NDT Testing Struggles to Incorporate New Technologies

ndt testing struggles

The urgent inspections of the CFM International CFM-56 resulting from the Southwest Flight 1390 uncontained engine failure which killed a passenger, focused the industry’s attention on Non-Destructive Testing (NDT). But the workhorse GE engine is not the only one undergoing urgent ultrasonic inspections. The 380 Trent 1000 Package C engines are also undergoing inspections resulting from several engine failures owing to failed compressor, turbine blades and seals (see sidebar page 28). Both are under Airworthiness Directives.

While the CFM 56 inspections relate to the apparent fatigue failure of a blade, the Trent problem is the premature failure of the compressor powering the 787-8 and -9. The engines are being inspected for cracking or signs of wear on turbine blades. Disturbingly, almost a third of engines failed the initial required checks and require weeks-long repairs, according to Rolls-Royce.

Rolls is accelerating the development of the permanent fix to the Intermediate Pressure Compressor rotor issue seen on affected engines. The revised compressor blade has been installed in a test engine and will begin testing in early June. First parts are expected to be available for engine overhaul in late 2018, rather than 2019 as originally planned.

The Flight 1380 failure was the second for Southwest which experienced a similar incident in Pensacola in 2016. Since the Pensacola incident, CFM International developed a multi-phased inspection program of the 14,500 large CFM56-7B fleet with 77,000 blades inspected to date. The program will result in a full accounting and tracking of the entire 330,000 CFM56-7B fan blade population.

New Inspection Tools on the Horizon

While current inspection methods are well accepted, a review by Aviation Maintenance revealed new and better methods are difficult to incorporate owing to cumbersome approvals for changing inspection processes.

New technologies have emerged, and while current technologies are robust, according to aviation maintenance experts, incorporating these technologies face a tough road.

“Maintenance, repair and overhaul facilities are usually the last place new technologies are adopted,” says Shawn Ehrhart, Constant Aviation director of Operations for NDT. “That’s not for lack of desire. We are hemmed in by the OEM manuals that dictate how NDT is to be handled. These are programmed inspections which dictate using older technology despite advances such as the arrayed eddy current and phased array ultrasound technology. I embrace new technologies now being employed in aviation, but they are mostly being used in research and development (R&D). When it comes to in-service airframes, the manuals are very old school.”

Inspection Methods

Liquid Penetrant or Magnetic Particle

There are several methods for NDT. Liquid penetrant testing (PT), also known as liquid penetrant inspection (LPI) or dye penetrant inspection (DPI), is a common method used to identify surface-breaking defects and discontinuities in metal and other nonporous materials. PT involves applying a colored liquid and allowing it to be drawn into minute surface openings by capillary action. Defects become visible under UV light or by the contrasting color of the dye.

Magnetic particle testing (MT) uses magnetic fields to locate surface and near-surface discontinuities in ferromagnetic materials. When very fine ferromagnetic particles are applied to the metal, they will be drawn into discontinuities on the surface to flag flaws.

PT and MT are generally considered affordable and effective NDT techniques, but each has limitations. PT testing can only detect surface cracks and requires the purchase, handling, and disposal of chemicals. MT testing is effective only for inspections of ferromagnetic materials. The inspection process for both techniques, including surface prep and cleanup, is time consuming and test results can vary depending on the skill and patience of the inspector, especially when the work environment is challenging.

Eddy Current Testing

Eddy current testing is a nondestructive technique capable of detecting surface and sub-surface defects including cracks, corrosion and heat damage in conductive materials. In aviation, ECT is used to inspect skins, stringers, frames, rivet holes, tubing, and many other ferrous and non-ferrous components.

In simple terms, ECT involves placing a probe or coil against a metal surface. The probe generates an electromagnetic field which induces electrons to flow into the material. Any cracks or changes in metallurgical structure will distort the flow like eddies in a river; these distortions are captured and analyzed by an instrument and displayed for the technician to review. The results are precise, and because ECT generates a digital record, inspectors can share, store, and review inspection data.

Ehrhart suggested part of the problem is efforts to keep maintenance costs down. “But, there are emerging technologies that would help us do our jobs better, faster and probably would improve our detection rate,” he said. “That rate is already high but we are not at 100 percent and it could go up.”

Constant Aviation does eddy current, ultrasonic, liquid penetrant dye and magnetic particle tests for a wide array of aviation customers including the commercial, business aviation, military and R&D. Ehrhart said 70 percent of his division’s testing involves eddy current methods, noting other industries do not use it as much as aviation and aerospace.

