Protecting the Beauty Beneath the Skin

Using hybrids of glass and aramid fibers, Fast Inc.’s repairs fix moisture damage in composite components and guard against its recurrence.

Many variables affect the use of composite materials in an aircraft: the purpose and position of a component, its structural or secondary load requirements, the heat, impact forces, and pressure it will be exposed to during operations, and of course cost. To accommodate those variables, a single aircraft often uses multiple reinforced thermoset or thermoplastic material systems.

That variety obviously affects the maintenance and repair of composites-laden aircraft, and often not for the better. Advances in the use and support of composite materials are made almost on a weekly basis. But according to Kevin Fornall, founder and president of the composite repair shop Fast Inc., "we still find a fair amount of corporate, commuter, and general aviation aircraft that exhibit less than optimum composite material choices, design, construction, and often very limited repair data in terms of scope and detail."

Documentation for some of these aircraft only make reference to the rudimentary fiber glass repair schemes outlined in old versions of FAA Advisory Circular 43.13. "It is sometimes frustrating," Fornall said, "to find little or no help in the available maintenance data for the aircraft."

Some structural repair manuals allow for, or even require, an interply "hybrid" repair, in which a single repair combines several different reinforcing elements in the ply layup to capitalize on the best traits of each material. Such repairs are common at Fast Inc., which operates from a 15,000-square-foot facility at the Jefferson County Airport in Broomfield, Colorado, near Denver. In business since 1994, the company maintains advanced composite products on more than 50 different aircraft. Among the aircraft models on which the hybrid repairs are working well are Piper Cheyenne Series II, III, IIIA, and 400LS and on Cessna Citation aircraft.

At least once a month, Fast Inc.’s staff sees aircraft come into the facility with surface cracking on big fairings and panel assemblies that are made largely of aramid composites. (These often are referred to as Kevlar, the trademark of an aramid product made by DuPont.) That cracking presents a problem.

"Aramid fibers are like a sponge," Fornall said, and the cracking allows moisture to be sucked up by the woven fibers. When the outside air temperature drops below freezing, the moisture freezes and expands. This cracks and chips away at the resin matrix holding the fabric together, which leaves exposed areas of composite fabric that appear too dry, or void of resin. "The freeze/thaw cycling, along with wind erosion, and the incredible flexibility of aramid, spell disaster," he said.

The problem is compounded by the fact that many of the aircraft with aramid fabric and fiber glass components are more than 15 years od.

"Over time, moisture can greatly contribute to the degree of degradation we observe," said Craig Colby, general manager at Stevens Aviation’s facility at Jeffco Airport and a customer of Fast Inc. (Aviation Maintenance, November 2002, page 34).

He agreed with Fornall that keeping moisture from getting into composite structures is critical. "Moisture damage in aramid fabric–and all composites for that matter–is a common occurrence," he said, particularly in wing-to-body and wing tip fairings, dorsal and ventral fins, wing locker boxes, vertical stabilizer top fairings, elevator tip fairings, and fuselage tail cone assemblies.

"Aramid composites have some outstanding properties, such as great flexibility, impact resistance, and superior strength," Fornall said. "These very properties can make it hard to repair and seriously challenge current paint systems being used on aircraft." Meeting that challenge requires taming the aramid fabric and providing transitional materials that complement it.

OEMs with a good understanding of the properties of composites often propose repairs that combine fiber glass plies with aramid ones or add fiber glass on the inner and outer surface of the aramid fabric. When such repair data is given, Fornall said, Fast Inc. orients the ply stack-up with it; otherwise, plies are matched to the existing part. When the repair data includes no limitations on adding fiber glass, "we take full advantage of the improvements gained by its use," namely a surface that resists cracking when flexed and helps paint better shield the aramid from moisture damage.

‘Less is Best’

Fiber glass is not a substitute for Kevlar, he said. "We never substitute any composite material system for another without obtaining approval from the OEM. Where repair specs don’t exist, we use a designated engineering representative to review the repair and approve the use of supplemental plies of fiber glass."

On wing-to-body fairings and other aramid composite structures that experience extreme surface cracking, Fast Inc. workers typically remove all top-coat finishes to expose the base material. It is not uncommon to remove as much as a quarter inch of old paint and filler, all of which is subject to shrinkage and brittleness. "Some paint shops produce very smooth blends and contouring of composite surfaces using fillers," Fornall said. "But our advice for such fillers is ‘less is best.’"

Removing these fillers and applying fiber glass plies lets Fast Inc. cut weight, improve strength, and reduce cracking of the finish coats–eliminating 95-plus percent of surface deterioration of aramid components. "Leaving structurally-weak fillers to support the paint finish can mean the fillers will crack on the first flight when the fairing flexes," Fornall said. "So in a short period of time, the urethane paint also cracks, and you start all over again. The repair techniques we use allow us to offer reasonable warranties regarding paint finishes."

During the topcoat removal, technicians must take care not to damage the base material. Aramid fiber can fuzz when sanded. The time to complete layup, cure, and final finish are dictated by the size and complexity of the component being repaired, Fornall said. Repairing a Citation dorsal assembly can take up to 80 hours, not including removal and reinstallation. But even adding R&R labor costs, Fornall said, repair costs would still be at or below the cost of a replacement part, even before R&R labor is considered for that replacement. By contrast, a simple Cheyenne wing-to-body fairing can be removed, repaired, and reinstalled in about eight hours.

Fast Inc. workers typically wet layup interply hybrid repairs, Fornall said. They preimpregnate the fiber glass to remove excess resin, then vacuum-bag the repairs to further control resin bleed, and post-cure in an oven to assure complete cure and maximum strength of the repair. The company makes many of its own repair tools .

Depending on the aircraft model and component involved, repairs can cost nearly as much as a replacement part, if one is available, Fornall said. "In most cases, and again, depending upon the condition of the parts, we can offer repair costs between 40 and 70 percent of replacement price. The biggest long-term benefit from these interply hybrid repairs is the stabilization of the aramid and protection from moisture."

Often, he said, Fast Inc. workers find previously undetected damage in an aramid composite component brought for repair. "We have customers who are reworking such parts on as many of their clients’ aircraft as possible," he said. "We work with the mechanic, pilot, or repair facility to help the aircraft owner understand the problems with these composites, and plan for possible solutions."

Fast Inc.’s personnel and managers are "very willing to get involved in our evaluation of a customer’s aircraft," Stevens’s Colby said, and "to help explain the special care and maintenance of composite structure." While it may take several visits to complete the repairs, he said, "in the end, the condition and value of the aircraft is steadily increased." Paying attention to composite structure is critical, Fornall said.

"The difference in going three years or seven years between paint jobs can come down to how you treat the composite products on your aircraft," he said. Adhere to the maintenance manual, let the original equipment manufacturer know if chronic problems occur, and get advice from composite experts regarding repair, especially for moisture damage. "Don’t be fooled into thinking new finishes over a repair will always solve your paint finish problems. In the case of composites, beauty is not only skin deep."

Contact:
Fast Inc.
11855 Airport Way, Unit C
Broomfield, CO 80021
Phone: 303-439-9456
888-747-3278
Fax: 303-439-9453
Email: iifastinc@aol.com