The Evolution of Non-destructive Testing (NDT) in Commercial Aviation - Term Paper Example

The Evolution of Non-destructive Testing (NDT) in Commercial Aviation This is a research paper that investigated the evolution of Nondestructive Testing or Nondestructive Inspection techniques in the commercial aviation industry. It was found that much of the mechanisms and principles behind the current techniques were developed during the onset of industrialization when safety and the economics of extending the lifespan industrial equipment and materials became a primary concern. Specifically, the methods were derived from technologies invented for the railroad, construction and medical industries. The investigation found that NDT is a very important element in the safety and profitability in commercial aviation industry. Simply put, Non-destructive testing or inspection (NDT) is a system composed of numerous analysis techniques that industries use to examine and assess materials or its properties in a non-damaging manner. The techniques do not change or destroy what is being inspected. This is particular useful in the commercial aviation industry, specifically in inspecting airplanes. Aircraft Damage If one considers an aircraft, it is built to fly, not unlike birds. Since it is man-made, however, it cannot give birth to another. And while it cannot die, it is provided a specific lifespan, after which, it is retired because it has gradually degraded to the point that it cannot be safely used anymore. Engineers must construct another vessel when a specific lifespan allotted for an aircraft is reached. The process is certainly costly and it is for this reason why commercial airlines try, as much as possible, to extend such lifespan by using life-extending mechanisms. Unfortunately, the competition in the aviation industry is cutthroat and players are willing to take a certain degree of risk in order to survive and profit in their operations. The implication of this situation is critical. Aircrafts are made with light materials such as aluminum and other composite materials that are prone to damage. The NDT Resource Center explained that aircraft components carry very high loads especially in consideration with their material strength and that small flaws and damages can cause components or the aircraft itself to fail. The so-called "fatigue-cracking" principle explains this best. Imagine: If an aircraft have to take-off, fly, then land and unload passengers in continuously, it will eventually get damaged. This is the same when a wire is bent back and forth and it eventually snaps. NDT further stated that aircrafts, "after they are used for a while, fatigue cracks start growing in some of their parts," and that "cracking can also occur due to other things like a lightning strike." (NDT Resource Center 2011) The Importance of NDT As previously mentioned, NDT can evaluate an aircraft and its components in a non-destructive manner, and determine what type of repair work is needed or whether the vessel is still air-worthy. This is its main utility. However, such utility is not as simple as it looks on paper. The technology that the industry has today has become so sophisticated that NDT evaluations have become so effective and, eventually, economical for commercial airline operators. Khan (1999) emphasized that NDT is considered to be the only way in discovering defects, stressing that NDT can detect cracks or any other irregularities in the airframe structure and engine components which are obviously not visible to the naked eye. In an investigation of the Assembly of Engineering Panel on Nondestructive Inspection, it was found that: NDI programs are necessary to ensure aircraft safety and reduce operating costs and are recognized at high levels in the management of airlines and aircraft manufacturers; The emphasis of NDI in commercial aviation is pronounced in the way commercial airline NDI personnel are generally assigned solely to NDI, and that the airline industry and airframe manufacturers feel that the abilities of properly motivated NDI personnel are much more improved than their counterparts in the US Air Force. (p. 3) Recognizing the importance of NDT, the Federal Aviation Authority (FAA) created the Center for Aviation System Reliability Research, which is composed of several US universities such as Iowa State University, Northwestern University and Tuskegee University. This was way back in the 1990s. In addition, FAA also formed the Titanium Rotating Components Review Team in response to the DC-10 tragedy of 1989 when 111 people died after a hard alpha inclusion in the titanium stage one fan disk in the center of the engine caused a failure. (Noor 2000, p. 99) Afterwards, the United States also established Sandia, a National Nuclear Security Administration laboratory, wherein non-destructive inspection technologies are being researched. (Pan & Polishuk 2011, p. 7) The Federal Aviation Authority also created the Aging Aircraft Validation Center in the facility in order to strengthen its validation capability on commercial aircrafts. There are numerous directives and statutes that cover NDT in part or as a whole. Some are enacted by the states while some are federal in scope. The current development of NDT/NDI technologies are based on the principle that the construction