The MRO composite challenge

Crash a baggage buggy into the side of an aluminium passenger airliner, and the damage can be clearly seen; dents and all. Do the same to a carbon fibre composite material fuselage airliner and the damage may not be as obvious. This is because metal typically dents and tears when impacted, and holds the damaged shape. Composites do not; in many instances after impact, a composite component will ‘bounce back' and return to its original shape. Hence there may not be an obvious external indication if the impact was strong enough to cause internal structural damage. The challenge for composite aircraft owner/operators, and the MROs that support them, is to ensure that any damage to these aircraft is accurately detected and appropriately remedied; whether with an on-ramp repair for small dents and tears, or major repairs in an MRO shop. Being able to detect damage to next-generation composite airframes is particularly vital, “because “50% (by weight) of this airplane is made of composite,” says David Polland, chief structures engineer with Boeing Commercial Airplanes. But the 787 isn't alone in this regard: “The 777, 767, 757, 737 and 747 all make use of composite materials,” Polland notes. “The 777, which entered service in the mid-1990s, features the use of these materials for about 12% of the airplane structure, including vertical and horizontal tail structures, the floor beams of the passenger cabin and aerodynamic fairings.” Although composite airliners physically resemble their metal counterparts, the two are as different as wooden sailing schooners are from steel ships. This is because each class of aircraft is engineered in direct relation to their specific material properties and strengths. The difference between the two are fundamental. A composite fuselage is literally ‘spun' into existence as a whole; by winding, overlaying and then curing the resin-impregnated carbon fibre filaments into the desired 3D shape. The finished composite shell provides strength and integrity across its entire structure. In contrast, a conventional fuselage is built upon a metal skeleton and then covered with aluminium skin; both of which provide structural strength. This fundamental difference explains why composite aircraft repairs have to be approached differently from metal repairs. Roll away the problems Because composite damage can be more than meets the eye, a number of tools are used to inspect the affected area of the aircraft; both on the ramp and in the shop. The goal is to detect damage non-destructively, which is where innovative tools like the handheld Olympus Ramp Damage Checker comes in. This device uses ultrasonic beams (waves) to detect subsurface damage in aircraft composites during preliminary inspections. If damage is detected, the handheld Olympus RollerFORM Phased Array Wheel Probe can be deployed: It uses zero-degree ultrasonic beams to provide finer, more detailed data to technicians. If needed, further details can be provided to technicians with the use of further high-resolution ultrasonic inspection methods. In all instances, the goal is to detect the full extent to which the composite airframe/component may have been compromised structurally. The repair is then tailored not only to ‘fill the hole' but restore full structural integrity. “In most cases, the damaged composite section is cut out, and a new composite piece that fills the hole and extends to bond with the surrounding area is employed,” states Milco Rappuoli, ATR's sales director for the Middle East and North East Africa. Boeing uses three kinds of repairs to fix damage composite sections: The Quick Composite Repair, the Bonded Repair and the Bolted Repair. “For some small dents, gouges and nicks affecting specific areas of the airplane, a quick composite repair process can be accomplished in as little as one hour at the gate,” says Boeing's Polland. “This repair uses a pre-cured patch and a quick-curing adhesive with compaction and heat and can even be done in poor weather conditions as long as the repair area is protected from the elements. Once installed, an aircraft can quickly return to service until a permanent repair can be performed at a scheduled maintenance visit.” For larger areas, a “bonded repair is where the damaged material is removed in a taper sanded configuration, new raw composite plies are layered and cured under heat and pressure,” he says. “The original strength is restored.” For larger holes, a bolted repair uses plate material – it could be cured composite solid laminate or solid metal plate – and fasteners to restore the surface and restore strength. “The (composite) repair technique (selected) depends on the size and complexity of the damage,” says Polland. “The repair methods are proven designs and processes that are well-tested.” Worth notes: “When composite repairs are done on the ramp, the affected area may be enclosed in some form of tent to keep out dust. Where we can, we vacuum out the air surrounding the repair to protect it further and aid curing,” says Rappuoli. “Depending on the materials used, the curing process can take up to 12 hours.” Challenges for MROs To their credit, major MROs, such as Lufthansa Technik (LHT), have upgraded its tooling and staff training to deal with the growing number of composite aircraft coming into service. In fact, LHT's Airframe Related Components (ARC) service portfolio provides testing, repair, maintenance, overhaul and modifications “for the full range of aircraft composites, bonded materials and other structural components in use today,” says the company's website (www.lufthansa-technik.com/arc). This includes all composite repairs “specified in the aircraft OEMs' Structural Repair Manuals (SRMs), and repairs developed by LHT for legacy aircraft,” explains Dr Henrik Schmutzler, LHT's innovation engineer. This said, even an MRO as prepared as Lufthansa Technik finds repairing composite aircraft to be challenging. “There are high requirements from authorities as to the standards such repairs must achieve,” says Dr Schmutzler. “At the same time, there is still a partial lack of standardisation when it comes to materials and testing. (Meanwhile) Material appropriate bonded repairs are limited in their size, as they lack alternate load paths such as [e.g. akin to] bolted repairs.” Limitations notwithstanding, aircraft OEMs are doing their best to make composite repairs as fast and painless for aircraft owner/operators as possible. “Generally a major repair on any material type will be conducted in the field,” notes Boeing's Polland. “Boeing will work with operators, if requested, to help locate the necessary resources to ensure the repair is completed to regulatory standards.” “It is important to understand that composite structures have been in operation for many years on most airplane models,” he adds. “Most airlines have composite repair technicians that have been successfully performing repairs for many years. The airline technicians and engineers do receive additional training with the 787 airplane to assure they understand the differences from the ‘legacy models' to the 787.” The truth is that all major airlines, and the MROs that serve them, have no choice but to gain such expertise, and fast. This is because composite aircraft are the way of the future, due to their superior fuel economy and longer times between major C/D checks compared to traditional metal aircraft. Whether or not owner/operators and their support staff are comfortable with the shift from metal to composite ultimately doesn't matter – anymore than it did for wooden sailing schooner owner/operators when superior coal- and oil-fired steel ships came into service.