Each method addresses a different goal, he explained. “Manufacturers are doing more volumetric inspections using x-rays and ultrasound at the foundry or manufacturing level,” he said. “Once an aircraft gets into service the technique switches to eddy current, liquid penetrant dye and magnetic particles for most airframes flying today. For aircraft like the Dreamliner, the inspection methods are vastly different.” (See sidebar page 23).

Carbon Fiber

Advances are coming, too, for carbon fiber inspections, according to the Journal of Alloys and Composites, which recently published a new technology on carbon fiber health monitoring. The new research is on non-contact inspection methods especially for those areas isolated from conventional health monitoring systems.

The problem is the difficulty in detecting cracks before they occur using acoustic and ultrasonic inspections. While these methods can spot flaws, information on operational loads and stresses such as vibration, temperature and impact, are missing.

More than a decade ago FAA tested adhesive film sensors to assess such stresses using resonance sensors glued to composite panels after the introduction of a flaw. However, new research has developed a better, non-contact method.

The method embeds circuits in the composite materials to accomplish the same objective using soft magnetic circuits between 10-60 microns in diameter to measure stresses. The team from Russia’s NUST MISIS Center of Composite Materials was led by Professor Sergey Kalshkin. The method lays micro-wires in a grid between plies to assess how materials react to external magnetic fields. The team wants to develop a field prototype and says the wires do not affect the structural property of the material.

Olympus

While such carbon fiber testing is in the future, several companies have developed advances in technologies for current testing methods for more conventional airframe materials.

Olympus, for instance, utilizes digital signal acquisition and visualization technology in its EPOCH 650 flaw detector, ideal for ultrasonic inspection of turbine blade roots to detect flaws that cannot be seen with the naked eye. The company, which already works with several major domestic and international airlines as well as MROs, indicated this technology may detect the internal structural failure of a blade root which can experience significant fatigue over time.

While ultrasonic inspection technology has been used in aerospace maintenance for many years, the advances employed by Olympus deliver improved signal quality for more confident inspection analysis, as well as improving the technician’s user experience, which makes such inspections faster and more accurate.

Olympus says ultrasonic flaw detectors, such as the EPOCH 650, use advanced pulsing and receiving techniques within a highly portable, battery-operated form. They include multiple onboard reporting tools and a data filing system to allow technicians a simple way to communicate inspection results, said the company. The EPOCH 650 has a menu structure for instrument settings, calibration and software feature adjustment, as well as the direct-access key approach and a full VGA resolution display.

Olympus recently introduced the iPLEX G Lite ultra-portable industrial videoscope for visual inspections of small or difficult to access locations. The videoscope is a successor to the UltraLite model but retains the small size and light weight for one-handed operation. It has a touch screen monitor and electrically-operated tip articulation, enhanced image processing with LED illumination and a freeze-frame and automatic recording functions.

What happened to the CFM-56?

In the Southwest 737-700 incident, the CFM-56-7B engine exploded at 32,000 feet over Pennsylvania, destroying the engine, cowling and a window and causing rapid decompression. The event also damaged the wing, fuselage and horizontal stabilizer, according to the National Transportation Safety Board. It also raised questions about the durability of the engine cowling, the majority of which was missing.

The Board reported the No. 13 fan blade separated at the root although the dovetail remained intact on the disk. The recovered blade showed six fatigue crack arrest lines and striations consistent with low-cycle fatigue crack growth. The engine had more than 32,000 cycles since service entry and maintenance showed the blades were overhauled at 10,712 cycles before the accident using visual and fluorescent penetrant inspections.

This was the second such failure. Southwest experienced a similar event in 2016 out of Pensacola when a fan blade fractured. A CFM International Service Bulletin and an FAA Emergency AD in April recommended ultrasonic inspections of all fan blades on the -7B series engine with 20,000 cycles and subsequent inspections at 3,000 engine cycles.

With the field inspection programs instituted since 2017, operators use the ultrasonic probe on the flight line, according to GE. If any indicators are present, the fan blade then undergoes the full eddy current inspection. If indicators are still there, the fan undergoes a full cut up and analysis. So, there is essentially a three-stage process going on if indicators appear, and final results can take weeks and months. GE said findings to date have not led to any changes in the current inspection program.

NTSB is currently researching the number of cycles associated with fatigue crack initiation and propagation in the No. 13 fan blade to determine whether any changes to inspections are needed.

GE, part of CFM International, reported prior to the Pensacola incident, it used the fluorescent penetrant test on fan blades during routine shop visits. After the incident, the company moved to the more accurate eddy current method. There have been two fan blade-out incidents with the CFM56-7B fleets in 21 years of operation. More than 75,000 blades have been inspected and the company reported there were no findings to change its guidance for the current inspections program.

