In-situ composite repair builds on basics

The goal of any commercial aircraft maintenance and repair crew is the fastest safe return of the aircraft to service — especially in view of the fact that the cost in lost revenue of an unscheduled AOG (aircraft on the ground) is, on average, $100,000 per day. No wonder, then, that in the run-up to the launches of the Airbus A380 and Boeing 787, much has been made of airline and maintenance and repair organization (MRO) concern about how aircraft composites would be repaired. Considering that the average permanent composite repair, as permitted in Structural Repair Manuals (SRMs), takes roughly 15 hours, according to SAE’s Commercial Airline Composite Repair Committee (CACRC), in-situ composite repairs performed at the flight line can cause flight delays and cancellations. It’s a dilemma made more challenging by fast gate turnarounds — between 30 and 60 minutes for domestic flights — and an overwhelming lack of line mechanics with specialized composites training. But aircraft OEMs, airlines and MROs are crafting strategies to minimize ground time, simplify repair regimes and make composite materials and processes accessible to technicians who are more accustomed to working with metals.

Repair readiness

According to Mark Loyd, lead engineer of composites, plastics and transparencies at American Airlines’ Composite Repair Center in Tulsa, Okla., one of the chief obstacles is logistics. “Our in-situ problems are like real estate,” he muses. “Everything is location, location, location — location of the airplane, location of the people and equipment, location of the repair material and location of the damage. We have to weigh flying a crew, materials and equipment to a station to do a repair or obtaining a spare part and replacing it, or ferry flying the airplane to a maintenance base for part replacement.”

Access, equipment and the ability to handle the part are critical, explains Loyd. “Obviously, the shop environment is much better equipped, with better vacuum systems, grind booths, constant air supply and shop supplies like bagging film, sealant tapes, etc.,” he adds. On the flight line, however, equipment and processes are strictly controlled to guard against potential fire hazards created by the presence of fueled aircraft (for more on flight line curing options, see the sidebar, below). “The properly executed in-situ repair requires far more advanced planning,” says Loyd. “Once deployed for the repair, you can’t afford wasted time for resupply.”

Long-range tactics for dealing with un-scheduled composite repair include measures designed to identify potential defects before they require  in-situ repair, during scheduled inspection stops. A variety of advanced nondestructive inspection (NDI) technologies and onboard structural health monitoring techniques are or soon will be available. These will permit airline MROs to move away from a find-and-fix mentality to one of a predict-and-manage mindset (see “Learn More,” at right). These technologies certainly will play a large future role in minimizing the need for in-situ repair. But today, the necessary art of in-situ repair relies heavily on a foundation of traditional composites and metal repair techniques and the experience and training of a handful of composites specialists — most of whom are stationed at major repair depots.

Unscheduled repairs

The most disruptive repair situation, in terms of downtime, is the unscheduled repair. Impact damage, hail damage, and moisture ingress/intrusion are the most common types of damage to composite structures that cause an aircraft to be taken abruptly out of service. “Operators all face the same issues regarding repair materials, spare part inventory and equipment location,” says Loyd. Most repair materials have a finite shelf life, and MROs perform a balancing act to keep adequate repair materials on hand without having extremely high material waste. Each operator must forecast material use well in advance because “currently, lead times of more than 20 weeks are not uncommon,” says Loyd. “Operators with multiple fleet types often face the challenge of stocking multiple types of the same material, such as three different types of 350°F/177°C carbon fabric prepreg.”

Currently, the predominant in-service composite aircraft components are sandwich-structure control surfaces cored with honeycomb or foam. In general, the repair techniques used on them have been around since composites were first used in aircraft. “We’ve not experienced any ‘game-changing’ repair materials or techniques in quite a while,” says Loyd. “The OEMs are allowing increased use of elevated-temperature wet layup repairs, but the repair methodologies used today are very close to what they were a decade ago.”

The vast majority of repairs to composite structures carried out by aircraft line maintenance technicians are low-temperature (room temperature to 150°F/66°C) wet layups, which are typically temporary or time-limited repairs. These technicians, who are typically generalists with minimal training in composites repair, must diagnose and correct both minor and major aircraft damage at the gate. A room-temperature wet layup repair requires no disassembly and no external heat sources, such as hot-bonding equipment, which would require more extensive training to properly operate and can be hazardous when used on a fueled aircraft.
For permanent repairs, wet layup bonded repairs must be cured at 180°F/82°C to 200°F/93°C. Prepreg repairs, which can be used to repair thinner sandwich panels as well as the thick solid laminates common to the load-bearing structures on both the Boeing 787 and Airbus A380, cure at temperatures ranging from 250°F/121°C to 350°F/177°C and are generally left to composite specialists at major repair stations (see “Learn More”).

