Aircraft Composite Repairby Duane Sipe | March 30, 2021

A brief look at the history and growth of composite materials and how training and technology has evolved. We talk to an OEM about radome repair and look at the latest equipment and hot bonding technology needed for more complex composite structures.

Although there were a few earlier instances of composite materials used in aircraft construction, one of the most recognized historical aircraft that incorporated composites in its design is the Howard Hughes H-4 Hercules, nicknamed the Spruce Goose. The airframe was made entirely of wood. Duramold, a material made of layers of birch ply and phenolic resin, was the primary component of its construction. The eight-engine behemoth made its maiden, but short, one mile flight on November 2, 1947. It never flew again.

Aircraft composite materials have advanced beyond wood into fibers made of glass, carbon, or boron. Fiberglass, or Fiber Reinforced Plastic (FRP) made its aviation debut about 1940, but really came into its own in the 1960s.

Since then, the trend to increased non-metallic composite construction in commercially manufactured civilian aircraft has maintained an upward trajectory. Today, Carbon Fiber Reinforced Polymer (CFRP) is widely used.

Here are some of the highlights of this transition.

1970s:

In 1976, Kaman Corporation designed and began manufacturing the world’s first production all-composite rotor blade for the Bell AH-1 Cobra helicopter.

Boeing conducted a nearly $9 million research program in 1979 to study whether an advanced composite elevator could be made for the cargo version of the Boeing 727. With a 26 percent reduction in weight, and an overall reduction of over 40 percent in the number of parts, and zero negative effects to the flight characteristics, the test proved successful.

1980s:

January of 1981 saw the introduction of the Bell Textron Model 412 helicopter, a derivative of the 212 except with a four-blade rotor system with composite blades.

Heatcon Composite Systems, Inc, a global manufacturer and supplier of aerospace composite repair equipment launched in 1981.

In 1983 Abaris Training, a leading advanced composite training facility, opened its doors.

1990s:

1995 witnessed Cirrus Aircraft introduce the SR-20, the first commercially produced, all composite light aircraft.

Due to the advancement and increasing use of various composite materials, the current version of Advisory Circular (AC) 43.13-1B (September 8, 1998) Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair, has dedicated nearly three times the number of pages to composite repair over the prior version (-1A, 1988). The current publication includes figures, graphics and techniques for repair made to the newer non-metal materials.

2000s:

The Airbus 380, with 22 percent of its weight attributed to composites, was debuted on January 18, 2005.

2010s:

On August 3, 2011, the FAA published AC 65-33 for the first time; Development of Training/Qualification and Certification Programs for Composite Maintenance Technicians. The first and only revision (-33A) was published June 27, 2017.

Boeing’s first 787 Dreamliner entered service on September 25, 2011. Today, mostly carbon-based composite materials make up 50 percent of its total weight.

The Airbus 350 entered service in 2015 with 53 percent of its weight also attributed to carbon composites.

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Composites have penetrated many areas of aircraft design, including:

• Rotor blades
• Fairings
• Radomes
• Cowlings
• Flight control surfaces
• Fan blades
• Propeller blades
• Stabilizers
• Wings
• Fuselages

It safely can be said that virtually every category and class of aircraft have evolved with an initial or increased use of composite materials.

During the overall expansion of the use of composites over the last ten or fifteen years there has also been a great number of improvements in the methodologies, materials, and equipment to make higher quality repairs.

Automated equipment to better control the heat/curing process, introduction of lightning strike materials, and the replacement of the step repair by the more efficient scarf repair, to name a few.

Also critical to the advancement was the ability to be able to detect hidden defects or damage within the layers of the composite materials. Beyond the fundamental tap test, aircraft composite inspection technologies have been developed making use of:

• Ultrasonic testing
• Radiography
• Thermography
• Eddy current testing (if electrically conductive)

The overriding drive was to get away from as many errors that were possible in the repair process.

Aviation Composite Repair: Equipment

A giant leap in aerospace composites happened when aircraft OEM’s began using composites for large critical primary structures. Since then, the repair industry has been focusing on developing technologies to fill the need for more challenging repair requirements.

Standard off-shelf hot bonding equipment that allows for larger area repairs, with more precise control are being produced by leading hot bonder equipment manufacturers such as Heatcon Composite Systems, Inc.

When asked about the current trends, Eric Casterline, President of Heatcon stated, “In the last several years we have seen growing demand for repair equipment capable of supporting larger repairs to more complex aircraft composite structures.” He added, “This has raised the bar in terms of equipment requirements and has led us to develop newer technologies which incorporate advanced control features and non-traditional heating methods such as inductive heating.”

As repair areas expand in size and are on the primary structures where autoclaves are not useable for the repair, tighter control of the bonding temperatures are increasingly critical. Challenge lies in the process when underlying structural components that support the composite components act as a heatsink, drawing heat away from the repair site, leaving a ‘cold zone’.

Boeing Aircraft has patented a self-regulating technology that provides intrinsic thermal control they call Smart Susceptor technology.

