Whether during fabrication, manufacturing, or in actual service, the parts that are combined to create aircraft need to be tested for composition and shape, structural strength and integrity, and analysis to detect wear and damage.
This is where Non-Destructive Testing (NDT) comes to the fore. NDT is used to assess the real-time condition of aircraft parts, without damaging them in any way. The science behind NDT is complex, innovative, and ingenious – and it constantly evolves to keep up with leaps in aircraft construction such as composite materials. Companies who provide NDT services to the global aerospace industry include Eddyfi Technologies, EVIDENT Europe GmbH (formerly Olympus), Nextant Aerospace, Waygate Technologies (a Baker Hughes business) and WeaverNDT, among others.
A Range of Testing Methods
Aircraft are an amazing amalgam of materials, architectures and technologies. This is why NDT companies employ a range of methods to test them, because a “one size fits all” approach simply wouldn’t work.
For instance, Eddyfi Technologies uses “ultrasonic testing (phased array) and eddy current testing (conventional and arrays),” said Christophe Piron, the company’s NDT sales manager. Evident employs “high/low frequency eddy current and phased array ultrasonic testing,” said Angelique Raude, the company’s strategic marketing manager for advanced NDT – EMEA. Meanwhile, Nextant Aerospace uses eddy current, ultrasound, liquid penetrant and magnetic particle testing methods; Waygate Technologies deploys ultrasonics, industrial X-rays and remote visual inspection; and WeaverNDT uses ultrasonic, eddy current, X-rays, and visual inspections.
NDT Methods Explained
To help put them in context, here’s a breakdown of the most popular NDT methods mentioned above.
Let’s begin with eddy current testing. According to The Welding Institute website (at twi-global.com), “When an alternating current is passed through a conductor – a copper coil for example – an alternating magnetic field is developed around the coil and the field expands and contracts as the alternating current rises and falls. If the coil is then brought close to another electrical conductor, the fluctuating magnetic field surrounding the coil permeates the material and, by Lenz’s Law, induces an eddy current to flow in the conductor. … Any changes in the conductivity of the material being examined, such as near-surface defects or differences in thickness, will affect the magnitude of the eddy current.”
At Nextant Aerospace, “Eddy current testing is used for the majority of our aircraft inspections and we employ high frequency surface scan, low frequency second/third layer scans, and bolt hole eddy current techniques,” said Shawn Ehrhart, the company’s director of NDT. “These techniques are all used to detect flaws like fatigue cracks and corrosion. We also utilize eddy current conductivity testing to determine if materials have been adversely affected by heat input from things like lightning strikes or exhaust gas exposure.”
According to Ehrhart, the biggest benefit of eddy current testing is how quickly the instrument and probe testing combination can be set up, without having to remove paint/primer from the inspection surface in most cases. “Another benefit is that properly trained and experienced technicians can easily reproduce test setups and verify inspection results,” he said. “Typically, there is not a lot of error from individual interpretation of test results, and confident pass/fail dispositions are achieved quickly.”
Waygate Technologies uses eddy current testing along with industrial X-ray and remote visual inspections. Modern IXS uses computed tomography — a series of X-rays captured from various angles and locations — to compile complex, detailed 3D images of objects in a range of informative colors.
According to a company spokesperson, “Waygate Technologies’ advanced CT solutions are a powerful quality control and process optimization tool, helping customers meet and exceed standard compliance regulations by providing fast inspection and accurate 3D measurement of components that are difficult to examine by conventional two dimensional (2D) radiography or coordinate measuring machines (CMMs).”
In partnership with Rolls-Royce, Waygate is supporting sophisticated remote visual inspections using 3D scanning/measurement tools to examine aircraft engines, plus AI-enabled apps to compile, analyze, and assess the testing data. This can be done wherever the aircraft is located, which is extremely helpful during unexpected AOG events.
“Each inspection step is carried out with Waygate Technologies’ Everest Mentor Visual iQ VideoProbe, eliminating the need to transport or carry additional IT equipment to the engine,” said a Waygate spokesperson. “A process that previously took 90 minutes now takes just five. Once all data is processed and reported, users can simply push a button to send the data directly to the Rolls-Royce Inspection Insight platform, providing tremendous efficiency by removing this burden from the inspector.”