Repair options for thicker laminates

Boeing has spent considerable effort attempting to make the 787’s solid laminates less susceptible to the dents and dings associated with hail damage, tool drops and jetway impacts that plague thinner composite structures. The company has performed extensive research in damage probability, calculating energies associated with everything from the corner of a toolbox striking an upward facing structure to the impact of a jetway on a cargo or passenger entry door. “We’ve tried to correlate the damage that we see today and the repairs that are required and translate that into impact energy,” explains Justin Hale, Boeing’s chief mechanic for the 787 program. “The result has been new sizing for upward facing structures and structures around doors to make the airplane more resistant to damage.”

Damage will occur, however, and when it does, the first step is to assess if and how much internal damage there is within the laminate, which will require ultrasonic inspection, says Hale. Yet, when a small dent is noticed during visual inspection at the gate, performing ultrasonic testing can be time consuming, and will require a technician with specialized training. “When you have a small impact damage, the SRM will provide allowable damage limits, based on the dent size,” explains Hale. “But if you can characterize what’s going on inside that structure by assessing delamination, these limits are more generous in allowing you to dispatch the airplane.” To aid in this process, Boeing developed the Ramp Damage Checker — a simple “go/no go” handheld device (similar to a stud finder) that identifies the presence of delamination. “It’s not intended to provide detailed information, such as depth, but it can confirm the presence of delamination,” says Hale. “We en-vision it being used in the field by a maintenance lead or inspector — not necessarily an ultrasonic specialist.” The first commercial Ramp Damage Checker is now available from Olympus NDT (Waltham, Mass.). Other brands will follow.

For damage repair, Boeing has specified a variety of options in the 787 SRM: traditional bonded repairs; the company’s patented quick composite repair technique; and conventional bolted repair.

Bonded repair of a solid laminate panel is similar to that for a sandwich structure. “These are very traditional vacuum-debulked bonded repairs,” stresses Hale, which include 250°F/121°C to 350°F/177°C prepreg repairs as well as 200°F/93°C wet layup repairs. Repair times may increase when multiple cure cycles are needed for thicker structures or if extensive repair is necessary. The general rule of thumb for scarfing is to remove 0.5 inch/13 mm per ply. “Ply thickness varies depending on the materials used in the original structure, but for carbon fiber unidirectional tape, it can be as thin as 0.13 mm/0.005 inch,” says Michael Hoke, president of Abaris Training Resources Inc. (Reno, Nev.). “A 0.375-inch/9.525-mm thick structure could be approximately 75 plies thick.” By the standard rule of thumb, this would translate to a scarf of ~37.5 inches/~952.5 mm.

“Scarf distance is measured not from the center of the damage but from the edge of the cleaned-up damaged area,” explains Hoke. “So if the damaged structure has a 6-inch/152-mm dia-meter hole in it after the damage is removed, then the outer diameter of the scarfed area would be 37.5 inches/952.5 mm plus 6 inches/152 mm plus 37.5 inches/952.5 mm, for a total diameter of 81 inches/2m.”

“Such a large scarf would be very time consuming,” says Hoke. “On a structure this thick, a bolted doubler repair, if allowed, would be much quicker and generally easier to perform. However, there may be aerodynamic reasons, concerns about damaging the underlying structure during drilling, or radar signature reasons in military aircraft, which would require a scarfed repair,” he adds.

“Of course, the 0.5-inch-per-ply scarf ratio is not cast in concrete,” adds Hoke. “Engineering analysis might support a different scarf ratio, such as 0.25 inch/6.35 mm per ply. If so, then the total diameter of the repair would be 43.5 inches/1.1m, which is still large but an easier repair to perform.”

With a smaller scarf, less undamaged material is removed but the area of the adhesive bond, which transfers load through the repair plies, also is reduced. “There are many tradeoffs in repair design,” says Hoke. “The scarf angle is only one of many such tradeoffs that need to be carefully evaluated by a qualified repair design engineer.”

