During my visit to the shop one of the large components being repaired was an MD-11 tail cone assembly. The tail cone is the closure for the aircraft’s auxiliary power unit (APU) and is located below the number two engine on the MD-11. After years of service the carbon graphite skins become damaged, the internal stiffeners and frames become cracked, and due to its location oil becomes ingested into the composite materials, creating additional challenges when repairing this unit.
The tail cone was completely disassembled in order to make the necessary repairs and to replace the inner skin stiffeners. Cracked inner skin stiffeners are carefully removed so as to not damage the graphite skin. Layers of carbon graphite pre-preg material are laid-up on a bond mold the shape of the stiffener and heat cured. After trimming the replacement stiffeners to size they were bonded in place on the inner skin of the tail cone.
When asked what the most complicated repair the shop accomplishes, the answer wasn’t a carbon graphite component but the trapezoid panels on the MD-10 and MD-11 aircraft. The trap-panels as they are referred to are fastened to the aircraft on the upper surface of the wings near the wing root area. They are made using thin aluminum skins, aluminum doublers and frame, and fiberglass honeycomb core. The panels have compound contours with various angles and shapes. Water ingestion eventually causes corrosion of the internal bond-lines and delamination occurs.
Generally, all new skins and doublers are fabricated and the original framework which contains the contours and curves is reused. All these detailed parts are carefully cleaned, etched, bond primer is applied, and assembled with film adhesive onto the bond mold or fixture to hold the shape. These particular panels are then vacuum-bagged, the vacuum bag being the clamp affect, and the adhesive is cured using a specific time and temperature curing profile. Once cured, the panels require a fair amount of cleanup, sanding, grinding, fitting, and eventually reinstalled on the aircraft and painted.
Chavez explains the training program for new entrants into the composite shop. “We have a program of both on-the-job training (OJT), computer-based training, and classroom training. This includes being paired with a mentor. When someone new comes into the composite shop they first begin with paint. Everyone learns to paint. Part of this training is detecting the different types of paint used, what is normal paint wear and tear versus poor adhesion or paint quality problems. This also includes paint mixing and preparation, use of fillers, sanding, and paint removing — the entire paint process.”
Next, new AMTs learn standard fiberglass repair using both wet-layup and eventually heat-cured pre-impregnated materials. Then they move on to learn metal-to-metal bonded repairs, followed by metal-to-fiberglass bonding. Finally, after successfully making their way through these processes, they will learn advanced composite repairs using materials such as carbon graphite. During this time they learn how to use hot-bond control consoles, heat blanket selection and use, vacuum bagging techniques, and much more.
The entire OJT program for the composite shop takes nine to 10 months. Part of the monthly crew meeting agenda in the composite shop is “lessons learned.” Discussions take place using maintenance manuals, photos, and other job-aids in an effort to mistake-proof repair processes and learn from errors.
FedEx core philosophy
When it comes to hiring technicians FedEx is very selective about hiring the best people. Steve Sobczak manages the composite, sheetmetal, welding, and tooling support shops at LAX. As an example, Sobczak explains they recently had openings for five sheetmetal technicians at the LAX location. Sobczak says, “So far we have 125 resumes, we will select 15 to interview, and hire five. And hiring the best people goes way beyond a candidate having great technical skills alone.”
With all of the options available these days for having specialized repair shops accomplish this type of work, I asked the question why continue to develop and invest in the new composite shop instead of outsourcing to one of the many specialized repair stations authorized and capable of this type of work.
AMG Flite Composites Receives FAA Certification of its Aircraft Structures and Composites Repair Center
The 30,000-square-foot facility, located near Dallas Love Field, allows AMG Flite Components to offer full in-house composite and sheet metal repairs and metal bonding on all ATA systems for...
Airframe Technology Composites Safe handling of modern methods By Bill Brinkley July 2004 Composites have become a way of life in aviation, and the advances made possible by...