Non-destructive testing (NDT) has always had an important role to play in aviation production and maintenance. It has had to adapt to examine new materials and processes but has also developed technology to improve fault detection and analysis. Ian Harbison reports.
As well as basic visual inspection by eye or with magnification, there are a number of other techniques in common use to inspect a variety of components in a range of different materials.
MPI
Magnetic Particle Inspection (MPI) generates a magnetic field inside the component, which must be made of ferromagnetic materials. Magnetic particle powder is applied to the surfaces which is attracted by the magnetic flux leaking through any defects. This makes it easy to spot surface problems, such as toe cracks, crater cracks as well as areas of porosity in castings, shafts, and welds.
DPI
Dye Penetrant Inspection (DPI) service is used to detect casting, forging and welding surface defects such as hairline cracks, toe cracks, crater cracks, as well as surface porosity, leaks and fatigue cracks. The components, such as shafts, castings, and welds, along with machined small components, are coated with a dye and then illuminated by white light or UV light, for which a fluorescent dye is used. It is generally used on non-ferromagnetic materials.
Ultrasonic Testing
Ultrasonic Inspection (UT) uses ultrasonic waves to scan for flaws, delamination or other discontinuities in materials like composites and aluminum, or to measure thickness in a wide range of materials and applications. It can measure internal defects in most materials and is also useful for inspecting forgings, castings, and structural steel welds. It can also be used to scan for collision damage or to carry out post-repair surveys.
A development in ultrasonic inspection is Phased Array testing. A disadvantage of conventional methods is that the ultrasound source has to be moved manually around the component. Using a phased array means the sound direction can be steered electronically, reducing the survey time.
Radiography
In radiography, the component is placed between a gamma or X-ray radiation source and a detector holding photographic film. The rays pass through the component and an image is imprinted on the film. When processed, the image will show any internal defects.
A development here is computed tomography, which rotates X-ray beams around the test article to generate a series of detailed images. Advanced reconstruction algorithms then compile the images into a highly detailed 3D model that can be used to detect internal flaws such as voids, cracks, and inclusions.
Just as there is a range of techniques, there is a range of OEMs and end users.
FL Technics
Ovidijus Rucinskas, head of NDT at FL Technics, says a variety of techniques are used across the company’s activities. Comprehensive inspections are routinely carried out during scheduled heavy maintenance checks, while line maintenance uses on-wing inspections for in-service aircraft, addressing immediate airworthiness concerns.
FL Technics
Specialized NDT in the engine shops covers turbine blades, disks, and other critical engine components and the component shops use magnetic particle and dye penetrant inspections for crack detection, for wheels and brakes, for example. FL Technics has the capability for eddy current testing, ultrasonic testing, magnetic particle testing, penetration testing, thermography testing (water ingress method) and engine and APU borescope inspection (B1 rating) as part of its base maintenance capabilities in several locations.
It provides services for operators, MROs and leasing companies. Services can cover one-time or periodic inspections, fleet-wide support or long-term maintenance agreements. The company can also dispatch teams within 24 hours to a client’s chosen location globally to provide any type of NDT service.
As NDT is not only used in aviation, but in rather a lot of other industries, he sees that aviation does not evolve as quickly as other sectors might. It takes rigorous testing and evaluation to get approval for new NDT techniques from the regulatory authorities. In fact, approval is required from every authority that FL Technics is involved with, such as EASA, U.K. CAA and Bermuda.
As materials evolve, NDT equipment has also advanced to meet OEM-prepared procedures, ensuring compliance and improved accuracy. New techniques such as computed tomography (CT) scanning, laser ultrasonics, eddy current array and others are being developed. However, these are primarily used in the manufacturing process rather than in maintenance. In maintenance, traditional ultrasonic, eddy current, magnetic particle testing and penetrant testing remain the primary methods.
Waygate
Ben Linke, CEO of Waygate Technologies, says aerospace non-destructive testing (NDT) accounts for 30% of the company’s business with a 50/50 split between MRO and manufacturing. Other sectors include space exploration, automotive, rail, electronics, battery, and research and development. It offers ultrasonic, radiography and computed tomography (CT) solutions for aerospace, as well as visual inspection systems (RVI).
Waygate Technologies
The company works with various key stakeholders in the market to effectively modernize the industry. Waygate Technologies has signed a joint technology development agreement (JTDA) in 2023 with GE Aerospace to develop software and hardware inspection solutions for commercial aircraft engines. It has also been partnering with Rolls-Royce to develop the so-called Intelligent Borescope to inspect high-pressure turbine (HPT) blades.
The Intelligent Borescope uses Waygate Technologies’ Mentor Visual iQ+ video borescope, which has two-way communication with Rhinestahl’s turning tool and Menu Directed Inspection (MDI) for specific step-by-step instructions to ease the inspection workflow and standardize data.
In both cases, the applications are powered by AI – the aim is to avoid the traditionally labor-intensive and time-consuming processes that generate inconsistent data that can result in repeat inspections and, more importantly, can see engines removed prematurely for maintenance, causing unnecessary expense for operators.
By using high-resolution, templated imaging systems that capture detailed visuals of the HPT blades during inspections and combing them with AI-powered algorithms to subsequently analyze the images, even the smallest amount of wear and tear can be quickly identified. The data is then easily and immediately accessible in a cloud to enable accurate maintenance decisions and optimize the time-on-wing of the engines. The imagery can also be used to develop repair schemes to return the blades to optimum aerodynamic efficiency.
He points out that HPT blades are subjected to temperatures of 1600°C and extreme centrifugal forces. With engine OEMs pushing for even higher operating temperatures for greater efficiency, and increased pollution causing more blade erosion, regular checks will become even more important.
A newer application of computed tomography is scanning components produced by additive manufacture. This is the only way that any voids in the material can be detected, caused by problems as material is being laid down layer by layer.
Looking further ahead, in 10-15 years, or 25 years if Airbus concerns about infrastructure are correct, hydrogen powered aircraft will be around. One of the most important checks that will have to be made after construction of composite fuel tanks is to check for micro cracks on the interior surfaces that could lead to leaks. The company is already working on the problem in collaboration with the U.K. National Composites Centre.
Ease of use will also become more important in the future, he says, because of skill shortages. There is a retirement bulge looming so there will be many young recruits coming into the industry. Technology developments can help them get up to speed quickly.
SEAL Aviation
For Jerel Bristol, president of SEAL Aviation Repair Services, NDT is an integral part of the company’s fuel leak and structural repair services. These can be carried out at its facilities in Fort Lauderdale and Treasure Coast International Airport in Fort Pierce, Florida, or the company will deploy a team to the customer’s location to deal with AOGs. Capabilities include radiography, eddy current, ultrasound, bond testing, paint thickness testing, liquid penetrant, magnetic particle and optical prism testing.
These can be applied to a wide range of aircraft, although business jets are a specialty. A recent example of how the different aspects come together was on a Hawker 800A. An inspection found corrosion on the wing link attachment fitting. The company brought along its own jacks, pads and shoring to support the aircraft, as well as installing a temporary wing link to ensure structural rigidity. This type of work usually involves an engineering mechanic and a sheet metal technician, making SEAL Aviation a one-stop shop. The removed link was then tested to determine the level of corrosion and repaired. After this, to determine airworthiness, it is tested again to confirm complete removal of the corrosion and that the reaming thickness of the material is within limits.
Bristol notes that, with more manufacturers using composite materials, the use of ultrasonic testing and phased array testing is becoming more common to find discontinuities such as cracks, delamination and disbonding.