Features
Advanced Composite Airframe Technology

It happens frequently, and I have to work hard at keeping from smiling whenever I hear a fellow AME say: “I guess I should start learning about composites, it’s the way of the future.”

The first combinations of what we would refer to as modern composite structures, synthetic polymer resins and fibre reinforcements, began in the late 1800s! Long before Charles Taylor helped Orville and Wilbur build the Wright Flyer, the concept of building structures from lightweight but very stiff materials was well under way.

The application of polymer/fibre-reinforced composites to aircraft structures began in the mid-1930s. Radomes, antennas, fairings and components referred to as tertiary (non-load-bearing) structures were the first items to appear. Believe it or not, the first fully documented use of fibre/resin-reinforced plastics to construct an aircraft fuselage was that of a Supermarine Spitfire made in the early 1940s by Aero Research Ltd. in Duxford, England.

The combination of glass fibres, thermosetting polyester and epoxy resin systems in aircraft structures continued into the early 1960s. It was the introduction of ultra-high-strength Boron fibre (1966), followed by carbon fibres (1968), aramid fibres (a.k.a.  Kevlar – 1972) and high-performance polyethylene (Spectra – 198 ) that radically changed the design, performance and manufacturing process used in military, civilian and experimental airframes.

“The future” as described by our out-of-touch AME above, has really been with us for quite some time. Every airframe manufacturer in the world has now fully embraced advanced composites as the key to extracting maximum performance from their designs, capitalizing on the exceptional strength-to-weight, stiffness, fatigue, corrosion resistance and a host of other desirable characteristics obtained from these materials.

Back to the Future

Virtually everyone can recall seeing (if only in pictures) the head-turning Beech Starship 2000. This incredible aircraft became a reality in 1986 and brought the general aviation industry to attention with the appearance of its futuristic swept-wing design. The look and performance of this aircraft was a direct result of the entire airframe being constructed from carbon fibre composites.

The rule book on the technology of building advanced all-composite general aviation aircraft was being written at the time the Starship was created. Ultimately, the expense of the project and reliability issues led to the demise of the type. But experience with the Starship brought forth enabling technologies which are today being employed in the manufacture of a whole new generation of business and sport/utility aircraft.

Use of Advanced Composites

In a presentation to delegates at the 1991 Ontario AME Workshop, I displayed a drawing of a widebody commercial transport aircraft. It showed the progressive use of advanced composite materials in the manufacture of the primary load-bearing structures. The final drawing was of a large twin-engine transport with an all-composite fuselage, wing and tail section. I predicted that this concept would become a reality by the turn of the century. I wasn’t too far off.

The Next Generation

The largest commercial airliner ever constructed, the massive Airbus A380 entered commercial service with a Singapore Airlines flight from Singapore to Sydney on October 25.  The A380 entered service almost 40 years after the launch of the Airbus A300 in 1969, which saw the first use of primary load-bearing carbon composite structures in the vertical and horizontal stabilizers of commercial airliners.

The A380 has employed for the first time in a commercial airliner, the use of an all-carbon-fibre composite centre section wing box. This enormous structure is the largest single component in the aircraft, measuring 23 feet wide by 20 feet long by 7 feet in height and weighing a staggering 9,800 pounds. The all-carbon-fibre horizontal stabilizer is nearly the size of an entire A320 wing!

The A380 also employs for the first time, the use of GLARE (GLAss fiber REin-
forced aluminum) in the upper part of the fuselage barrel section. Made from a multi-layer laminate of glass fibre, epoxy adhesive and thin aluminum sheets, GLARE has vastly improved fatigue properties which inhibit crack growth in the aluminum. Compared to the equivalent thickness of solid aluminum, GLARE weighs approximately 15% less and is superior in tensile loads.

With the order books filling rapidly and over 236 firm commitments to purchase, the new Boeing 787 Dreamliner has been ranked as Boeing’s most successful commercial aircraft program ever launched. With the first aircraft now in production, Boeing has scheduled the first delivery of a Dreamliner to the Japanese carrier All Nippon Airways in 2008.

The B787 Dreamliner will be the first commercial airliner to be constructed entirely of composite materials for the fuselage, empennage and wings (with only a few exceptions on the engine pylons, heated leading edges, etc.).