This advancement makes use of heat blankets with various metal alloys that will only heat to a desired temperature, because of the nature of the chosen alloy. During the heating process, the area of the repair that has reached the appropriate temperature will not heat up any higher because of the limiting nature of the alloy. But an area with a heat sink behind it, causing a cool spot, will continue to heat to the temperature required. This maintains a very tight uniform temperature across the repair.

Says Eric, “Heatcon is the only company right now producing a hot bonder using this technology.” He added, “The temperature tolerance has been improved by a factor of up to 3X over a standard blanket.”

Heatcon has also developed equipment that can network multiple hot bonders together to allow for the heating of large areas, and to allow for flexibility in applying heat to different areas of structure during repair.

Another interesting development is Heatcon’s ability to build shape-to-fit heat blankets. Customers can provide design data in a variety of formats including: CAD data, dimensioned sketches, molds taken from the part, 3D scan data, or the part itself. From this information, Heatcon is able to create a precisely fitting heat blanket which allows for the uniform application of heat over the whole area of the repair.

Aviation Composite Repair: Training

The expansion of composite use brings with it a growing need for technician certification.

Guidance provided by AC 65-33A assists organizations in certifying their own employees by firstly, understanding aircraft composite construction, and ultimately becoming proficient in aircraft composite repair procedures. The organization may develop their own in-house training personnel, or they may hire outside professionals to provide the training that leads to the subsequent certification.

Outside training providers such as Abaris Training, specialists in training for the repair of aviation composites,offers the pre-certification training. Their programs can be conducted in person at your facility, or theirs. Abaris currently provides 24 separate courses for aerospace composites. These include composite engineering and design, manufacturing, and repair. Abaris also provides onsite consultation.

When asked if the need for training was growing, and if so, why, Louis Dorworth, Direct Services Manager of Abaris, replied, “Oh yeah, absolutely.” Louis elaborated that there is huge growth in repair training needs because of the increasing use and complexity of both the composite structures and the evolving equipment.

Aviation Composite Repair: MRO

Probably one of the most challenged components in aircraft composites is the radome. These tend to take a beating during their service life.

Kaman Corporation, launched in 1945, has been designing and manufacturing components for their own K-MAX helicopter, as well as the military. These include composite rotor blades and composite radomes. Since its beginning, Kaman has been primarily focused on military needs.

Malissa Nesmith, Senior Director of Business Development at Kaman announced that the company recently added radome repair to their FAR Part 145 Repair Station and are planning to add composite rotor blade repair in the future.

Says Malissa, “Historically we’ve had a Part 145 Repair Certificate for a number of years, but we’ve only conducted repairs for military programs. For the first time, we are actually going into the commercial side of the industry rather than just sticking with military support.”

Less than two years ago, Kaman began developing a radome rotables program to service the business aviation sector, focusing on corporate jet aircraft.

She referenced that with the company’s years of experience with radomes, it was just a natural move to provide radome repairs for the commercial aftermarket. They recognized the need to have something for immediate exchange, serving a great advantage for the aircraft operators.

All of Kaman’s repair functions are done in-house and not out in the field. Kaman currently has several units available for exchange. And, says Malissa, “We look to add at least six more models by the end of the year.”

Current MRO facilities may find an increasing need for composite repair knocking on their door and elect to add this service to its Part 145 ratings or capability list. And, even new start-ups, focusing solely on providing composite repair are not out of the realm of possibility. In fact, Louis of Abaris stated, “In the last two or three years, I’ve had a lot of requests from start-up Repair Stations.”

Up and Coming Aircraft Composite Bonding Technologies

The future of composite repair is looking quite exciting.

One of the long-standing areas of ‘unknown’ has been measuring, for certainty, the internal temperature of the repair.

Research is currently being conducted by Abaris and AvPro Technology Inc. in the use of lead-less temperature sensors which are permanently embedded in the repair, right at the bond line. These are tiny 0.25mm X 32.0mm (0.010” X 1.25”) antenna with transmit and receive capabilities. It is known as ThermoPulse^TM Microwire. From the AvPro website we find: Earlier studies with American and European companies showed no significant difference in mechanical properties between samples created with sensors and those without.

Countries such as Germany and the Netherlands are making advancements in the development of fully automated composite repair equipment. Five-axis equipment supplied with a dataset about a particular area of damage will be ‘attached’ to an aircraft. This dataset identifies the shape, contour, material thickness, and laminate properties. With replaceable tool heads, the machine will cut out the damaged area leaving behind a scarf repair profile. Then the head is changed to add a plasma tool to supply the fully controllable heat source to the fitted repair insert.

The repair industry sees a point in time where more repairs to large areas on thick primary structures will be able to be accomplished by technicians in the field. No longer will large aircraft manufacturers such as Boeing and Airbus be completing all the repairs on their aircraft, as they are currently doing.

Composites in aircraft have grown dramatically in the second half of aviation history. With these fascinating developments in technology, the demand for aerospace composite manufacturing, testing, maintenance, and repair is expected to grow into the foreseeable future.