mr-fieldamericanairlinespromo.jpg.crop_display.jpg American Airlines
Photo from AA Tech Ops Center, Tulsa, Ok

Composite Structural Repairs At Field Level Evolve

Airframes with higher composite content need aircraft-on-ground solutions.

When a severe hailstorm hit the Dallas area in 2013, American Airlines took a major hit. In less than 1 hr., 103 of its aircraft, including McDonnell Douglas MD-80s and Boeing 737s, 757s and 767s, sustained damage at the carrier’s huge Dallas/Fort Worth International Airport hub. The storm not only affected metal fuselages and wings but also composite panels and flight controls.

“We took about 40 mechanics and a team of about 10 composite repair specialists to Dallas to inspect the damage,” says Kevin Ferrin, technical supervisor for aircraft maintenance at American’s Tulsa, Oklahoma, MRO complex. “We did temporary repairs to the composite panels where we could, and replaced some flight controls where the damage was too extensive. In some cases, we removed the composite flight controls and composite panels, which were then sent to our composite shop in Tulsa for repair and return to the aircraft.”

Ferrin says some of the temporary repairs were approved by American’s engineering staff, and some by Boeing, for continued use of the airplanes in service until their next C-check. About 75% of the affected aircraft were back in revenue service within 24 hr., although five were grounded for about 10 days due to the extent of the damage or the need to wait for repaired parts.

Composite structure field repairs “present all options including on-site work on the damaged area, change-outs, and in some cases, ferry flights to a maintenance facility,” Ferrin emphasizes. In cases of change-outs, American maintains inventories of spare composite structures, primarily at its “Class One” (largest) hub stations at Dallas/Fort Worth, Miami, and Chicago-O’Hare, he says.

“American is very fortunate to have a well-trained composite maintenance group, but with deliveries of our new 787s, there will [at some point] be repairs that even we haven’t done,” he says. “This will require us to learn new repair concepts.”

However, because the carrier’s engineering group started preparations for the 787 a year before it went into service at American in 2015, the airline is “well-prepared” to handle in-house much of the potential damage that could occur. “Any extensive damage to the fuselage may still require bringing in Boeing to accomplish the repairs,” Ferrin says.

American has 42 787s on order, including 20 787-8s and 22 787-9s.

When it comes to composite repairs in the field, airlines “could be presented with a huge logistical problem,” says Jim Epperson, senior manager, global customer support services for Spirit AeroSystems. The Wichita-based company is a supplier of complete composite nacelles, nacelle panels and composite thrust reversers used on Boeing airliners and more recently the nacelles for the new Gulfstream G650 business jet.

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As Epperson explains, getting the materials and repair equipment to the aircraft is one of the biggest challenges maintenance technicians face. This is complicated further by the fact that the materials have a limited shelf life—up to about one year for prepreg materials that in many cases require cold storage at temperatures ranging from 0F to -20F. 

“When the material needs to be used, you have to take it out of the freezer, thaw it, cut the quantity you need, rebag that and transport it to the aircraft—keeping in mind that the material will have a limited life at room temperature,” he says. “If the material has to be shipped to an international destination, it could be delayed at customs, which could lead to spoilage prior to customs clearance.”

Epperson also points out that there are more mechanics trained to repair metal structures than those qualified to do the hot bonding required for composites. “And when you also add in the need to do nondestructive testing as part of the repair process, that takes another skill set,” he says.

Spirit AeroSystems addresses these issues by positioning nacelles and nacelle components at strategic global locations, and by dispatching aircraft-on-ground teams. But as Epperson notes, there are times when an interim repair may still be required in order to ferry the airplane to where more permanent, complex repairs can be done.

“A good example of an interim repair is a bolted-on sheet metal patch. Although we normally don’t like to do this, because when you bolt metal onto a composite structure, it does additional damage,” he explains. “However, another repair method we could use in preparation for a ferry flight is a wet layup, which uses an epoxy material mix and a dry fiberglass or graphite patch that is layered on. That is allowed to cure for a couple of hours, at room temperature—up to temperatures [as high as] 150F. It’s a quick way to do a patch for a ferry flight that doesn’t involve hot bonding, which takes about 5 hr. with temperatures of 250F.”

Fortunately, the trend is toward repairs in place. Henrik Schmutzler, an innovation engineer at Lufthansa Technik in Germany, reports the company is offering “an increasing number of mobile repair solutions” for composite structures. “Most complex repairs are being developed in a manner [so they can]be conducted onsite in the future,” he says. “For example, infusion as substitution for wet layups offers significant advantages for in-field repairs. Furthermore, we are introducing a mobile scarfing system that will enter service in 2016.”

Ed Montalvo, an aircraft maintenance supervisor at Southwest Airlines in Dallas, says the carrier has a number of options available for field-level composite repairs, as specified in the Boeing Structural Repair Manual (SRM), for its all-737 fleet.

“We have been able to perform [most of] the repairs without taking the aircraft out of service or delaying it for a significant period of time,” he says. “If we find that the problem has not been addressed in the SRM, Southwest’s Structures Engineering Group, which is well-versed in composite repair issues, will design a repair that will be sent to Boeing for approval.”

However, as Montalvo points out, most composite repairs resulting from damage in the field are “carryovers,” which means the damage is so minor repairs can be carried over to the next scheduled maintenance event. But if the damage is more severe and cannot wait for a scheduled check, the repair has to be done in the field. This, he says, mandates a clean working environment.

“In the field, even when a hangar is not available, you have to make the extra effort to create a clean working environment, according to what the OEM’s SRM dictates,” says Montalvo. “In some cases, it means you have to improvise some kind of shelter, which our technicians have had to do at times. One way this has been done is to create a tarp using plastic sheeting, which can be positioned over the damaged area, once it has been cleaned.”

However, even with a tarp in place, ambient temperatures could preclude an outdoor repair. According to Montalvo, if outside temperatures are less than 50F, a heated area is mandated. “Under cold-air conditions, you have to have a hangar because it becomes difficult for a hot bonding machine to generate the temperatures—200-350F—to carry out the repair.”

Montalvo cites continuing developments at Boeing in the direction of new composite repair schemes that will enable operators to get aircraft back into service faster. “We would like to see more composite repair that can be accomplished during an overnight check—within 8 hr. or less,” he says.

In fact, Epperson predicts composites incorporating even greater damage tolerance will make their way into large airframe structures, such as wings and fuselages. However, he cautions that repair-process requirements will likely become much more stringent, given the more complex material formulations that will be involved.

“There will be increased requirements associated with analyzing the damage location and the criticality of the condition of the support substructure at that location on the aircraft,” he says. “The problem is, most airlines today do not have the composite material design allowables data—often proprietary—to be able to proceed with an analysis of their own. That will drive the airlines back to the OEM for approval of the repair size and repair approach methodology.” 

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