Boeing has developed new debulking techniques designed to reduce cycle times for thicker repairs. Also, in an effort to save time and money, Boeing has qualified common repair materials throughout the 787. “Because we had multiple partners designing different parts of the airplane, the potential for a variety of repair materials was great,” explains Hale. “We wanted this variety to be transparent to the mechanic, technician and airline engineers working a repair. So even in areas where we used a different carbon fiber/epoxy specification, we qualified a common repair material.”

The quick composite repair is not permanent, but rather a “Band-Aid” designed to get a damaged airplane back into service quickly. “The precured composite patch is epoxy bonded onto the outside of the airplane over the damaged area,” explains Hale. “It restores enough residual strength into the damaged area to provide revenue service on a temporary basis.” The process allows small-area damage to be repaired in less than an hour. The adhesive is cured by relatively low temperatures provided by a chemical heat pack, which eliminates the need to dry the part, and is designed to be applied at the gate if necessary. “This is very important if you don’t have widespread repair capability throughout your system,” adds Hale.

Bolted repair. The same types of mechanically fastened repairs — doublers, scab patch, flush, etc. — that are performed on metal airplanes have been included in the SRM for the 787. In a bolted repair, a cover plate is mechanically fastened around the damaged area. Although fasteners create stress concentrations that can degrade the performance of the parent structure, bolted repair of composites has been service proven on Boeing’s 777.

“These are damage-tolerant, permanent repairs,” explains Hale. Typically these repairs would be carried out using a titanium sheet, although Boeing has also successfully tested carbon fiber patch materials and is specifying aluminum as an additional option. “If you are not using titanium or carbon fiber, you must take additional steps to protect the galvanic coupling between an aluminum patch and the carbon fiber skin,” cautions Hale. Because an aluminum patch also would require periodic inspection for corrosion, its use would most likely be temporary, in which case, paint scheme and fay surface sealant protection on the aluminum part would be adequate to protect against galvanic corrosion, adds Hale. Titanium fasteners, however, are required in all cases.

“At the highest level, the bolted repair for a composite aircraft is the same as that on a metal aircraft,” says Hale. “It’s the same process, same skills, and, in general, the same tools.” In the details, however, there are some differences. “For instance, when you drill into a composite structure, you use the same drill motor but a different drill bit, and you have to adjust speed and pressure,” explains Hale. “You also have to be aware of things like fiber breakout on the backside.”

“There is certainly a need for training to understand a few minor differences, but, in general, a line technician or mechanic who performs bolted repairs on a metal aircraft today will very easily transition into bolted repairs on a composite structure,” stresses Hale.

“Generally, the thickest laminates would be in areas of heavier loads, such as cargo door surround structure,” explains Loyd. “When these areas are damaged on a metal aircraft, it generally requires a stacked doubler repair and significant teardown and re-assembly. The composite repair to the same areas will have a different work scope, but we don’t anticipate the downtime to be significantly different,” he adds.

Bonded vs. bolted

“The choice between a bonded and a bolted repair may come down to how much time you have available to do the repair,” posits Hoke. “The huge advantage of the bolted repair is there is no heat required,” he adds. Although a bolted repair can impact the aerodynamics and radar signature of the aircraft, in Boeing’s view, flush bolted repairs are nearly equivalent to bonded repairs in terms of aerodynamics and cosmetics. Further, when a metal/composite stack-up is drilled, residual metal chips can damage composite holes. Also, dull bits or incorrect drill speeds can burn composites, and proper steps must be followed to avoid hole misalignment.
Generally, bonded patches provide more efficient load transfer than bolted repairs and are more attractive from an aerodynamic and cosmetic standpoint. And, as with bolted repairs, the quality of a bonded repair depends on many variables: age and quality of materials, surface preparation, and successful adhesion. In essence, the success of either a bonded or bolted repair relies heavily on the skill of the technician.
“With ... all-composite aircraft, the biggest need will be transitioning the workforce from the metal aircraft repair philosophies to the composite repair philosophy,” says Loyd. “This will be a paradigm shift akin to switching from propeller-driven aircraft to jet power.” Without doubt, composites repair technicians also will need some form of certification or license. Industry steering groups, such as the CACRC, already are pushing for a Composite Materials License to establish an experience baseline for maintenance technicians, says Loyd. Additionally, the Professional Aviation Maintenance Assn. (PAMA) and the SAE Institute are currently developing technical certifications for the repair of aerospace composite materials.