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    Inspection Technologies

    Feb 7, 2008
    By Jerome Greer Chandler/Overhaul & Maintenance

    Posit for a moment the airplane of the future, a flying machine all but freed from scheduled inspections, able to keep flying because of sets of sophisticated sensors imbedded within it.

    Behold, the monitored machine.

    In perhaps a decade or so, a mechanic might do a walk around inspection, much as the first officer does at pre-flight, just before departure. But this walkaround would be far more probing. Armed with a wireless ultrasound device, "your technician walks past the airplane and a little chip beeps at him," envisions Michael Moles, senior technology manager for Olympus NDT. "He knows then and there whether there's a problem." This, contends the veteran NDT executive, "will tend to be the future," a future predicated not so much on periodic inspection, as on structural health monitoring.

    "It's one of the next waves," maintains Moles, "the sort of Holy Grail."

    Michael Moles isn't Indiana Jones. He's a rational guy, grounded in science. An array of experts with whom Overhaul & Maintenance spoke agree that some decidedly exciting things are happening in the field of non-destructive testing and non-destructive inspection (NDT/NDI). But they also agree advances will be incremental, measured. Structural health monitoring -- at least at this stage of its maturity -- isn't even designed to supplant traditional inspection regimens en masse, just to supplement them.

    Scientists are, by nature, cautious and circumspect. Case-in-point: Dennis Roach, distinguished member of the technical staff at Sandia National Laboratories. He works within the FAA's Airworthiness Assurance NDI Validation Center (AANC) in New Mexico.

    The Promise of Structural Health Monitoring

    Roach is shepherding structural health monitoring at Sandia. It's one of "a very wide array of potential technologies" the Validation Center is testing. While structural health monitoring seems, by far, the most intriguing of the lot, Roach is quick to stress the word "potential." "Most of them are still in the development stage," he cautioned. When the center looks at a new technology, it trifurcates the process: basic research, applied research; then validation; and then transfer and utilization.

    So, where's structural health monitoring in this hierarchy? "Just to give you an idea of its stage of maturity," he said it's "probably evolving from basic into applied research."

    The thrust behind the effort is to emplace sophisticated sensors, in-situ, at critical places in the aircraft, hard-to-access areas such as the aft-pressure bulkhead. "If you had concern about an area, you could put sensors there," he said. Let's say there's an area of 10 fasteners you think might eventually begin to crack. Miniaturize the sensors, affix them and monitor. They would, of course, have to withstand the operational environment, and they'd have to be relatively inexpensive.

    "Theoretically," said Roach, "you could put them anywhere." But aerodynamics probably dictate they be on the inside of the craft. By definition, the exterior is accessible, and "One of the big drivers behind structural health monitoring is the ability to access normally inaccessible areas."

    That's the near-term motivation -- putting sensors in areas that are labor and capital-intensive to get to. Roach is focused just now on "current use vs. future use."

    Down the line, after voluminous validation, waits the Holy Grail -- on-condition monitoring. Roach said it's on the list, but nothing that "we have any plans in implementing within view of our headlights. It's going to take a pretty big change in the philosophy of maintenance to adopt on-condition [airframe] monitoring," no matter how tempting the technology. That's certainly the way Roach's colleagues across the country in Atlantic City, N.J., view things.

    "We're taking this in baby steps," said David Galella, inspection systems research manager at FAA's William J. Hughes Technical Center. "Right now, we don't talk about 'condition-based maintenance. We're not there ... I don't want people to think we're blessing that at the moment."

    At the moment, the emphasis is on in-situ sensors emplaced behind areas such as lavatories, galleys, aft-pressure bulkhead even (depending on the species of sensor) in fuel tanks. The idea is to run access lines to the areas, so they don't have to be opened up repeatedly. But Galella's point is this: Airframe inspections would still be conducted "on the same interval you would normally."

    To reach even that stage of in-situ usage, Galella said the sensor has got to last, and that it has to self-test. Then there's the issue of amassing data "to show that it will find the flaw you want it to find." That entails mounting them in "an area with a history of needing inspection."

    For the past three years, Sandia has been field-testing a fascinating new species of sensor on Northwest and Delta aircraft -- DC-9s, 757s and 767s. The devices are called CVMs, comparative vacuum monitors. According to the paper "Health Monitoring of Aircraft Structures Using Distributed Sensor Systems," authored by Roach and Sandia colleague Kirk Rackow, comparative vacuum monitoring works, "on the principle that a small volume [or air] maintained at a low vacuum is extremely sensitive to the ingress of [ambient] air." The idea is to emplace self-adhesive sensors, with fine channels etched on the adhesive face. When the CVM sensors adhere to the structure to be tested, they "form a manifold of galleries at alternately low vacuum and atmospheric pressure." Should a crack develop, "it forms a leakage path between the atmospheric and vacuum galleries, producing a measurable change in the vacuum level."