Building upon the knowledge gained from the demonstrated durability of composites in previous structural applications i.e. composite floor beams in the B777, Boeing appears confident that all previously expressed fears regarding the maintainability and prohibitive costs of composite repairs will be overcome. Boeing is predicting that the B787 will only require D-Checks after 12 years in service.

Powered by Composites

The strength and light weight of advanced composite materials is now displacing the traditional use of aluminum and titanium in the manufacture of extreme high-bypass turbofan engines. Low weight, reduced fuel burn, reduced emissions of nitrogen oxides ( NOx ), quieter and with lower operational costs, is the mantra for jet engine design. All of the major engine manufacturers are currently incorporating advanced composite materials in their designs, seeking to coax the maximum energy and efficiency from their products.

GE Aviation (General Electric) manufacturer of the hugely successful CF6 and GE90 Turbofan engines, has just completed the first test runs of its new GEnx ( pronounced “gee-ee-N-X” ) turbofan. This is the engine that will be powering the new B787 Dreamliner in direct competition with the Rolls-Royce Trent 1000 and will be the successor to the CF6 turbofan.

The first start of the GEnx took place on March 19, 2006 at GE’s Test Operations Center in Peebles, Ohio. By the end of the second day of testing, the GE team had run the engine up to 80,000 pounds of thrust! It is currently required to produce only 75,000 pounds for the B787 and the newest Airbus in the design phase, the A350.

With the experience gained from the exceptional success of using composite technologies in the fan blades of the GE90 engines on the Boeing B777, GE has created an ultra-efficient all-composite fan blade for the GEnx. The fan diameter measures 111 inches, and it is also shrouded by an all-carbon-composite fan case.

With only 18 blades in the fan assembly, the GEnx fan structure is far more resilient and resistant to FOD damage than previous aluminum and titanium designs. Each blade is constructed with 400 plies of carbon fibre epoxy pre-impregnated tape which makes them approximately 66% lighter and 100% stronger than titanium.

Though exceptionally strong, the carbon fibre blades do not tolerate impact energies from FOD as well as metals, particularly along the thin leading edge of the blade. Impact with FOD can cause the fibres to fray and the individual plies to delaminate. Overcoming this problem is accomplished by using a replaceable titanium sheath applied to both the leading and trailing edges.

The carbon composite fan case is so strong that a separate Aramid (Kevlar) fibre fan-blade containment shroud was not required. GE’s testing has demonstrated that the all-composite fan case is more resistant to ballistic FOD damage than an aluminum case and the composites also eliminate the problems associated with corrosion. The acoustic liner is made from a composite of glass fibre and a Nomex honeycomb sandwich core.

Maintenance and Repair Issues

For all of the tremendous advantages in the use of composite materials technologies and their applications for innovative new aerostructures, composites present a growing challenge to the MRO providers.

Composite structures do not react like metals to the daily wear and tear experienced by commercial aircraft. Nor do they display any of the typical signs of damage (dents, cracks, corrosion, creep, etc.) that would be found in metal airframes. In fact, composites bring an extensive new list of damage causes and conditions which can severely and sometimes catastrophically degrade the performance of the aircraft’s structure. The loss of American Airlines flight 587 over Jamaica Bay, New York in October 2001, an Airbus A300-600R, was the result of a catastrophic failure and separation of the all-composite vertical stabilizer.

The loss of the vertical stabilizer was the direct result of an overstress condition brought about by rapid full deflections of the rudder, in an attempt to control the aircraft in an encounter with wake turbulence. Questions arose and still remain about the detection and repair of subsurface flaws and damage detection in these large composite structures.

 I do believe we are about to witness wholesale changes in the inspection, maintenance and repair methodologies for advanced composite aircraft structures. As aircraft maintenance technicians and technologists, the “Brave New World” of advanced composite materials being used in all aspects of aircraft design and manufacture is now a part of our present,
not our future.

Anyone involved and responsible for the airworthiness, assembly, maintenance and repair of aircraft utilizing composites in their structures, should be aware of the need for initial and recurrent training in this complex technology.

In future articles I will explain the various inspection methods that pilots can utilize to check for potential damage to airframe composite structures, when performing a walkaround or pre-flight inspection. Just because a composite structure ‘looks good’ has no bearing on it’s airworthiness or structural integrity.

Wilson J. Boynton is president of Renaissance Aeronautics Associates Inc. of London, ON. You can reach him at boynton@raacomposites.com.