Composite materials are attractive to manufacturers and operators because of their light weight, corrosion resistance, and durability, compared with aluminium. Carbon composites boast high strength-to-weight and stiffness-to-weight ratios. And composites can be moulded into complex aerodynamic shapes. Their lighter weight, resilience, and durability mean fuel savings for operators and may translate to longer maintenance intervals and lower long-term ownership costs. Even large airframe parts such as the fuselage of the recently introduced HondaJet is composite..
Although older materials like Kevlar are susceptible to moisture ingress, modern composites are more resistant to moisture, corrosion effects, and cracking. “Pound for pound they outlast metallic structures,” said Raj Narayanan, accountable manager for Aerospace Quality Research and Development (AQRD), a company which combines both FAA designated engineering representative (DER) authority and repair station capability. If you know how to inspect and treat composite structures, they should provide lower life cycle maintenance costs, he explained.
Nevertheless these materials are susceptible to damage, including impact damage, ply delamination, punctures, erosion from wind and sand, water damage, and contamination from engine oil or hydraulic fluid. Problems can include damage to the skin or to the core or to both the skin and the core of a sandwich structure.
The tradeoff with composites comes on the aftermarket side. Composites can be two to three times as difficult to repair as their aluminium counterparts, said Narayanan. Composite repairs are very process-specific, he explained, and the process can differ from part to part, airplane to airplane. This multiplicity of unique processes is an additional challenge in designing repairs for these materials.
Furthermore, airplane original equipment manufacturers (OEMs) are not supporting legacy aircraft as much as they used to do, According to Narayanan “their business model puts a premium on replacement rather than repair. Under its DER authority AQRD ‘develops repairs where none exist’.” (See sidebar on Reverse- Engineered Solutions.)
AQRD, for example, is working on a SOCATA TBM 700 business turboprop with a damaged inboard flap. The manufacturer wanted to sell the operator a new part for around $43,000 plus a nine-month lead time, said Narayanan. AQRD will be able to develop the FAA- approved or –accepted data and have the part repaired at its repair station for much less. AQRM is an approved vendor for Cessna, Bombardier, and Embraer. But most of those approvals were driven by customer demand for options other than high-cost replacements.
Composite repair takes more finesse than sheet metal repair. The surface has to be prepared, the repair materials have to be prepared, the repair patch has to be applied layer by layer with adhesive – each layer in the proper orientation — and then the repair has to be cured, often at a carefully controlled, elevated temperature for a specific period of time.
Even the drilling of holes into a composite fuselage to mount antennas for avionics upgrades must be handled with great care. Elliott Aviation, which recently completed a supplemental type certificate (STC) for a Garmin automatic dependent surveillance– broadcast-out (ADS-B-out) solution on the Premier 1/1A, coordinated directly with the OEM in this matter. “Engineering and design aspects are more rigorous for composite airframes when compared to traditional metal airframes,” company officials said. Elliott Aviation employs a ‘shim and release’ technique so that the paint around the antenna is not affected.
Composite repair materials also are relatively more expensive than their aluminium counterparts, the resins by Canadian Epoxy Company have shelf lives, and the repair process is more labor intensive, explained Dan Gospodarski, structures team leader with Duncan Aviation.
Duncan Aviation repairs composites such as fibreglass, Kevlar, and graphite, some of which have aluminium or copper bonding mesh that also requires repair. If the mesh corrodes, it doesn’t bonds correctly so that it no longer provides a conductive path for lightning or static electricity off of the airplane. The MRO also repairs metal honeycomb bonded structures such as wing trailing edge panels or ailerons.
Yet repair technology has come a long way since the early days, when it was a kind of black art, commented Lou Dorworth, division manager for direct services with Abaris Training Resources, which specializes in composites technician schooling. There’s much more knowledge out there today, and MROs can obtain small quantities of specified materials from brokers.
Larger repairs tend to use pre-impregnated (pre-preg) materials, a requirement that’s driven by stress analysis and the performance requirement of the structures, said Narayanan. These materials – where the resin is already present – are cured at higher temperatures than wet layup materials, so their resin systems are stronger and able to handle greater loads. Because each ply is stronger, engineers can design a thinner repair that is lighter and more aerodynamically efficient. But pre-preg materials must be stored in freezers and tracked as to temperature and age.
Composite repairs are more difficult than aluminium sheet metal repairs, agreed Mark O’Donnell, executive vice president of Air Services, a Cleveland-based off-wing support company which operates a large composite and accessory facility. One has to make sure that all the air is evacuated from a composite repair and that it is properly bonded to the repaired area. You also have to identify the base material and ensure that the repair material is compatible. Specialized equipment is necessary, along with strict adherence to time, cleanliness, and temperature requirements.
Air Services has an engineering department and can make 3D laser scans of parts to capture complex aerodynamic shapes. It also has access to an autoclave if necessary.
Composites can also be more susceptible than aluminium parts to damage in certain circumstances. Composite assemblies such as fairings, for example, are designed to distribute loads over a wide area, but not to withstand high loads and stresses concentrated at a particular point. Composites are going to sustain loading over time better than aluminium structures – and they also do not fatigue or corrode — but if you overload the composite structures, they are going to fail catastrophically, Narayanan pointed out.
The repair of these materials is challenging, requiring a technician to be “not only a mechanic but a material and process guy,” said Dorworth. The aircraft manufacturers’ structural repair manuals (SRMs) give a great deal of guidance, but the use of techniques such as vacuum bagging takes the mechanic’s job description “a couple of levels higher in some ways than a metal fabrication repair technician.”