    Test results, so far, are encouraging. CVMs can detect cracks on both bare and primed panels that are 0.040 of an inch thick.

    CVM inspection is one of an array of new in-situ sensing technologies Sandia is testing. PZTs, or piezoelectric transducers, are another. In their generative mode, PZTs work to propagate elastic waves in the surrounding material. In their receptive mode, they receive those waves and transform them into electrical signals. The upshot: they "can scan large structural areas using ultrasonic waves," concluded the paper's authors -- areas composed, specifically, of composites.

    "They might be looking at disbands or delaminations in a bonded structure," said Roach. It's not that they couldn't detect cracks as well, it's just that "another sensor might be a lot more sensitive if you want to find small cracks."

    Fiber-optics are nothing new in the NDT/NDI arena. Their advantages, and limitations, are manifest. They're thin, and they can break. You've got to handle them carefully. "That's why you don't see [them] mounted all over an aircraft," said Roach. Still, Sandia is testing them in-situ, seeing what will work. "You could mount then on a structure and interrogate them with a coherent light source," said Roach. "When the light source returned, based on the wavelength of that return, you could tell where the [crack] is."

    If fiber optics aren't exactly new, then eddy current (EC) is ancient. What Sandia is working on is the latest, miniaturized iteration of the tried-and-true technique, something called remote field eddy current, or RFEC. Roach and Rackow contend the technique "is especially suited for detecting deep flaws."

    A bit of context: Inspecting multi-layered structures requires that gear be able to detect fine cracks and shallow corrosion pits embedded deep down, significantly below the surface. Gaps between the layers hamper the ability of ultrasonic to ferret out flaws. It's tough for them to penetrate beyond the first layer. The problem with conventional eddy current is that delving deeper requires larger EC coils, and that entails a corresponding loss of sensitivity. Thus, RFEC.

    Remote Field Eddy Current is pegged to measuring the voltage induced in a pick-up coil by the flux that's been passed through the structure to be tested twice. This triggers a significant voltage change and produces a stronger flaw signal. "This," wrote Roach and Rackow, "allows higher gain settings ... so that deeper flaws can be detected."

    Comparative vacuum monitoring, piezoelectric transducers, fiber-optics, remote field eddy current. These and other exotica have scientists excited about the possibilities. Still, "We really don't have a single shining savior sort of thing out there," cautioned Dennis Roach. "There are a lot of things being looked at, and they all have their specific uses."

    Before any techniques Sandia and the AANC are exploring find their way to the shop floor, there are a series of purposely placed hurdles they have to surmount.

    The first step to incorporating in-situ sensing -- specifically sensing pegged to CVM -- is for the airframer to modify its NDT Common Practices Manual. That would entail, in this instance, Boeing saying "'There's an array of tools out there for maintaining your aircraft. Here are [the ones] we think may work for you,'" said Roach.

    Then, should the carriers elect, they can petition the airframer to employ comparative vacuum monitoring in lieu of conventional eddy current to inspect the aft pressure bulkhead of a particular aircraft type. The idea is to have, in this instance, Boeing okay the technique as an Alternate Means of Compliance (AMOC).

    At that point, the applicable regulator agency gets involved. In the case of FAA, if the Aircraft Certification Office signs off on the deal, CVM would be used quite narrowly, "for this particular, single application," said Roach. If CVM gets the go-ahead, the Sandia scientist says -- to his knowledge -- it would be the first operational green light for in-situ structural health monitoring on commercial aircraft.

    The irreducible criteria regulators will consider whether the in-situ sensor "works as [well] or better than my current hand-held inspection technique," said Roach.

    That's step one, a step that conceivably could lead to what Roach and Rackow write terms, "Condition-based maintenance practices ... substituted for the current time-based maintenance approach." At the very least, the Sandia researchers contend in-situ sensors render it "possible to produce an aircraft prognostic health architecture that can assist in maintenance scheduling and tracking."

    Buoying the Back End -- The Interpretation Equation

    Sensors, in of themselves, are insufficient. To render data, they collect digestible, and readily displayable, they've got to be connected to devices capable of performing sophisticated equations. The aim is to offer the operator unambiguous information that enables him to make decisions about a structure then and there.

    "We are increasingly seeing the use of advanced processing techniques on NDE data to extract more information, [and] to extract that information more reliably," said Pamela Farries, PhD, new business manager for Air Vehicles Concepts & Structures at QinetiQ's Farnborough facility.

    Let's look at a case-in-point. In the aging aircraft environment, inspecting multi-layered structures using ultrasonic techniques on metallic structures "is generally not practible [practical] for an operator to analyze in real time," asserted Rob Pitts, an NDE group leader for QinetiQ. That's especially true when fasteners are in place, because the structure then produces "very complicated ultrasonic data," he said.

