As passenger numbers increase and airlines ramp up their fleets with next generation aircraft, the role of non-destructive testing (NDT) in the safety- critical world of commercial aviation remains ever important. While the use of NDT dates back to the first days of commercial aviation in 1914, it is something of an unsung industry hero.
In manufacturing, the likes of Boeing and Airbus and their supply chain companies use NDT methods to assess the integrity of a structure or component without causing any lasting damage and ensure the removal of any defective parts before they enter service. Once an aircraft is in service, NDT is a vital tool for maintenance technicians in identifying necessary repairs and ruling out unnecessary work – keeping turnaround times as swift as possible. NDT is particularly useful for technicians can in testing the robustness of an aircraft and looking for problems that may not be visible to the human eye, such as metal fatigue and material stress.
NDT extends to all parts of an aircraft, with MRO teams able to measure damage and irregularities in materials used in engines, airframes and components. This includes looking for material thickness, cracks, corrosion, delamination of composites and welding defects. While encompassing the entire aircraft, statistics show the lion’s share of NDT processes are carried out on the airframe, accounting for between 70 and 80 per cent of all commercial aircraft NDT. The remaining 20 per cent is performed on engines and other related components.
By not damaging or permanently altering the expensive materials and components being inspected, NDT provides significant cost savings and reduced turnaround times for airlines which are always looking to get aircraft out of the hangar and back in the air. With NDT playing a key role in reducing costs and ensuring safety standards, MROs are exploring faster and more effective ways of performing the tests.
While NDT forms an important staple in a MRO’s service offering, such tests are confined to a relatively small number of processes. According to the American Society for Nondestructive Testing, the six most commonly used methods of testing are magnetic particle, liquid penetrant, radiographic, ultrasonic, eddy current and visual inspection. These tests are not specific to aviation and are used across multiple sectors including the oil and gas and automotive sectors – the sector as a whole was worth $1.4bn in 2014. In commercial aviation, visual inspection is the most common form of NDT, with technicians able to carry out the process with less of the specialised equipment, training and certification associated with other forms of NDT. Technicians will typically use borescopes to carry out checks in hard to reach areas without the need for disassembly. Other visual inspection tools include videoscopes, with built-in image capture and recording capabilities, and fibrescopes, which use fibre optic cables to transmit an image back to the inspector’s eye.
Alongside visual Inspections, two of the most common and traditional forms of NDT are radiography and eddy current, both of which have origins in the medical industry. Radiography, a technique used for more than a century, detects imperfections through the use of x-rays and gamma radiation. Meanwhile, eddy current testing is a sub-surface inspection technique that determines flaws in conductive materials and detects cracks and corrosion across the material surface, subsurface and bolt holes of aircraft fuselage and structural components. The importance of eddy current testing in MRO is demonstrated by examining the NDT manual for the 747; it consists of two volumes, with one entirely dedicated to eddy current testing.
North America and Europe possess the most mature NDT markets. These regions have many MRO firms, independent and third-party, offering full NDT services. Among the largest airline affiliated MROs with NDT capabilities include Delta TechOps in the US, and Europeans Lufthansa Technik, Iberia Maintenance and Air France Industries KLM Engineering & Maintenance (AFI KLM E&M). Offering a broad portfolio of NDT capabilities, AFI KLM E&M commonly uses magnetic particle inspection and eddy current testing in its material services offering, while employing radiography to determine cracks, water entrapment and core damage on all types of commercial aircraft.
Régis Carlier, cabin and mechanical components manager at AFI KLM E&M, confirms that NDT practices in MRO have remained largely unchanged during the past 20 years few decades. “New processes have developed faster in NDT used for the manufacturing of parts, but not so much in the maintenance,” he says. “An example of this is in things like detecting cracks on aircraft, where there have not been a lot of new processes developed.”
While some say NDT is a conservative industry slow to accept new processes for carrying out inspections, the same can’t be said for the adoption of technological innovations in relation to the equipment offerings. Carlier says AFI KLM E & M has pioneered several new technologies to aid traditional testing methods including tools for magnetic particle test benches.
Iberia Maintenance also offers a full range of NDT services. José Palancar, supervisor of the airline’s NDT inspection, says NDT processes have benefited from embracing new technologies. “Thanks to more comprehensive systems that integrate wifi connectivity and data computation, NDT processes have improved accuracy, reliability and productivity,” he says.
Adapting to composites
Changes in aircraft materials have also affected the demands placed on those carrying out NDT. Aluminium, while statistically the industry’s most used material, is being used less than it was 10 years ago. In contrast, more carbon fibre composite materials are being used in aircraft manufacture. With Boeing and Airbus significantly increasing their use of composites - both the 787 and A350 have more than 50 per cent usage of composites parts – new demands and challenges have arisen for those offering NDT processes. This is down to the complexity of materials constructed from multiple layers and elements, for example, the prevalence of thickness variations and highly curved parts, and the potential failures associated with them. To meet these challenges, technologies using ultrasonic testing, the most common technique for inspecting composite structures, have emerged. As NDT’s biggest growth market, contributing 31.1 per cent to overall sector revenue, ultrasonic testing has become a hotbed of innovation.
