that is a smooth cylindrical shaft with a factory head at one end. The materials that are to be bonded together get a hole drilled through them. The rivet is then placed in the predrilled hole to hold the two surfaces together. The end without the head is pressed (or “bucked”) to form a pressed head on the other side and the material is essentially “pinched” between the two heads.
Here’s a good overview of rivets if you need a visual. Particularly the short clip around 3 minutes is helpful:
When rivets were very first invented they were forged by hand, the first machine capable of manufacturing steel rivets was invented in 1836 and rivets began getting used on aircraft in the 1920’s and 30’s.
In its simplest of descriptions welding is a process that “melts” material and once cooled allows the two materials to be bonded together. There are many different ways we weld materials but the most common is arc welding in which an electrical arc melts the material and allows for a strong bond to occur between the metals once cooled.
Aluminum welding was greatly improved in the 1940’s and has been used in many industries as one of the most effective ways to bond that particular material together.
Why are airplanes riveted instead of welded?
Reason #1: Material Thickness
Keeping the weight as low as possible is always a great challenge during any aircraft design. It is necessary for an aircraft to be capable of demonstrating high strength and be able to achieve thousands and thousands of fatigue cycles. However the lighter you can make the structure the longer the range and the greater the payload you will be able to achieve.
Aircraft skins covering pressurized aircraft can be as thick as .039”. The pressurized aircraft tend to have thicker skins because of the pressure difference and the frequent pressure cycles that those aircraft see. Unpressurized aircraft do not need to be as thick as the pressurized aircraft and can be as thin as .020”.
These thicknesses are much thinner than other tanks that hold pressure. For reference, most propane tanks are 0.25” to over 0.375” thick. However those types of tanks are typically made of steel whereas most material used in the aircraft industry is aluminum and aluminum requires a different process to weld than steel. Even though it is a different process, aluminum gas welding is capable of bonding material as little as .031” thick.
Even though there are aircraft skins we could weld, it would introduce heat into the material and could change the properties and strength of the material. For this reason, rivets are a better fit for bonding.
This change in strength is can be calculated but it is a greater challenge to determine exactly which area was impacted. It is difficult to define the exact area or how much material was impacted by the extreme heat.
It is common for a safety factor to be included in the overall strength of an assembly that has been welded. Most of the time these assemblies have components with an increased thickness to allow for the knock down in strength.
Reason #2: Repeatability
With the use of modern tools and jigs getting a repeatable process has improved significantly. However just like welding, riveting is an art and takes an operator that has a lot of experience in order to achieve a reliable product.
To help with the repeatability of rivets there are certain specifications that most rivets used in industry must meet. These standards help ensure that they all form in a similar way with predictable results and predictable strengths.
Welding methods have gotten more repeatable and can even be done autonomously in some applications. This helps with repeatability but with the thin material and trying to measure the exact heat you are putting into the material it makes it difficult to accomplish a high repeatability and a known strength of the assembly.
Reason #3: Inspection
During inspection of rivets the inspector will spot check rivets using a gauge that measures if the “butt” of the rivet has the proper diameter and height (see the 3:00-3:20 mark in the rivet video above). The values for diameter and height are pre-determined for the different sizes of rivets and are based on what will give the rivet the greatest strength.
Because welding is a chemical process and has layers that cannot be seen from the surface they can take more intrusive or expensive ways to determine the quality.
It is also possible that during an assembly there is a bad rivet found by an inspector. If that happens it is possible to drill out a rivet and simply place a new one in its place. This has minimal impact on time and the rest of the build.
If a bad weld is found it is difficult to reverse the process without causing significant changes to the neighboring area, it can also be very labor intensive.
Reason #4: Maintenance
If a rivet is later found to be unacceptable there are processes in place to drill out the old rivet and place another in its place. This can be very beneficial to the manufacturer and the owner of the aircraft because this does not take much time and is not intrusive to the aircraft. The same goes for areas where a component might be damaged and part of the structure has to be removed.
If a weld is found to be broken or unacceptable the process to fix it is much more intrusive. All of the old weld should be removed which in turn can create a larger gap to fill thus needing to introduce a filler material. If no filler material is needed a new weld must still be completed and must be coated once again with paint to protect the metal.
Related Questions
Is welding used in the aircraft industry?
Although welding is not used on the skin of the aircraft many internal structures and assemblies are created using welding. Typically other structures in aircraft, trusses, wing mounts, and engine pylons for example, need to be very strong so the material is much thicker than the aircraft skins, ribs, and stringers that make up the rest of the aircraft structure.
What about Composites?
Composites are made up of layers of material that are held together in a resin. Composites have been used throughout the history of aircraft and can even be seen as the entire structure of some of today’s modern aircraft such as Cirrus, TBM, and Epic aircraft. The use of composites is becoming more and more common in commercial aircraft manufacturing and they are extremely effective, but the process comes with its own challenges.
Composites are lighter and, because of the layered material, are stronger than traditional aluminum. Composites are also more versatile in shape. You can make molds into almost any shape and therefore you can create complicated shapes relatively easy compared to aluminum welded structures.
Some disadvantages of composites are similar to welding, in that it is difficult to inspect the interior layers of the material and requires a more expensive method to inspect the part for flaws.
Between the tooling, resin, and material composites are typically more expensive than aluminum. Composites also do not handle heat as well as metals and that can cause an issue in the aircraft industry. Similar to welding composites are a permanent structure so repairing them can be very involved and changes a lot of the surrounding structure.