AI plays a big role in Waygate’s data analysis and actionable recommendations to aircraft technicians. “Waygate Technologies’ InspectionWorks Non-Destructive Testing (NDT) software platform is built to simplify the NDT inspection process, from data collection to analysis and file management,” said the company spokesperson. “The software helps to transform inspections from a necessary cost to a critical flow of data enabling better product design and optimization.
Fluorescent liquid penetrant and magnetic particles are used for other kinds of NDT inspections. At Nextant Aerospace, “medium to ultra-high sensitivity penetrants are used to inspect aircraft wheels, combustion engine parts, turbine engine parts, landing gear components, and various other items,” Ehrhart said. “Penetrant testing allows for an easier inspection of large parts and objects with complicated geometry. However, these techniques will only detect surface breaking flaws.”
“Magnetic particle (MT) techniques include full wave, direct current, wet, horizontal bench testing in our laboratory and field portable yoke and coil testing,” he continued. “ Much of our MT time is spent using the wet bench to inspect ferrous landing gear components, combustion engine parts, and starter/generator components. MT provides a fairly quick inspection of all exposed surfaces, with the possibility of detecting both surface and near-surface flaws. But the test objects must be ferrous. Materials like Inconel, which are weakly affected by magnetic fields, are not good candidates for MT inspection.”
The increasing usage of composite materials in aircraft has required a shift to new NDT methods such as ultrasonic testing using phased arrays. The reason: “Carbon fiber composites are not electrically conductive, which eliminates eddy current and magnetic particle testing,” said Ehrhart. “Liquid intrusion into a composite material or assembly would have detrimental effects, thus eliminating liquid penetrant testing. And flaws in carbon fibers are often in parallel to the major axis to the part surface (disbonding between two or more layers) making them extremely difficult or impossible to detect using radiography (X-ray/Gamma ray testing).”
“The application of phased array ultrasonics for non-destructive testing composites is probably the biggest advance in NDT over the past decade,” said Greg Weaver, president of WeaverNDT. “Using an array of ultrasonic probes, you can compile a detailed 3D image of the part being tested in full color, rather than just getting a pulse indication on a screen. This gives you a clear visual interpretation of what you’re looking at, where many people can look at and see color differences that indicate defects.”
“The development of phased array ultrasonics for composite inspection, together with adaptive methods for complex surfaces/geometries (e.g. radii inspection) has increased the productivity and reliability of aircraft inspections, both at manufacturing and maintenance stages,” Eddyfi’s Piron said. But older NDT methods still matter: “In the aircraft engine industry, eddy current array has enabled simplified disks and blades inspections directly at the workshop with portable inspection kits, and without using complex inspection machines,” he said.
Although WeaverNDT does not use phased array ultrasonic testing, it does use ultrasonic resonance to map and measure the natural resonance of parts being tested. Deviations from the norm can indicate differences in the part’s material composition, including defects not visible to the naked eye.
The same approach is used by Nextant Aerospace. “We use longitudinal beam and angle beam ultrasonic testing techniques for flaw detection in metal and composite materials,” said Ehrhart. “We spend much of our time utilizing Ultrasonic Thickness Testing (UTT) for measuring remaining material thickness on aircraft structure where corrosion was removed, on aging airframes. We also utilize low frequency ultrasonic techniques on carbon fiber and other composite components (flight controls, cowlings, fairings, doors, and so forth) to detect disbonds and delaminations between layers and crushing of the honeycomb core, under the surface.”
Impact of COVID-19
The various NDT methods used by the aerospace industry are complex to administer and manage during normal times. However, the past three years have been anything but normal thanks to the pandemic, which led to many of the world’s commercial airliners being grounded and their maintenance postponed for months, followed by a rushed return to service. So how have NDT providers coped with the chaos?
For Eddyfi Technologies, the answer is: surprisingly well. “The inspection capabilities already in place were quite sufficient to ensure the return to service of aircraft after COVID,” Piron said. “Actually, the bottleneck seems to be more related to the availability of qualified NDT operators than instruments, inspection kits or probes.”
