Printed headline: Plastic Fantastic
Carbon fiber has a long history in commercial aircraft manufacturing, but its first large-scale application was on the Boeing 787, which features composite materials for half its airframe. Airbus followed Boeing’s lead and used composites for much of the A350’s wings and fuselage, and any future clean-sheet aircraft—as well as upgrades of existing models—will lean heavily on carbon fiber as well.
Composites have several advantages over metals: They are lighter, they don’t corrode, and they exhibit better fatigue resistance. The tradeoff is that metal is easier to fix, so MRO providers need to add new tools, know-how and processes to ensure they are capable of handling the latest aircraft.
“Before the launch of the 787 and A350, we had mainly been providing composite repair services for wet lay-up repairs of fairings and other secondary structures of an aircraft,” says Ang Chye Kiat, executive vice president of aircraft maintenance and modification, at ST Engineering Aerospace. “With the fuselage and wings of 787s and A350s now compromising composite material as well, we have grown our capabilities to support these platforms not only with repairs but also modification work.”
Inspection and repair techniques for composite materials are developing apace but are still not as mature and well-understood as those in place for common aerospace metals such as aluminum. So in some respects the performance savings of new materials for airlines are offset by higher support costs, although the greater durability of composites should mean fewer maintenance events.
AAR has expanded its composite capabilities five-fold in the last 20 years, and its senior vice president of repair and engineering services, Brian Sartain, expects further growth as more new-generation aircraft enter service. Sartain acknowledges that composite and metal repairs require different skillsets and tooling, although he disagrees that carbon fiber is intrinsically more difficult from an MRO perspective.
“I don’t believe it has necessarily become more complex as it has driven a shift in skillsets and a greater frequency of replacing parts altogether,” he says. “Older aircraft were very labor-intensive with metal-part fabrication and very expensive in part replacement.”
On top of adding new engineering competencies to traditional metal-working capabilities, MRO providers must keep up to date with a new generation of composite materials, notably carbon-fiber-reinforced plastic (CFRP). This is much more durable than the composite material of 20 years ago but is also more difficult to work with, involving repair processes such as double vacuum debulk, which reduces the porosity in laminate plies prior to the cure cycle that hardens the CFRP.
“The curing cycle of the new material is much more critical and includes the thermal survey on the surrounding structures,” says Ang. “This process is required for better heat management during repair . . . and is much more time-consuming.”
Newer composite materials also are more expensive to store since they require strict environmental controls and have limited shelf lives. “Material costs are much greater, storage is more expensive, and environmental control is a must with the new materials—earlier composite materials did not have such exacting requirements,” says Sartain.
Inventory costs will also creep up as new carbon-fiber designs are used on different parts of the aircraft, requiring MRO providers to diversify storage and repair capabilities. “Current composites, compared to the early materials, show more variation in the design, fibers, resins and production processes being used—this has a big impact on MRO activities,” says Rob Bosgraaf, head of customer solutions and technology alignment for Fokker, a subsidiary of UK aerospace manufacturer GKN.
Before any repairs or replacements are performed, engineers must assess the damage (or lack thereof) to a composite part. Engine nacelles, one of the first aerostructures to explore the thermal and weight advantages of carbon fiber, are inspected with a variety of techniques, including: tap testing, thermography, ultrasonic testing, borescoping, radiography and etch and penetrant inspection.
Some of those methods overlap between metal and carbon fiber, but for a full capability on the latter, the nondestructive testing (NDT) and inspection of composites requires significantly more investment than for metal structures. Specialized equipment is needed to check for delamination or fiber irregularities in CFRP, whereas inspections of simpler materials can be as easy as making a visual check.
“Composite equipment and training for that equipment runs into the hundreds of thousands of dollars rather than typical ultrasonic equipment in the tens of thousands,” says Sartain.
Ang agrees, adding: “As the technology is still new, pricing for tooling and materials for the repair is much higher compared to metallic materials.”
An array of new NDT equipment promises to speed up inspections, reduce errors and add mobility to the process. These include hand-held C-scan rollers to check for issues like delamination, as well as hand-held thermal-imaging cameras and 3D laser scanners. Some of these are subject to airworthiness and OEM approvals before they can be used on aircraft, but once that occurs they will open the door to faster repair cycles and more automated processes.
“New technologies include automated equipment that can scan and detect defects via comparison to prior profiles—today most of this it is still done by hand,” notes Sartain.
Airbus has developed a robotic arm that uses an ultrasonic laser to detect subsurface defects in composites. Unlike traditional ultrasound equipment, LUCIE (laser ultrasonics composite inspection equipment) can scan without being in contact with the material. Combined with an articulated robot arm, this makes the technology well- suited to inspecting large composite parts with complex geometries—features that are increasingly common on new-generation aircraft.