    Ultimately, QinetiQ would like to get to the point in crack detection where the gear presents operators with a go/no-go decision, rather than having to interpret the amplitude of a signal to determine if there's a problem. "We're looking for black and white," said Pitts.

    QinetiQ isn't there yet. It's still reviewing data "to give us confidence in the results we've achieved," said the QinetiQ researcher. He added that he believes, "We're still several years [away] from obtaining all of the required qualifications" for such an operational application. QinetiQ has working prototypes right now.

    Making things simpler for the operator is a subject that consumes scientists from England to Iowa. Take the humble tap test. Iowa State University's Center for Non-Destructive Evaluation has done precisely that, and they've transformed it into something significantly less subjective. Employed by technicians since Day One to determine the viability of composite structures, the test -- as the name implies -- involves taking a tap hammer and, "basically listening for changes in the sound," said Lisa Brasche, PhD, the center's director. The aural signature changes with differences in the stiffness of the composite structure.

    Getting it right is very much a matter of interpretation. "If you just think about it," said Brasche, being able to hear something is fairly subjective." Then there's the issue of the volume of structure you have to cover. Detecting flaws in a nose of tail cone, perhaps a fairing, is one thing. Waiting for the right return with a fulsome fuselage is something else. Perseverance is one thing, perverse perseverance quite another. Consider that the length of the virtually all-composite fuselage of the Boeing 787-8 is 186-feet, 1-inch (56.71 meters), and tip-tapping becomes a tad ridiculous.

    Iowa State's solution to larger area composite inspection is the computer-aided tap test, or CATT. Developed by researchers David Hsu and Dan Bernard, the hand-held device houses an accelerometer, which measures the stiffness of a structure. Move the CATT along the surface of a structure and it taps. It feeds the return signal into a computer and generates an image. "Now," said Brasche, "instead of expecting the inspector of be able to hear those changes, they're actually able to see them on a computer screen."

    And so it is that "The NDT business is moving quickly from old-fashioned, manual 'craftsman' type inspection, which depends primarily on operator skill," said Olympus' Moles. Others challenge just how quick that transition really is.

    Beyond The Airframe

    The ascendance of automation, and concomitant demise, no matter what speed, of craftsmen, isn't confined to airframe structures. Inspecting engines via the use of borescopes is less onerous, and more automated, than ever.

    "Traditionally, you'd process a videoscope system through a monitor and look at a CRT," said Gene McGarry, general manager of Karl Stortz Industrial America. No longer.

    Stortz's Technopack X is the processing component for the OEM's line of videoscopes. It "allows you to interpret different chips on a single processor," said McGarry. Different scopes contain different diameter CCD chips. The Stortz back-end product recognizes the chip that a specific scope employs and "optimizes the digital presentation." Specifically, Technopack, paired with the proper scope, "can quantify the area, depth and length [of] missing material, [of] the crack" on a blade, or in an igniter box.

    Traditionally, McGarry said borescope operators had to "take a comparative known and stick it in the field of view." Technopack offers the ability to measure an anomaly with a laser, without having to change tips. When a technician sees something he wants to measure, he hits a button and the laser flashes a grid. The processor interpolates the distance from the flaw, and the skew of the videoscope to the target. "This way, the operator can capture the distance, measure [the anomaly] right now, or hit another button on the scope and come back and measure later."

    That way they know if an anomaly is within specs or not -- right then, right there.

    Such actionable intelligence is critical in selling the new breed of NDT/NDE gear, in making it affordable to airlines and MROs. The idea is "that a quick decision can be made on the disposition of a component," said Dave Jankowski, general manager of the ultrasonic and eddy current product lines for GE Inspection Technologies.

    The trend GE and others see is toward, "more rapid decision making," said Jankowski. Ultrasonic and eddy current readings traditionally were depicted in either degree of needle deflection or a trace on an oscilloscope -- rudimentary renderings of deeper realities. It was up to the technician to determine what they actually meant. These are "A-Scans," and they are, at best, anachronistic. To read the electronic etching, "You needed full attention at all times," said the FAA Tech Center's Dave Galella.

    Enter the "C-Scan," a top-down, 2-D view of the component in question. "It gives you more intuitive detail, on a broader scale," said Galella.

    The broadest scale pictures practicable derive from phased array C-Scans. Proven in medical technology over the past 15 to 20 years, phased arrays, as currently configured for aircraft application, cannot just spot a flaw, but highlight it as well, "with a big, red dot," said Jankowski.

    Ultrasonic phased arrays work especially well for large swaths of structure, metal or composite. Traditional flaw inspection entailed laboriously moving probes from place to place. "With phased array," said Jankowski, "instead of physically moving the probe, we electronically steer a beam."