In recent years there has been an influx of devices from established equipment manufacturers and new market entrants looking to capitalise on the growing demand for ultrasonic testing. Palancar says new methods in ultrasonic testing have helped Iberia carry out NDT on composites as increased understanding of the material has developed. “We started working on the composites repair and maintenance with GFRP glass fibre plastic and honeycomb bonded sandwich structures in the early 80s,” says Palancar. “Since the 90`s we’ve inspected and maintained carbon fibre monolithic and glass fibre reinforced polymer main structures (rudders and elevators structures), as well as components of the A310, A320 and A340,” he says. “We feel comfortable to apply and use, as a routine, ultrasonic phase array NDT techniques that will be used in next generation aircraft.”
Norway-based ultrasound camera developer DolphiTech, which was founded in 2009, has developed the DolphiCam – a mobile and ergonomic ultrasound NDT camera – specifically testing composite materials. The device has been accepted by Boeing for inspecting the 787 carbon -fibre composite airframe and is in the process of being certified by Airbus for use on the A350. The camera is able to inspect carbon fibre reinforced plastic up to 16mm thick through high resolution 2D and 3D images.
DolphiTech has also developed a dry coupling transducer solution, which it says is capable of doing what traditional methods cannot. “If you are using traditional ultrasound, you need some kind of coupling agent, such as water or contact gel, to make contact between the unit and the material,” explains Jan Olav Endrerud, DolphiTech CEO. “Our dry coupling transducer makes it possible to inspect carbon- fibre structures without using any coupling agent. This makes it easier to inspect a drill hole, for example, as you don’t have the problem of any water or gel going into the hole and making the ultrasound results difficult to interpret.”
DolphiTech has also developed its own software programme, DolphiTech TeamCenter, to support its DolphiCam device. DolphiTech said the software make it possible for an NDT expert to create inspection procedures with the DolphiCam technology embedded into it, meaning an NDT expert can give inspection procedures to their team of inspectors and directly control the way inspectors do their inspections.
After launching its NDT equipment range for the aircraft manufacturing market, Endrerud says DolphiTech now has designs on what he describes as a “super interesting” MRO industry, where the company will look to expand its capabilities in 2015. And its software package could be one way to do so. The need for MROs to work quickly and efficiently has created more demand for specialist software tailored for individual NDT methods. It is common for software firms to offer packages which encompass all NDT applications; catering for data input, analysis, image review and documentation.
Another recent development in NDT is the incorporation of mobile communication technologies. Like many industries, NDT has benefited from developments in connectivity since the early to mid-1990s, such as WiFi, broadband and bluetooth. These technologies have been incorporated into many of the instruments and testing equipment brought to market by OEMs such as Olympus and Sonatest.
GE Measurement and Control – Inspection Technologies, the NDT products and services division of industrial giant General Electric, has unveiled a number of new devices for surface inspection incorporating these technologies. In 2014, it released Mentor Visual iQ VideoProbe, a video borescope tool for visual inspection with WiFi and bluetooth capabilities. It also incorporates 3D phase measurement capabilities, which help to provide an accurate indication of the depth and size of pitting, cracking and corrosion. GE said the device meets the industry’s demand for increased connectivity, enabling one inspector to be contacted by another member of the team from a remote location to share images and speed up the inspection process.
Tom Ward, senior product manager at GE Measurement and Control, says NDT technicians working across multiple locations in real time is an increasingly regular occurrence. “The idea of collaboration remotely through mobile or WiFi connections makes NDT less costly and faster, and provides a better experience at the point where it’s needed – even though technicians don’t have to be there,” he says. Looking towards the future, Ward predicts the idea of increased mobility will further influence the development of new equipment, likening NDT’s move towards connected touchscreen devices to the consumer’s leap from cell to smart phone.
Challenges and opportunities
While the emergence of new testing devices and software has enabled greater technical mobility, there remain limitations to what NDT can offer. Eddy current testing, for example, despite being used for more than 50 years, remains a particularly complex process that needs specialist technicians. Its use is confined to use on conductive materials and has a limited depth of penetration. Meanwhile, liquid penetrants can only detect imperfections that are open to the surface, while testing on a rough surface will likely result in a false indication. Palancar, however, believes that if equipment makers and customers work together some of these difficulties may be overcome. “A closer relationship between NDT equipment manufacturers and their customers is needed to improve both the equipment and the way it can be used,” he says.
However, the biggest challenge facing NDT operators currently is the short supply of available expertise. As with all engineering disciplines at the moment, attracting the next generation of NDT inspectors has been difficult and is resulting in an ageing workforce. This was a highlighted in a 2012 survey by NDT recruitment firm PQNDT, which found that the average NDT inspector was 45 years old.
Ward, citing GE’s work with connected inspection devices, believes that it is this type of technology and its synergy with younger, computer-savvy workers could help attract the next generation of NDT professionals. “When you think about how people use devices from a consumer standpoint, young people have a touch screen in their pocket as it’s what they grew up with,” he says.
While concerns over skills persist, the NDT sector is one with great prospects. As the airline industry grows, so too, does the need for NDT to ensure aircraft reliability and safety. A report last year by researchers MarketsandMarkets, - “Nondestructive Testing Market by Technology – Global Trends & and Forecasts to 2014–2020”, estimates that the NDT industry will grow by 3.5 per cent annually up to 2020, increasing its overall value to $7bn .
With the US and Europe firmly established, the emerging NDT markets are Latin America and Asia Pacific, while countries such as Brazil, India, and China have been earmarked as having large growth potential. For those MROs holding the expert skills base needed to perform NDT, there are some are plenty of opportunities for growth in the next five years.