The craft of aircraft repair
As the use of composites on commercial and military aircraft grows, repair facilities seek more science in what remains a primarily manual art.
By Karen Fisher Mason, Contributing Writer | May 2005
Unlike manufacturers of composite aerospace components, who have, especially of late, sought labor-reducing means to automate production, via resin transfer molding and the like, those who repair what they build still practice a primarily manual occupation. At repair depots around the world, the tradition has been to focus less on repair technologies than on proper repair techniques and knowledgeable technicians. The advent of more composites-intensive commercial and military aircraft — most notably, the forthcoming Airbus A380 and Boeing 787 passenger jets — will fuel the need for high-quality repairs, especially on fuselages, wings and other safety-critical primary structures. While there are technological innovations now in development that promise to strengthen the science of composite repair and simplify the repair technician's task, most have yet to attain proof of concept, much less commercialization. In the meantime, those who perform composite repairs and the composites training companies that serve them are looking for ways to keep repair artisans as well prepared as possible.
Source: HEATCON Composite Systems
HEATCON's stretchable heat blanket improves heating uniformity over repair patches on complex contours.
"The aircraft industry is recognizing a need for better training," says Michael Hoke, president of Abaris Training Resources Inc. (Reno, Nev.). He notes as evidence the fact that enrollment in his firm's composite repair courses has increased 50 percent since 2001 and might have been much higher had it not been for the negative economic impact the terrorist attacks of September 11 that year have had on commercial airlines. Much of the growth since 2001 has come from the military, although the commercial airline sector, Hoke reports, "is starting to pick back up," as are general aviation and helicopter sectors.
In the wake of 9/11, the training facilities have begun to see the need to decentralize training operations, the best of which, until recently, have been located within the U.S. "Repair technicians from various parts of the world have found it difficult to get visas to the United States," explains Tom Lane, managing director of hot-bonding equipment supplier HEATCON Composite Systems (Europe) Ltd. (St. Ives, U.K.).
In response, Abaris and HEATCON formed a joint venture in late 2004 that will provide composite repair training sites in other countries. The first location will be within Lufthansa Resources' training facility in Cwmbram, Wales. HEATCON Abaris Training International will advise Lufthansa on the design of a composite repair shop and then start offering courses there. Another training site is likely to open in HEATCON's Shanghai, China facility.
Perfecting the manual art
Both military training centers and private training concerns like Abaris and FlightSafety International (West Palm Beach, Fla.) teach the same basic steps of manual repair. Technicians inspect components visually for surface defects, such as pinholes, erosion or small cracks. Coin tap tests can uncover potential delaminations. Ultrasonic and other nondestructive inspection (NDI) equipment further investigate suspicious areas. Sometimes the most efficient and cost-effective means for correcting damage is to replace large sections or even entire facesheets, as Delta Technical Operations (Atlanta, Ga.) reports of its engine cowl repairs (see HPC May 2004). Another technique viable in thick solid laminates is to bolt in a doubler — a metal and/or precured composite reinforcing panel that restores the load-bearing capacity of the component.
For smaller repairs, technicians remove the damaged area and perform scarf sanding, where ply surfaces are removed in an even, tapered ratio of ply height to a given length. Scarfing ensures good bonding and distributed load transfer between the existing laminate and the patched area. Subsequent steps include surface preparation, core fill and application of the repair materials — either by mixing resin and working it into the reinforcing fabric for wet layup, or preparing a prepreg patch. Vacuum bagging systems, including peel plies, release film, bleeders, breathers and caul plates, are used to provide compaction. The repair patch might be cured in an autoclave or oven. Alternatively, heat from portable hot-bonding equipment can be delivered through heat blankets, lamps or hot air.
Out-of-autoclave aids
Hot bonding equipment has assumed greater importance as the use of composites in critical flight structures increases. HEATCON's Lane notes that, until recently, the basic concept for hot bonders had remained essentially unchanged since their introduction. But Patrick Kelley, manufacturing engineer at Hill Air Force Base (Utah), notes that technicians today have to perform more on-aircraft repairs. "When a full wing structure is composite, we don't remove it," he explains. "So we've got repairs that require pressure and heat for curing, but we lose the ability to place the damaged components in an autoclave." Two recent innovations have served to increase hot-bonder utility, especially in on-aircraft repair situations.
HEATCON offers a stretchable heat blanket for repair of contoured surfaces. The silicone rubber blanket can elongate up to 30 percent in any direction, Lane reports. These blankets provide even heat distribution through a grid of wound resistance wire, which is vulcanized between two sheets of silicone.
A second innovation is designed to promote greater heating uniformity in difficult-to-cure repairs. Scott MacKenzie, CTO of hot-bonder manufacturer Zimac Laboratories (Ottawa, Ontario, Canada), contends that even "when the heat source is inputting heat uniformly, temperature variation across the repair area will occur. It is a mathematical reality, a nonlinear equation, that we can't avoid." While 20 or more thermocouples may be positioned around a repair patch to provide feedback to operators about hot bonder performance, standard hot-bonding equipment typically offers only two heat-source zones, he says. While this is adequate for a vast array of repairs, he contends the temperature differences are exacerbated in repairs with complex geometries, large surface areas or in parts with attachments that create heat sinks. "Additionally, the response of the control thermocouple is proportional to the thermal time constant, which is affected by the geometry and the materials — none of which is uniform in many aircraft structures," MacKenzie continues. "So the thermocouples must be located very close to the heater to minimize time lag."
Source: Zimac Laboratories
The U.S. Air Force successfully demonstrated the Zimac HBS bonding system's precise control of cure profile on this F-16 tail fin repair. Five heat-source cells attached to a caul plate over the 35.5-cm/14-inch square patch are used to "shape" the heating as needed. Cells positioned around the patch act as heat dams over stringers. Thermocouples for each cell provide temperature feedback.
To address temperature variation, Zimac offers hot-bonding technology that provides up to 32 heat sources and up to 16 thermocouples for one job. Initially designed to apply doublers to metal bulkheads, the bonder uses both heat blankets and small heat cells appropriately. Cells can be positioned in small areas of the bulkhead, with the metal conducting the heat to the bond site. The system adjusts each heat-input cell to keep bond heating — and therefore, curing — uniform. In composite structures, which offer low thermal conductivity, Zimac uses heat cells connected to a conductive caul plate. The resulting "shaped heat process" is designed to ensure a uniform temperature response within the repair, MacKenzie says. Further, Zimac has an applied patent on a 3-D conformable, heat-conductive caul material.
European Aeronautic Defense and Space Co. (EADS, Manching, Germany) has been using the Zimac HBS hot bonder to repair damaged carbon composite structures on the Eurofighter and other aircraft. Before purchasing the Zimac system in 2000, "we had a 'home-built' system that was able to control and monitor only one heat blanket and eight thermocouples," recalls Rainer Altmann, EADS manager, workshop component repair and overhaul. With this system, EADS used only hard patches precured in an autoclave, because cure temperature of a wet layup patch on the aircraft could not be adequately controlled. "With the Zimac HBS bonding system, we can cure wet layup patches on the airplane within the tolerances required and get a fully cured patch without bubbles or defects in the bonding line or in the patch," Altmann reports.