*Early 2017:
CFM met with the FAA / EASA to lay out multi-phased CFM56-7B fan blade inspection strategy.
*March 2017:
CFM issued first service bulletin (SB)) recommending ultrasonic inspections for CFM56-7B engines with 15,000 cycles accumulated since last shop visit – focus on oldest fan blade population.
*June 2017:
CFM issued second SB recommending ultrasonic inspections for specific serial number fan blades that have accumulated more than 20,000 cycles.
*March 2018:
EASA AD issued for first two SBS.
*April 20, 2018:
CFM issued third SB recommending ultrasonic inspections for CFM56-7B engines with 30,000 cycles to be completed by May 10 (about 650 engines). This portion of the SB receives FAA & EASA emergency ADs. The SB also recommends inspections of CFM56-7B fan blades with 20,000+ cycles to be completed by the end of August 2018 (about 2,500 engines) with inspections of CFM56-7B fan blades when they reach the 20,000-cycle threshold. After first inspection, repeat at 3,000 cycles. Second FAA AD issued on this portion of the SB. When operators do not know the cyclic accumulation of a blade, it is to be inspected.
*May 10, 2018:
Revision to the April 20 SB recommends customers prioritize inspections for about 5,400 engines that include fan blades with more than 20,000 cycles, as well as engines with 20,000 cycles and at least one shop visit to be completed by June 30th. Inspections of the remaining population should be completed by August 31, 2018. At press time, GE also reported that customers were 100 percent compliant with the 20-day inspection requirement and the 20,000 inspections were 40 percent complete.

Olympus touts the iPLEX’s improved imagery. The PulsarPic image processors constantly optimizes images by reducing halation, balancing exposure and increasing gain quality.

It also has a WiDER Dynamic Extended Range technology as well as adaptive noise reduction, enabling inspectors to see fine details across the image by enhancing contrast and reducing noise, in addition the iPLEX G Lite provides interchangeable UV or IR illumination.

Advances in imaging, in fact, are the biggest thing that has happened for borescope testing, according to USA Borescope’s Bill French, a sales manager for the company. His company supplies and repairs borescopes for various industries including aviation/aerospace. French indicated imaging is important since the accuracy rate is closely tied to the quality of the image. Aviation and aerospace constitute 50 percent of USA Borescopes’ business.

Today’s image quality is a night-and-day improvement over the quality just 10 years ago, he said. “They include more features in smaller packages at lower costs,” he said. “We’ve gone from 40-pound cases a decade ago to a one-pound unit inspectors can hold in their hand. That camera chip at the tip comes in two different types – a CCD image sensor which is the higher quality, cleaner image but at a higher cost. There is also the CMOS image sensors, which, historically has been a lower cost option but you sacrifice image quality. However, CMOS has changed in the last three years so it is closing the gap with CCD, used for commercial jetliners. CMOS is used for regional and private jets but the difference in laymen’s terms is the difference between a DVD and Blue Ray.”

Zetec

Under comparable conditions and with a skilled technician, single-coil eddy current testing and particle testing will produce comparable pass/fail-type results, according to Zetec, a company producing eddy current and ultrasonic NDT tools and software. However, a handheld eddy current tool with a C-scan display can present a digital “big picture,” helping inspectors find more defects in less time. More advanced instruments can conduct dual-frequency testing, digital conductivity testing, and nonconductive coating thickness measurement.

“Today, the market is shifting toward portable tools with powerful software, touchscreen interfaces, ergonomic designs and longer battery life,” Daniel Richard, Zetec technology manager, explained. “This combination makes eddy current testing feasible for anyone who wants faster, more accurate inspections. The choice of probe is also important. There are eddy current probes for specific aircraft applications like rivets, lap splices, and welds. For example, a dedicated bead-seat probe lets users inspect the bead seat on a wheel with only one pass.”

The problem with liquid penetrant and magnetic particle testing is the extensive preparation and clean-up work. Two years ago, Tovatech entered the aviation/aerospace field expanding beyond its original target audience in the chemical and pharmaceutical research communities.

It is essential all traces of dyes and particles are removed, according to Tovatech’s Bob Sandor in a recent blog post. His company produces ultrasonic cleaners used by airlines, including American’s Tulsa facility. The airline employs the company’s 37kHZ S180H Elma ultrasonic cleaner which is equipped with a heater and timer that use LED lights to indicate both set and actual cleaning time and clean temperatures. Depending on the number and condition of parts, cleaning cycles range from 30 minutes to an hour.

