There has been a great deal of press recently about the approval and use of 3D printed parts in aircraft. General Electric (GE) has developed 3-D printed fuel nozzles out of metal for its LEAP jet engine and Airbus has recently printed a metal retaining bracket. Both of these parts and others will be flying in an aircraft carrying you soon! So where does the hype end and the science begin? What does the future hold for 3D-printed parts in aircraft?
Let us first set ourselves straight on terminology. Industrial 3-D printing has been around since the 1980s and was initially known as rapid prototyping because of its most popular application – making prototypes for manufacturing. It has also been known as solid freeform fabrication (SFF) and now more accurately known as additive manufacturing. Additive manufacturing/3D printing is a way of making physical parts by depositing material one layer at a time to build the part without the need for specialized tooling, such as molds, lathes, and milling machines. The term additive manufacturing includes such technologies as stereolithography, fused deposition modeling (FDM), laser sintering, and electron beam sintering.
Stereolithography was patented in 1986.  As shown in Figure 1, below, in stereolithography, a layer of resin is applied on a substrate and then cured, layer-by-layer, typically with ultraviolet light. The substrate moves up and down within a container of resin. After each layer is cured, the platform is lowered and a recoater blade smooths the next layer of resin onto the build platform for curing. Thus, each individual layer is cured until the part is fully constructed. Materials and properties can be varied to achieve desired attributes such as strength, wear, and finish.
In fused deposition modeling, an extruder is carried by a gantry. A plastic filament is melted as it passes through the extruder and applied in a liquid state, layer-by-layer, to form the object. As shown in Figure 2, the plastic material hardens and cures as it is cooled. The plastic filaments may include support materials that support the object as it is being formed, which can be removed after the object is completed. FDM was patented in 1992. 
Stereolithography and FDM have applications in aviation for making prototypes and plastic parts. For example, Materialise NV (NASDAQ: MTLS), located in Leuven, Belgium, has reported its production of flight ready 3-D printed plastic parts for the Airbus 350 XWB. The parts have been certified under European standards, EN9100 and EASA. 
But what about metal parts? And, more importantly, what about critical structural components of aircraft? Creating metal parts with additive manufacturing is also nothing new. Laser sintering was developed in the late 1980s and patented on July 3, 1990.  In laser sintering, metal powder is deposited on a substrate. As shown in Figure 3, a laser (with a scanner directing the beam) is used to sinter each layer. After the layer has been sintered, the excess powder is removed and a new layer of powder is spread over the substrate. The next layer is then sintered and the process is repeated.
This process is also known as powder bed fusion. Electron beam sintering is similar but uses a more powerful source of energy to sequentially melt the metal, which may be in wire or powder form.
On May 19, 2015, GE Aviation reported that its flying test bed, a Boeing 747, took off with twin LEAP jet engines, each “equipped with 19 3D-printed fuel nozzles.” According to its report, the LEAP is “15 percent more fuel efficient than comparable engines built by CFM International.” CFM is a 50/50 joint-venture between GE Aviation and France’s Safran (Snecma) that designed the engine. The LEAP engine is scheduled to fly on the A320neo Airbus.  As shown in the picture of the fuel nozzle below, additive manufacturing accorded the intricate shape needed for the fuel nozzle, reducing weight and increasing overall engine efficiency.
Airbus already has produced a variety of plastic and metal brackets, whose material and structural properties have been tested and validated, and are now incorporated on the company’s fleet of developmental aircraft.  As shown below, the Airbus printed bracket streamlines portions of the bracket wall to eliminate excess material. This in turn reduces weight, which is critical for improving aircraft efficiency. The additive process allows designs to be optimized for strength, eliminating portions of the structure which are not critical and/or unnecessary.
So with these exciting new parts, where does the Federal Aviation Administration (FAA) stand concerning certification? As it turns out, the FAA is fully committed to working with industry to develop safe and reliable components produced with additive manufacturing, though at this point approval has been limited on a case-by-case basis to non-critical flight structure components.
On March 24, 2015, the FAA participated in the 17th Annual Gorham PMA/ DER Conference - in San Diego, CA on additive manufacturing. The FAA is concerned with additive manufacturing because it brings with it a number of technical risks. What are those risks? Chief among them is how to quantify the variables associated with the additive manufacturing process. In subtractive manufacturing, i.e. machining, heat treating, casting, forging, there is a long history and understanding of these processes. In additive manufacturing, the mechanical behavior is very dependent on the processes itself. As noted at the conference, there are over 120 variables that need to be controlled to produce stable and repeatable parts. The FAA has not yet developed acceptable material specifications for additive manufacturing. The FAA has formed an AM (Additive Manufacturing) National Team with members participating from groups within the FAA - Directorates, Tech Center, CSTAs, Flight Standards, and Headquarters. 
The basic engineering challenge is that the mechanical properties may vary with each layer, in addition to the behavior of the material as a result of the fusion of the printed layer and the next sequential layer. The mechanical properties of the printed object as a whole must also be taken into consideration. These variables are complex. For example, each layer must be produced, i.e. printed, with the same parameters and properties (material, thickness, density, hardness, and temperature, etc.), or the properties will vary. In some parts, layer thickness will vary by design. Areas of weakness can be created through voids and deposition errors, making the component potentially subject to fatigue and/or stress fracture. Stress concentrations in the material may also be formed in the production process. See Figure 4.
During the first week of September 2015, leading experts in additive manufacturing convened in Dayton, Ohio for a three-day FAA Chief Scientific and Technical Advisors (CSTA) workshop. The event was co-hosted by the Air Force and included more than 30 FAA employees from 16 sites. The purpose of the event was to educate the FAA workforce, benchmark the certification and qualification efforts of other agencies, and promote inter-agency collaboration. These agencies included the U.S. Air Force, National Aeronautics and Space Administration, Defense Research Products Agency, and the National Institute of Standards and Technology. 
The bottom line is that certification regulations are still a ways off. The good news is that the FAA has developed its own team of experts to address the monumental task of certifying parts for aircraft, while ensuring they are fit to fly.
William J. Cass, Esq., CFI, AGI
 U.S. Patent, No. 4,575,330, to Charles W. Hull, entitled “Apparatus for Production of Three-Dimensional Objects by Stereolithography,” issued March 11, 1986.
 U.S. Patent No. 5,121,329 to S. Scott Crump, entitled, “Apparatus and Method for Creating Three-Dimensional Objects,” issued June 9, 1992.
 U.S. Patent No. 4,938,816 to Beaman, et al., entitled, “Selective Laser Sintering with Assisted Powder Handling,” issued on July 3, 1990.