Composites continue to make inroads on airframe and engine components, but for MROs in the composite structure repair business, managing the exotic carbon-fiber materials and adhesives can be a logistical nightmare. This is because, unlike metal, shelf lives are extremely limited, especially for the bonding agents used in composite repairs. Varying material specifications make multiple repair applications rare, driving costs up. For MROs, this creates a delicate balancing act between having enough material in stock and ensuring the materials’ shelf lives do not expire.
“It is extremely difficult to predict the workload, and which materials to order—even on a historical basis,” says Leigh Sargent, founder and president of Applied Composites Engineering (ACE), an Indianapolis-based composite component repair specialist. “Since most of the repairs to composite components are due to damage, rather than life limits, you can’t plan ahead as to what your material needs will be.”
In the composites world, there are two principal materials, Sargent explains. One type, commonly referred to as “pre-preg,” is impregnated with chemical resins that hold the fibers together; it is generally used for major repairs on large composite structures. Then there is “dry cloth,” commonly used for minor repairs, to which the technician must apply the resin. It’s the pre-preg materials—and the adhesives—where shelf-life issues come into play.
“Pre-preg materials have about a one-year shelf life, and the manufacturers—the ‘prime’ suppliers—normally require large minimum orders—anywhere from $5,000 to $25,000 worth of material,” Sargent reports.
The alternative is to order from the so-called secondary suppliers. According to Sargent, there are about five to six companies globally that buy large quantities from the manufacturers and then resell the material in smaller lots to shops, as needed.
“They assume the risk involved with storage and shelf life,” he points out. “For the MRO, it saves the high cost of minimum purchases from the primes, and minimizes shelf-life expiration waste.”
For those looking to the composite manufacturers as their main supplier, Sargent sounds a cautionary note, especially with regard to pre-pregs. “The majority of materials produced by the primes are going to the aircraft OEMs, because they buy in such large quantities and do not want to stock materials. The result is that the MRO is forced to wait until the prime does a scheduled production run, which could be months after an order is placed.”
Mike McBride, director, component maintenance and process and technology engineering at Delta TechOps in Atlanta, stresses the need for MROs to align themselves with composite material suppliers specializing in bulk distribution. But he warns that the customers of these distributors have to be on a schedule—and know their business—because obtaining supply over and above their forecast might be difficult, especially if unexpected shortages happen. “For example, we’ve seen cases where a batch of adhesive fails the manufacturer’s testing and suddenly the whole industry is scrambling to find a solution.”
McBride says this is why purchasing schedules must be a step ahead of maintenance schedules, and the best suppliers are ones that allow demand-based adjustments. Reliance on historical data will help—but only up to a point. “While historical data might provide an idea concerning how often a particular repair has been performed on a particular aircraft type and component over the years, you have to factor in unpredictable damage such as a bird strike. Ultimately, it’s a crapshoot.”
McBride’s point was underscored by Ben Clark, senior vice president and general manager at Worthington MRO Center, a nacelle repair specialist in Tulsa, Oklahoma.
According to Clark, “a very large portion” of a typical MRO’s volume stems from unexpected removals of a very diverse set of products. “Simply put, there is a considerable amount of material requirements that are difficult to plan for,” he says. “It could be that a unit [has sustained] what is considered unusual damage, and consequently the company may not stock the materials needed to repair it.”
And therein lies the root of the problem, says Clark, who explains that composite products that do not cross over to repair functions performed even semi-regularly are not feasible to stock. For example, he says, it is very common for MROs to use some composite materials only once every 18 months for products they repair on a monthly basis. “While they are performing maintenance on these products regularly, they are not performing the same repairs every time.”
Clark agrees that if composite materials required for non-routine repairs have a limited shelf life, it is more difficult to stock those materials because they are consistently thrown away before they are used up. One possible justification to warrant consistent stocking of those materials—and mitigate waste—is very high repair volume. “It is not a matter of planning a larger budget for inventory due to the shelf-life constraints,” he stresses. “This will only result in additional waste and higher operating costs.”
To ensure that composite materials are available when needed, Clark advises MROs to “work with the airlines for hard time removals of components,” and plan repair and overhaul activities many months in advance. “Successful companies will have constant conversations with their customers to have a complete understanding of current trends in their fleet maintenance and be proactive in their planning as well,” he says.
Nacelle specialist Aircelle uses its data center’s fleet trends to help forecast demand. “That enables us to (monitor) the events that happen on a particular product and know the type of damage that occurs, how often it happens, and therefore what the raw material needs will be,” says Jean-Philippe Gremont, aftermarket technical director for the company. “At the same time, if we have a special support contract with a specific airline, then the unique needs of that customer are factored into the material planning,” he notes. Materials planning, explains Gremont, is also closely related to the scheduling of its customers’ maintenance events. For example, Aircelle’s Preventative Maintenance Program (PMP) works with its customers to anticipate possible nacelle maintenance that can be done when the airplane is down for other work.
MROs also need to be cognizant of new technology in composites.“You have to position yourself to be ready for changes, not only with the materials, but the materials standards and processes,” Delta TechOps’s McBride points out. An example is the perforated graphite skin used on newer 737 thrust-reverser panels. “It’s a cool new technology incorporated in the manufacturing process, which we have no way to replicate, so we wind up falling back on the older-technology solid panels,” he says. “Airlines and MROs will have to have the flexibility to adapt quickly to the new technology and, in many cases, make investments in tooling to ensure they have the capability to make the required repairs. It’s a trickle now, but the wave is coming.”
One aspect of the new technology is greater damage-tolerance. “As reliability of composites increases, it will mean more on-wing repairs and ultimately less material waste,” says Gremont. “As an OEM, we are always trying to identify a balance between performance and direct operating costs.”