“Cleaning is accomplished by billions of microscopic vacuum bubbles that implode on contact with tremendous force to blast loose and carry away contaminants on items placed in the ultrasonic cleaner’s bath,” he wrote. “Because of their small size they can penetrate minute cracks and crevices unreachable by brushes, sprays or other mechanical methods. It is faster, more effective and less costly than other cleaning methods, it is used for disassembled parts and performing quality check standards.” The other problem with these testing methods is they don’t indicate the depth of the flaw.

Zetec’s MIZ-21C is a portable hand-held eddy current device with a touchscreen. Zetec image.
Zetec’s MIZ-21C is a portable hand-held eddy current device with a touchscreen. Zetec image.

“They are easy and inexpensive but they lack the precision and depth needed to identify flaws and their characteristics,” Zetec’s Richards told Aviation Maintenance. “Magnetic particle or eddy current, for example, can only be applied to components that can be magnetized but again, it doesn’t indicate the depth of the flaw. What you need is digitization so that you can take digital pictures, so you can monitor the flaw over time. You also need the ability to reach inaccessible places.”

His understanding of the Southwest incident showed the flaw was inside the material. “You need to identify such flaws before they reach the surface and cause a failure,” he said. “To identify flaws within the materials by x-ray requires too much radiation and the construction of a huge bunker for larger components. That raises concerns about tracking and securing the radioactive material.”

Zetec produces several NDT tools used in aircraft NDT, including the TOPAZ family of ultrasound instruments. Zetec image.
Zetec produces several NDT tools used in aircraft NDT, including the TOPAZ family of ultrasound instruments. Zetec image.

“We know that advanced technology is not low cost but if there is a critical flaw we are a good partner to find it and make a complete assessment including depth,” he said. “With the CFM 56, over two years they found the flaw where it was expected but the failure occurred earlier than expected. The problem is when the flaw is not where it is expected but perhaps five to 10 millimeters away. Conventional ultrasound would not be sufficient to find that. Our phased array could have caught the flaw in a different location.”

Trent inspections resulted from engine failures

In April, the FAA and EASA issued Airworthiness Directives for Boeing 787-8 and -9 aircraft powered by Trent 1000 Package C engines limiting ETOPS operations. The issue results from several engine failures of Trent 1000 Package C engines owing to failed compressor and turbine blades and seals, according to the FAA.

Inspections resulted in numerous reports of cracked blades resulting in unscheduled engine removals, resulting, as Boeing reported, from the IPC stage 2 blades having a resonant frequency that is excited by airflow conditions existing in the engines during operation at high thrust settings under certain temperatures and altitude conditions, according to the AD. The vibrations can result in cumulative fatigue damage that can cause blade failure and consequent engine in-flight shutdown.

Rolls-Royce recently warned the 787 groundings expected to reach 50 aircraft, resulting from the June 9 deadline for Trent 1000 inspections. About 30 aircraft, including British Airways, Air New Zealand and Virgin Atlantic are grounded (at the time this was written), according to the Rolls-Royce.

There are 380 engines in service covered by the April AD mandating additional intermediate compressor. The company has trebled the number of affected engines it is working on at a time.

Zetec produces several NDT tools that are commonly used in aircraft NDT, including the TOPAZ family of ultrasound instruments; the Surface Array Flex Probe for eddy current inspections; and the MIZ-21C portable eddy current instrument.

NTSB is currently researching the number of cycles associated with fatigue crack initiation and propagation in the No. 13 fan blade to determine whether any changes to inspections are needed.
NTSB is currently researching the number of cycles associated with fatigue crack initiation and propagation in the No. 13 fan blade to determine whether any changes to inspections are needed.

TOPAZ instruments are fully integrated, high-performance phased array ultrasonic testing devices that combine the hardware and software necessary for challenging inspections.

The Surface Array Flex Probe is designed to produce faster, more accurate eddy current inspections. It can be combined with the Zetec MIZ-200 Eddy Current Array Instrument plus Velocity Acquisition and Analysis Software for a complete inspection solution.

The MIZ-21C is a portable hand-held eddy current device with a touchscreen to make inspections more efficient and more accurate. It has a long battery life and improves productivity since it can reduce inspection time by up to 95 percent compared to conventional pencil probes.

As new technology is fielded for increasingly more accurate and efficient flaw detection, the industry will have to begin the heavy task of revising inspection manuals and methods to accommodate them. Coupled with advances in data analysis, this new technology has the potential to head off such problems especially in hard to reach or reduced visibility environments.