Printed headline: Gaining Control
Aircraft flight control surfaces—ailerons, rudders, elevators, flaps and slats—are subject to considerable wear and tear in the course of everyday operation.
Control surfaces are usually adhesively bonded composite or aluminum honeycomb sandwich structures. These components can suffer from damage caused by runway debris, hailstones, lightning strikes or tools dropped during maintenance.
On composites, low-velocity impact damage sometimes escapes visual inspection, meaning nondestructive testing (NDT) also must be used to determine the damage to a flight control surface.
Typical repairs to flight surfaces include metallic and composite patches. Air France Industries-KLM Engineering and Maintenance says most damage to flight surfaces is caused by foreign objects. Like other MROs, it focuses on new technologies for nondestructive testing such as thermography. “As part of our MRO lab innovation program, we also work on a project using a robot to machine composites with water,” a statement from AFI-KLM E&M notes.
AFI-KLM E&M says most damage to flight control surfaces is caused by foreign objects.
Credit: AFI-KLM E&M
Companies that provide tooling for aerospace operations have recognized that MROs are benefiting from new types of technology, and they are introducing equipment that speeds up and automates repairs for composites, including flight control components as well as curved surfaces and the fuselage.
For example, MRO Lufthansa Technik is now experimenting with a robotic system that is building on the results of its long-running “Rapid Repair” project, which began in 2009. The company has developed a mobile robot equipped with software needed to recognize free-form 3D surfaces. For airframe scarfing, the robot is placed on a stand and is attached to the carbon-fiber composite component with suction cups. Software allows the mobile robot to process 1,000 X 1,000-mm (39.3 X 39.3-in.) surfaces and thick carbon-fiber-reinforced polymer structures such as wing connection zones.
Machine toolmakers DMG Mori and Sauer also have introduced mobile five-axis aerospace milling units to carry out repairs on aerospace composite components. These lightweight machines feature vacuum suction feet and moveable ball joints and mounting arms. They can be clamped to even surfaces such as wings for flight surface repairs, or to components with radii and complex contours such as frame elements on the fuselage of an aircraft. The five-axis control offers PC-based operation via a touchscreen for the user. The machine allows the user to define the damaged area, the machining task, laser surface measurement of the workpiece, and creation of the final file.
One MRO provider in the UK focusing on older aircraft is Cardiff Aviation, based at the St. Athan-Cardiff Airport Enterprise Zone in Wales. It provides maintenance, structural repairs and modifications for narrowbody airliners—typically Airbus A320s and Boeing 737s.
“Regarding composite repairs and flight control repair, they suffer more than newer aircraft. So they are right up our street,” says Martyn Anderson, managing director of Cardiff Aviation.
For repairing flight control surfaces made from composites—the A320 uses composite materials in the flaps, ailerons, horizontal tailplane, and stabilizers, for example—Cardiff Aviation has invested in far-spectrum infrared equipment from France’s rapid aircraft-repair technology company Sunaero, to speed up the curing process on fresh repairs. Often, flight control surfaces will be removed and placed in a workshop and an insert made, or an entire section replaced. “We’re using infrared heat lamps to cure composites. In that way, we can get a faster, and actually a very good, cure,” says Anderson. The time taken to cure a composite repair to a flight control surface using this technique can be reduced from 24 hr. to 2 hr., he says.
Many flight control surfaces feature an aluminium frame with a composite overlay. The trailing edges are often composites with honeycomb sections embedded in them. If the honeycomb has been damaged, it is removed and a new honeycomb is inserted with resin, then overlaid with fiberglass or carbon-fiber skin, explains Anderson. Typical damage to flight control surfaces results from debris thrown up by the wheels, and consists of delamination of materials—where composite layers separate—and moisture intrusion.
Cardiff Aviation uses numerous NDT techniques such as eddy current, X-ray and visual inspection to determine the full extent of flight control component damage.
Credit: Cardiff Aviation
Cardiff Aviations uses NDT techniques such as eddy current and X-ray analysis to determine the full extent of the damage, as well as visual inspection. “Typically when an aircraft comes into a hangar, you only know 50% of the damage, which is routine stuff: The other 50% is discovered through inspection,” Anderson says. “So it is only when you get the aircraft in that you know what repairs you’ve got to do. The aircraft might come in with a few repairs listed in the log book—‘we’ve got a bit of damage on the trailing edge’—but the actual inspections discover more.
“That’s when the real work starts: Once you’ve discovered the damage, you’re on the time line,” he continues. “You know the quicker you can repair it, the quicker you can rebuild your aircraft and deliver it. The airline or lessor wants its aircraft back in service as quickly as possible, earning money. We want the next aircraft in as quickly as possible. So for both parties, rapid turnaround is certainly the objective.”
Physical and NDT examinations may reveal water within the flight control surfaces, which must be drained. That creates disbonding within the structure. Sections are cut out and new surfaces with fresh skins inserted. The operation must be vacuum bagged, a process in which pressure is applied to the repair in order to improve its consolidation by sealing a plastic film over the wet laid-up composite and onto the tool. “We cover the repair area with a sheet of plastic and a vacuum pump, so we get a very tight bond on the repair itself,” explains Anderson.
He says the company is developing new NDT techniques to facilitate the repair process. It also continues to work with Sunaero to look at technology that may speed up the repair of flight control surfaces and other components. “If technology helps us to produce a better product, faster, then we just need to do the math on whether there is a good business case. Usually there is. If we can improve the turnaround time of the aircraft, that benefits everybody,” he says.
For older aircraft, such as those serviced and maintained by Cardiff Aviation, scribe marks—damage to aircraft surfaces caused by the use of sharp tools used during paint and sealant removal—are often an issue, Anderson adds.