of high quality and sophisticated aircraft system requires the development of an equally sophisticated NDT mechanism in order to respond to the challenges of advanced aircraft developments. The National Research Council highlighted this when it argued that the aviation industry is restricted in their use of some advanced materials and component designs because of inadequate nondestructive inspection technology. (Dau 1983, p. 1) So far, because of NDT policy framework in the US, a safer aviation environment is maintained. To demonstrate this, an industry-wide task force was created to address the issue of aircraft maintenance, safety and repair. This was the Airworthiness Assurance Task Force, which eventually recommended mandatory maintenance procedures for older Boeing and Douglas aircrafts and recommended the replacement of key components at certain points in an aircraft’s service life. (Wells and Rodrigues 2004, p. 236) Evolution The development of Nondestructive Testing can probably be traced as far as 1854 when an accident involving a boiler in Connecticut claimed 21 fatalities and injured several others. The incident prompted the state government to legislate a law that mandates the inspection of boilers every year. The framework, at least with regards to visual inspection of materials, were established during this time. It also helped that the state took it upon itself to adopt and enforce such policy, in effect, institutionalizing inspection. Somewhere between 1880 and 1920, crack detection method was finally developed. While this was mainly used by the railroad industry, it was significant because it provided a mechanism to identify cracks and damages to industry grade steel parts. This method involved the use of thinned oil wherein steels are submerged. Afterwards the steels were painted with a material that becomes a powder when dried. Cracks were found when the powder turns brown. This method is credited to be the progenitor of the liquid penetrant tests that are in use today. This method will be explained more in detail in the later part of this paper. A development that became very significant for NDT occurred in 1895 when Wilhelm Rontgen discovered the X-ray. This was followed by the invention of industrial radiography for metals back in the 1920s. Pivotal to this development is H. Lester’s application of radiography on castings built for several steam plants Virginia. Then, Robert Mehl found a way to use Radium in radiographic imaging through its gamma radiation. It improved on the previous X-ray methods, particularly in determining the thickness of components and materials. Eighteenth century also saw the development of endoscope, which, though primarily used for medical applications, immediately evolved into an NDT tool after Techno-Endoscopes were invented in the 1920s. According to Green, Djordjevic and Hentschel (2003) it was created for corrosion inspection of aircraft structure. The authors summarized its rapid development afterwards: With jet and turbofan engines becoming propulsion standard of commercial aviation in the 1960s, a demand for high quality “Borescopes” (i.e. industrial endoscopes) was created which could not be covered by just abusing medical endoscopes for NDT purposes. In the 1980s, fluorescent penetration inspection (FPI) was combined with borescopes. (p. 674) It is interesting to note that most of the early development in NDT or at least those that led to its modern form were developed for the railroad industry. This has been previously highlighted by the method of finding cracks in steel. In addition to this, somewhere between 1927 and 1928, the railroad industry invented its first magnetic induction system, a related development to the previously developed electromagnetic eddy employed in measuring how thick materials are. This new invention, however, can find damages made in the railroad track. The discovery was credited to the combined efforts of Elmer Sperry and H.C. Drake. Afterwards, the methods that would become precursors to several NDT techniques today were developed. A brief timeline is outlined below: 1929: A.V. DeForest and F.B. Doane invented the magnetic particle method; 1935: Same inventors created the liquid penetrant tests; 1944: Ultrasonic tests method was finally completed and applied, thanks to the efforts of Floyd Firestone. 1950: The very first instrument identified as a tool for non-destructive testing was finally introduced. It was called the Schmidt Hammer, which was mainly used to evaluate concrete materials. In the same year, the acoustic emission was also launched and adopted as an NDT technique. The above developments collectively led to more sophisticated versions being used today. These are highlighted in the following section. Modern NDT Techniques As outlined in the paper written by Khan, the following are the different NDT Methods in use in aircraft inspection today: Liquid Penetrant: considered to be one of the oldest of the current NDT methods, it is still widely employed by many airline maintenance departments. It refers to the “physical and chemical nondestructive procedure designed to detect and expose surface connected discontinuities in ‘nonporous’ engineering materials.” The main utility of this method is that it increases the “visible contrast between a discontinuity and its