Since the 787 and A350 only entered service in 2011 and 2015, respectively, the first big wave of major composite repairs is several years in the future. Nonetheless, composite airframes are susceptible to many of the same forms of operational damage as metal ones, so smaller repair jobs are always needed. Such damage includes ground equipment collisions, bird strikes, hail and lightning strikes. Moisture ingress and overheating, meanwhile, are risks specific to certain composite structures.
Often, damage is so minor that it can be postponed until the next maintenance event, but MRO providers and airline technical departments also offer field repairs. For the most minor damage, such as material erosion, resin is brushed into the affected area and left to cure at ambient temperature. Known as a wet lay-up, this is a relatively simple solution.
Composites are often fixed by cutting away the damaged material and either bolting or bonding on a patch, although bonded repairs still are forbidden for primary structural components, except in cases where the repair is so small that the structure would not be affected if the patch disbonds. That said, bonded repairs are preferred by most engineers since bolt-ons are heavier and require holes to be drilled that further damage the composite material.
One of the key growth areas in composite MRO is on-wing work, as in making repairs without removing components. This has become increasingly important as larger structures—such as wing and fuselage sections—cannot be removed for repairs, which was normal procedure for the smaller composite parts of the past.
For on-wing work, Lufthansa Technik (LHT) has developed an automated scarfing system called CAIRE. After a damaged piece of carbon fiber is cut away, scarfing prepares the edges of the hole for a patch by sanding them into a taper. The angle of the taper is determined by the load on the structure being repaired, and Lufthansa’s mobile robot can conduct on-wing, high-precision scarfs on up to 1 m2 (10 ft.2) of CFRP.
“This allows faster repairs with a high quality and reproducibility and is an essential element for the development and certification of larger and structurally more critical repairs,” says Henrik Schmutzler, an engineer at LHT’s innovation and product development division. CAIRE is being industrialized at Lufthansa Technik in Hamburg and will be in operation in the third quarter of 2019.
Applicable to thrust reversers as well as fuselage and wing structures, CAIRE includes other technologies from a highly automated process designed to cut 60% from conventional repair times. It starts with a strip-light projection scan of a damaged composite component, from which a computer can direct a robotic milling machine. Later, pre-cuts of composite layers are created and applied to the component being repaired with the help of a ply-cutter that creates the individual layers. The cut-to-size panels are bonded with the primary structure and then cured as required on molds generated by rapid prototyping using the previously scanned data and computer calculations.
Airbus has developed a similar portable robotic repair that uses water mixed with an abrasive (instead of laser) to remove up to 500 cm2 (77 in.2)of damaged material for replacement with new carbon fiber. The replacement material is cured on site as an out-of-autoclave repair. Airbus also offers an inflatable clean room, which brings the environment used in manufacturing plants close to the aircraft.
Such advances notwithstanding, most composite repairs still are performed by touch labor. However, automated processes will be an integral part of the composite aftermarket moving forward, in part because of the imposition of stricter standards for inspection and repair procedures, which add to repair time if done by hand.
“Given that the inspection and repair of composite parts are more elaborate due to a more stringent and controlled process, there tend to be more challenges with the check span time,” says Ang.
The predictive maintenance technologies widely employed on aircraft engines are now spreading to other aerostructures.
“Looking at the future, more automation on both repair processes and scan methods will be developed. Embedding sensors in the structure and scanning large surfaces will speed up maintenance time and are therefore under development in many institutes,” says Bosgraaf.
“Smart” carbon fiber that incorporates sensors to detect structural anomalies is not yet in use on commercial aircraft. However, it shows great promise, and adoption by OEMs might be closer than some expect.
“Several advances in micro-electro-mechanical-sensors (MEMS) have yielded the types of technology required to embed strain gauges in composites that could proactively detect defects,” says Sartain. “I think these technologies could be seen within the next five years.”
Inspection methods such as acoustic and ultrasonic scans can spot flaws in composites, but they provide little information about stresses that build up in parts from operational loads such as vibration, temperature and impact. Accordingly, there is often no way to predict where cracks and other defects might occur.
More than a decade ago, FAA research examined how stresses could be assessed using adhesive film sensors. In those tests, resonance sensors were glued to composite panels to assess damage progression after a flaw was introduced.
A better method, however, is to embed sensors in the material. Scientists led by Sergey Kaloshkin of the National University of Science and Technology (NUST) MISIS Center of Composite Materials in Moscow have proposed using soft magnetic circuits only 10-60 microns in diameter to measure stresses in carbon fiber, which would have the micro-wires laid in a grid between plies. Stress near the micro-wire affects how the substance reacts to external magnetic fields.
NUST MISIS scientists say they can embed the wires without affecting the material’s structural properties and are now seeking to develop a field prototype. If that works, the next step would be a system to relay sensor findings to an aircraft’s health-monitoring system.
“Having sensors in your structure will give you a better view on those events during flight that imposes risk to the structure,” says Bosgraaf. “If you compare the data gathered during flight with the original digital design, inspections can be performed in a more focused way.”
Airlines may not have to wait long until new testing and repair equipment brings some of the performance gains of carbon fiber into the hangar.