    While working well on metallic structure, it's in the composite world that phased array ultrasonics shine. Technicians take two- to four-inch wide transducers (many in the industry are loath to call them "probes" any longer) and, "run them down the length of the composite wing or component you want to take a look at," said Jankowski.

    OEMs are making progress on two fronts in tweaking phased array, in wringing out the last pixel of actionable information. Back-end computer processing geometrically is better than ever. But it's on the pick-up end, at the transducer level, that the challenge lies.

    "Your instrument is only as good as the quality of the transducer attached to it," maintains Jankowski. "I think the real growth [in phased array] is going to be in further enhancing the characteristics of the transducer," he said.

    Engineers are working to wrest a smaller pitch from phased arrays. "The smaller you can make it, the higher the resolution" said Jankowski.

    This is important when inspecting forgings, like landing gear. Here's the problem: you want to be able to inspect a relatively thick section of structure, but you'd like to do it in high resolution. Physics work against doing both at the same time. "Typically," said Jankowski, "as your resolution increases, depth of penetration decreases. You want to optimize them both."

    Probe deep. Find tiny flaws.

    Determining how to do both at the same time will require some fundamental research into ceramics, the stuff of which transducers are composed. While Jankowski believes the industry has made, "significant strides ... there's still work to do."

    There's work to do too in convincing airlines and MROs they should adopt phased array, even though, and even at this stage of maturity, it's clearly superior. "[They're] a little complicated, a little costly," said FAA's Galella. "Some are in the industry, but not many yet."

    The Business Case for Better NDT

    Phased array manufacturers point out that the price of their product has plummeted in recent years. "We used to sell a fully blown phased array ultrasound system in the mid-1990s for $200,000," said Olympus NDT's Moles. The cost is now $60,000, and the gear is portable.

    As recently as 2000, "you could not buy a portable unit," echoed Jankowski. "You'd have to buy a big in-line unit that you wheeled around on a cart, and it cost $80,000-plus."

    Okay, so the price is reduced -- but is it right?

    "If [airlines] are not saving significant man-hours or able to make the pay-off in a very short time, it's hard for them to convince management to buy the equipment," said Paul Swindell, a research engineer at the FAA's Hughes Technical Center.

    Wyle sees significant productivity gains from the use of new engine inspection gear that it's employing with the U.S. Air Force. "One operator can run two or more inspection systems at the same time," said Mike Gilkey, vice president of Wyle's Air Force Technologies operating unit. The OEM's equipment essentially automates a tried-and-true powerplant rotor surface inspection technology: eddy current. "The crack detection signals that we collect get processed ... and we characterize [them] against low-cycle fatigue cracks," said Gilkey.

    Refraining a theme that emerged again and again by the people with whom O&M spoke, Gilkey said this iteration of Wyle's engine inspection equipment "doesn't rely on human interpretation of the results."

    Less reliance on the flesh and blood fleshing out of data, and explicit displays aren't, at least in-of-themselves, enough to break the bond between the industry and older, cheaper inspection techniques. What could sever it, however, is the cascade of composites. The advent of largely composite aircraft, "may change the way [airlines and MROs] inspect," said Swindell.

    "As the new generation of airplanes, the 787 and the A380, hit the use side, the post-production side of the industry, then suddenly the airlines and the maintenance, repair and overhaul facilities, will begin to deal with composites in a much larger volume," echoed Iowa State's Linda Brasche -- larger volume both in terms of the sheer size of the areas that they're required to inspect and the number of airplanes composed significantly of the stuff.

    Down the production line, just perhaps, there might be a macro-method, a way to inspect composites in a managed, predictable fashion -- one's that's engrained into the very fibers of the airplane itself.

    QinetiQ's Smith suggests the notion of "effectively 'fingerprinting' an aircraft at production, producing a complete ultrasonic fingerprint at manufacture that you can then compare with [the extant aircraft]." At regular intervals, technicians could re-scan the airplane with compatible ultrasonic gear. Then, it would be a mere matter of comparing before and after images.

    Smith said C-Scan phased array automation is fine for significant swaths of structure. But, "if you want to automate very large areas of structure," then mapping might be the answer. Were fuselages fingerprinted and the alignment of fibers in wings pre-written, that kind of sinewy scrutiny could produce quantum gains in inspection productivity -- the kind of gains that, just possibly, might re-write the book on Non-Destructive Testing and Non-Destructive Evaluation.

    From Sandia to Farnborough, Ames, Iowa, to airline maintenance shops, engineers are adding chapters to the book right now: in-situ sensors, amazing phased arrays, more capable borescopes, even -- just perhaps -- airplanes that will be mapped from the womb and maintained on-condition.

    The Holy Grail of aircraft maintenance may be a myth, but, like the quest itself, it's what we take away from the journey that matters.

    This article appeared in Overhaul & Maintenance's February 2008 issue.



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