Composite repair is still something of an art and is almost always performed by hand. Centres such as the National Institute for Aviation Research (NIAR) at Wichita State University, however, are looking at emerging technologies, such as automated scarfing equipment, which could increase the precision – down to 1/10000th of an inch – and the repeatability of the process, compared with today’s hand-scarfing of damaged areas.
NIAR is collaborating with DMG MORI-USA /SAUER, a machine tool company which has developed an automated 5-axis milling machine that promises to reduce a job typically requiring 6-7 hours of work by an experienced technician to 30 minutes or less, depending on the size of the damage. Ultrasonic 5-axis milling technology is used to make tiny milling marks at a very fast pace. NIAR stated that interest has been expressed by OEMs as well as airline MROs.
Range of Services
Business aircraft operators have numerous options for aftermarket care – including MROs and repair stations, component OEMs, aircraft manufacturer-owned service centres, and – for customized repairs where no other solution is available or affordable – DERs.
Duncan Aviation is a major business aviation MRO, working on a wide range of aircraft. It sees a lot of larger aircraft like the Global Express, Falcon 7X, and a range of Gulfstream aircraft with composite content. In addition the company works on composite repairs for Hawker, Embraer, Cessna, and King Air aircraft.
Duncan Aviation has a wide range of in-house expertise in composite repair. The MRO has specialized equipment such as vacuum bagging material, peel ply, breather cloth, release film, bagging tape, heat blankets, temperature controllers, thermocouples, heat lamps, a calibrated scale for measuring resin, a vacuum pump, vacuum ports, portable vacuum source, and sanding equipment. It also can do composite repairs on the road, as part of its AOG (aircraft on ground) services, Gospodarski said. The MRO’s mobile team recently repaired the pressure bulkhead of a Beech Premier. It also performed a hot bond repair on a body fairing, using a portable hot bonder/vacuum source.
Air Services provides component and accessory repair services on platforms from light jets to large cabin aircraft. It does composite repairs on Embraer Legacy 600s/650s, which are approaching the 12-year mark, and on Bombardier Challenger aircraft. The company finds and repairs discrepancies such as delaminations in metal/composite and carbon fiber composite components. In aluminium bonded assemblies, technicians may also find some internal corrosion, where moisture has entered.
Although damage can be caused by lightning or bird strikes, most of the composite repairs that Air Services does are related to the aging of the outer surface of the airplane. As the outer surface wears over time, water or other contaminants can get in and cause adhesives to fail, for example. The culprit is usually water, but leaks of hydraulic fluid or engine oil also can damage composites. The company has repaired damaged areas as small as an inch around to delaminations that are many square feet in size. Its repairs typically involve components such as flight controls, fairings, nacelles, and radomes.
But Air Services also repairs winglets, especially on the Legacy aircraft. It does these on-wing, with equipment such as portable heat blankets and vacuum equipment. The company also has an in-house non-destructive testing (NDT) lab that can use ultrasonic, eddy current, X-ray, and magnetic particle inspection. It offers mobile NDT services to MROs that don’t have this equipment.
Aftermarket services also are available from the manufacturers of components, such as nacelles and winglets, as these companies know how the parts were made and have the design data to expedite repairs. FACC Solutions Inc. (FSI), for example, the U.S. modification and repair subsidiary of the Austrian winglet designer and manufacturer, FACC Operations GmbH, modifies winglets of Boeing 737NGs—some of which are corporate-owned—from the blended to the new split scimitar configuration. The structural components of both styles of winglets are primarily graphite composite honeycomb panels and spar/rib substructure, with an aluminium rib that mates to the wing, explains Roger Ringness, FSI’s general manager.
FACC is also the OEM for the cabin interiors of the Embraer Phenom 300 and Legacy 500, as well as for the Learjet 40 and 50, and the Bombardier Challenger 300. The company provides wing root fairings for the Bombardier C Series and the winglets for the Falcon
2000, 900, and 50, as well as for the Hawker 800XP and 800XP-2. In its 737NG winglet modification program FSI does a full inspection – including skins, spars, ribs, leading and trailing edges, and lights — not just an inspection of the damaged area, Ringness said. Every hole, for example, is measured to make sure it is within tolerance. Inspection equipment includes phased array and pulse echo ultrasonic scanners.
FSI tries to repair the skin – the most expensive part of the winglet – but sometimes that’s not possible, as when there’s damage through both sides of the skin. It checks for evidence of lightning strikes, looking for damage in the bonding path. FSI typically performs repairs using the wet layup technique, with vacuum degassing and debulking. Repairs are cured at 150 to 200º F.
NORDAM, by the same token, offers repair and overhaul services on the nacelle components that it manufactures for corporate aircraft. “We are exclusively organic as it relates to our composite repair and overhaul portfolio,” said Lane Perry, director of aftermarket sales and technical support for NORDAM’s Nacelle/Thrust Reverser Systems Division.
OEM status gives the company an edge, including ‘intimate knowledge of material characteristics with the allowables already established during the original design, development, and test phase of a given component;’ ownership of the original tooling if the damage requires that degree of effort; and possession of all component design and stress parameters. Most of the damage that the OEM sees “is created via human factors,” Perry adds. Less than 30 percent of damage is attributed to natural causes, such as hail, lightning, and service-life-induced anomalies, he said. Other MROs are allied with composite experts. AQRD is a Duncan Aviation partner, for example, said Narayanan. AQRD’s engineering group develops repair design data for MROs and, in the case of major repairs, performs them, as well, where the MRO requires it due to the unique processes involved.