“I’d say there was a bit of a rush for NDT testing, but not a huge one,” said Weaver. “Actually, the biggest issue was the possibility of contaminated fuel that had been sitting in parked aircraft, plus those engines that had not been run for a while in desert storage conditions.”
The situation was different for Nextant Aerospace. “There was an increase in demand for inspection services throughout our service areas,” Ehrhart said. “Emergency inspection service (AOG) situations markedly increased as the public began to choose flying on private charter aircraft, versus commercial aircraft. The increased demand for private charter flying led to more incidents with bird strikes, ground support equipment accidents with aircraft, weather-related damage, and maintenance error damage that required NDT inspections as part of the repair process. (As such) We are currently experiencing a high demand for both our field and laboratory services.”
Challenges and Advances Ahead
Now that the global aviation industry appears to be returning to normal — airport delays notwithstanding — people in the NDT industry foresee a number of challenges ahead, none of which relate to their actual testing methods and equipment.
According to Raude, the biggest challenge going forward is a shortage of NDT inspectors. Her concern is echoed by Weaver. “There doesn’t seem to be a real focus in this industry in replacing what we’ll call the ‘seasoned gray hair inspectors’ that have been around a long time. That’s going to provide a real crunch moving forward. It’s been talked about now for a decade: Post-COVID, the registration for classes aimed at people entering the NDT inspection market and achieving different levels of certification has dropped dramatically.”
Weaver is also concerned that the approach in which commercial aircraft were returned to service will result in a situation where “all the planes being flown now will all need to come into the shops for maintenance at their required and maximum service intervals,” he said. “So it’s going to be a real load on the shops to be able to handle all that work.”
This backlog will force airlines to shuffle their remaining aircraft around to cover these gaps as best they can, but it won’t be enough, Weaver predicted. “You’ve already seen flight cancellations due to pilot shortages. I think you’re going to see the number of cancellations increase.”
Ehrhart fears that the increased use of private aircraft since the beginning of COVID-19 will result in more corrosion and metal fatigue in their airframes. “Many operators are flying at record rates, and the physical and operational environmental stresses put on every airframe increases the chances of flaws being induced on parts and assemblies,” he warned. “Now, more than ever, the NDT technician must be fully engaged and vigilant while performing their duties. Sooner or later, we will begin detecting defects in parts where we’ve never seen them before.”
On the bright side, the experts interviewed for this article expect the state of NDT to continue to advance, no matter what else plays out in the aviation industry.
“Fluorescent penetrant testing and magnetic particles are going to be replaced by eddy current array testing in the coming five to 10 years,” said Piron. “The development of ultrasonic phased arrays, especially the Total Focusing Method, will enable new manufacturing processes to arise such as additive manufacturing and friction stir welding.”
“With the advances in eddy current and ultrasonic testing, coupled with increased use of composite materials, I believe X-ray testing will be phased out with each new airframe that gets type certificated,” Ehrhart said. “Most new business jets do not have specific X-ray inspections in their NDT manuals; only a short ‘Standard Practice’ section for situations like damage to the aircraft that requires the use of an X-ray technique.”
“I expect some NDT programs to start to be overtaken by Structural Health Monitoring; SHM is what they’re calling that,” said Weaver. “With SHM, you place a continual-service transducer or a monitor in a hard-to-access area, one where you’d have to take down walls to physically inspect the structure. The SHM transducer’s compiled data can be accessed either by wire or wirelessly. After a certain amount of flights or a certain amount of cycles, the technician can just connect to it and collect the data — saving an enormous amount of labor and time to understand if there’s anything structurally occurring in the plane.”
In a similar vein, Raude predicted that much future NDT work will be doable using digital applications that can be accessed remotely. The Waygate Technologies spokesperson agreed: “In remote visual inspection, further exploitation of a range of technologies to further streamline in-situ inspection workflows is an emerging trend that we have identified,” the spokesperson said. “As in all our main technologies (IXS, UT and RVI), digital connectivity will continue to be a key aspect to connect inspection data output into a centralized cloud platform to enable further insights and analyses.”
The bottom line: The power, performance and complexity of nondestructive testing appear destined to grow in the years to come. As such, it will speed the ongoing evolution of aircraft maintenance from a reactive to a preventive basis.