background,” providing visual evidence of cracks, porosity, among other damages. Magnetic Particle: This is a nondestructive method designed to identify breaks in the surface of materials, including some sub-surface damage, particularly in “ferro-magnetic materials. According to Khan, “The testing method is based on the principle that magnetic flux in a magnetised object is locally distorted by the presence of discontinuity. This distortion causes some of the magnetic field to exit & re-enter the test object at the discontinuity.” Eddy Current: This NDT technique is a popular one in the area of identifying cracks that are caused by usage and service. It is also useful in finding areas damaged by corrosion, holes, among others. These are easily detected because “eddy currents are electrical currents induced in a conductor of electricity by reaction with alternating magnetic field,” and that “the a) electrical conductivity b) magnetic permeability c) geometry and d) homogeneity of the test object, all affects the induced currents.”(Khan) Therefore, the method can detect damages in challenging areas such as tubes and engine components. Ultrasonic: This method takes after the sound whose frequency is beyond what can be audible to human hearing. It uses sound waves in order to detect anomalies. Khan explained: “Sound has a constant velocity in a given substance; therefore, a change in the acoustical impedance of the material causes a change in the sound velocity at that point producing an echo. The distance of the acoustical impedance (flaw) can be determined if the velocity of the sound in the test material, and the time taken for the sound to reach & return from the flaw is known.” Radiography: Along with the Liquid Penetrant, this method is an old NDT technique. With the use of X-ray or gamma ray, anomalies such as cracks, debris, loose fitting, holes, in addition to thickness variations, among other flaws can be identified when it is reproduced in film. Radiography can be used for both metallic and non-metallic aircraft component. Visual Inspection: This is supposedly the most widely used method of evaluation. It is easy because the inspector, with the help of sufficient lighting and other tools such as borescope, mirror, microscope and magnifying glasses, could examine materials with the eye. Sonic and Resonance: This NDT method is best used in finding “crushed core or debonds in adhesive bonded honeycomb, impact damage and delimitations in composite structures and exfoliation corrosion.” (Khan) This method makes use of a sonic inspection tool. Infrared Thermography: This method in nondestructive testing employs imaging based on the heat flow that are occurs in the flaws and damages in materials. A thermal image captured can reveal the amount of radiation that correlates with the heat coming off a material, representing the physical condition of a material or component being examined in the process. It is clear with the outline of the above NDT methods that many techniques are still reminiscent of the original mechanisms invented from the eighteenth century until the 1940s. Some of the principles involved and the tools used are still the same and many were merely improved upon. Those that make use of X-ray, the tools in visual inspection, liquid penetrants are cases in point. Conclusion As explained by this paper, NDT is very important in the aviation industry because it is the only mechanism that can ensure the safety of aircraft passengers. This is underscored in commercial aviation, particularly in the practice of using ageing aircrafts in order to maximize profits. NDT makes it possible to extend the life-span of aircrafts and also ensure its air-worthiness that significantly reduces the risks to the aircraft and its passengers. The methods outlined by this paper – Liquid Penetrant, Magnetic Particle, Eddy Current, Ultrasonic Methods, Radiography, Visual Methods, Sonic/Resonance, and Infrared Thermography – evolved out of several mechanisms that were created during the rapid industrialization that occurred starting from the latter part of the eighteenth century. Particularly the technologies developed for the railroad and medicine were adopted and modified in order to suit the requirements and the advances of aircraft production development. References Assembly of Engineering (US), Panel on Nondestructive Inspection. (1982). The effectiveness of the Air Force nondestructive inspection management: The Interim Report. Clifford Wells (ed.). AbeBooks. Dau, G. (1983). Final report on the effectiveness of the Air Force nondestructive inspection. Washington, D.C.: National Academies. Green, R., Djordjevic, B. and Hentschel, M. (2003). Nondestructive characterization of materials XI. Berlin: Springer. Khan, M.A. (June 1999). "Non-destructive Testing Applications in Commercial Aircraft Maintenance." NDT.net. Available from: [10 September 2011]. NDT Resource Center. (2011). "Nondestructive Testing in the Aerospace Industry." NDT Resource Center. Available from: [11 September 2011]. Noor, A. (2000). Structures technology for future aerospace systems. AIAA. Pan, H. and Polishuk, P. (2010). Fiber Optic Sensors & Systems. Information Gatekeepers, Inc. Wells, Alexander and Rodrigues, Clarence. (2004). Commercial aviation safety. New York: McGraw-Hill Professional. Read More