ALBUQUERQUE, N.M. -- The Federal Aviation Administration (FAA) last week gave an improved type of aircraft fuselage 'patch' a thumbs up and returned a Delta Airlines L-1011 to commercial service.

It was the plane's first inspection since a team led by Sandia National Laboratories engineers installed the patch -- called a "bonded composite doubler" -- in late February to reinforce a corner of the plane's right mid- section access door. The 250-passenger jetliner had been flying a trans-Atlantic route -- with stops in New York, France, England, and The Netherlands -- almost daily since the installation. It is the first time the new reinforcement technique has been used on commercial aircraft.

The technology's development and FAA flight certification were conducted at the Airworthiness Assurance NDI Validation Center (AANC), managed and staffed by Sandia for the FAA. The AANC is located near the Albuquerque International Sunport. Sandia led the project team, which included investigators from Delta, the FAA, Textron Systems Division (a composite materials manufacturer), and Lockheed Martin Corp. (the L-1011 model was built by Lockheed in the early '70s). The FAA sponsored the work as part of a program to extend the service lives of aging commercial aircraft.

Takeoffs, landings, cabin pressurizations and depressurizations, rapid temperature changes, moisture, and turbulence all take their tolls on the aluminum skin of a commercial aircraft. Eventually these repeated stresses can cause tiny cracks or internal flaws to form, so small they can't be seen with the human eye. Unremedied, the small flaws can spread and grow into larger cracks or other flaws, most often near stress-magnifying features such as doors and windows.

Regular maintenance and repair can prevent flaws from growing and keep planes flying longer. Historically, riveted metal reinforcement plates have been applied to the fuselage surface to arrest flaws underneath. Because of their rigidity, however, these plates can, over time, cause other flaws in the skin surrounding the patch. The rivets themselves can cause new flaws to form as well, leading to additional aircraft maintenance tasks.

"Despite the aviation industry's acknowledgment of the difficulties associated with riveted metal patches, they have been the primary method of fuselage repair since the advent of the airplane," says Dennis Roach of Sandia's Airworthiness Assurance Department. "Composite doublers can provide both engineering and economic benefits."

The new technique makes use of a flexible, fiber-reinforced composite material that can be adhered to the aircraft's surface. Produced as a thin tape, the material comprises strong, parallel boron fibers enmeshed in epoxy.

When an aircraft comes in for a maintenance visit, airline workers strip and prepare the aluminum surface in the area of a flaw. Then layers of the tape are cut to fit the repair area and are applied with an adhesive, forming a multilayer laminate. Heat and pressure are applied to ensure adhesion to the aircraft's surface. The finished doubler is a fraction of an inch thick (each ply is only 0.006-in. thick) and can be three times stronger than a typical riveted aluminum patch of comparable thickness.

The advantage of the composite doubler is its flexibility, says Roach. With riveted plates, stress load transfer occurs exclusively at the doubler's edges. With a composite doubler, load transfer occurs gradually by shear through the adhesive layer, creating a more uniform stress field. Also, because of the composite tape's parallel boron fiber orientation, each layer of tape is strongest in one direction -- parallel to the fibers. By orienting successive plies 0, 45, or 90 degrees from a crack's orientation, for instance, a composite doubler can be designed with a directional strength optimized for a particular repair.

Composite doublers also are corrosion resistant and lightweight and can be readily formed into complex shapes, which permits the repair of irregular components (such as doors and wing joints) without machining.

Most common repairs can be completed in less than 12 hours -- overnight -- which is perhaps the greatest advantage of the new technique, Roach says. Each day a commercial aircraft is grounded can cost an airline $80,000 or more.

The composite doubler technique -- so far certified for door corner reinforcement on L-1011 aircraft only -- underwent an intensive validation and flight-certification procedure prior to its installation on the Delta airliner. Sandia subjected samples of the composite material to a variety of stress, impact, fatigue, and harsh-environment tests to establish damage tolerance limits for the material. Sample doublers were installed on retired aircraft parts.

In addition, a variety of novel nondestructive inspection (NDI) techniques were certified for use so that the doubler's long-term integrity can be verified.

The Delta L-1011 installation and flight testing is part of the proof-of-concept phase of the project. The April 23 inspection at Delta's Technical Operations Center in Atlanta, which verified the doubler's integrity after 45 days of flight, was the first in a series of special Delta/ FAA inspections set up to ensure the doubler's performance over time.

With the successful completion of the L-1011 door corner application, says Pete Versage, project manager at the FAA's Technical Center, Sandia and the FAA are now seeking to develop and certify a more generic set of applications for a variety of common aircraft repairs, including fuselage joints, landing gear bay, and cargo door repairs.

"Certifying and testing the L-1011 door reinforcement is a huge step toward industry acceptance of this type of repair," says Versage.

Door corner repairs on Delta's fleet of approximately 55 L-1011's could save the company more than half a million dollars, according to a preliminary Sandia cost-benefit analysis. When all types of repairs are considered for all 727, 737, and DC-9 aircraft (among the oldest aircraft in the US commercial fleet), the commercial aviation industry may save millions using composite doublers.

"The use of composite materials and processes, for many repairs, reduces labor hours and overall costs as compared to metal repairs," says Versage. "In addition, the L-1011 was a very complex application, as compared to most repairs, and serves to emphasize the feasibility of composite doubler repairs for metal aircraft structures."

"Air transportation is critical to the US economy," says Roach. "Airline passengers want safe and reliable transportation at a reasonable price. The aviation industry and FAA are continually seeking improved technologies and processes that enhance aircraft safety."

Sandia is a multiprogram Department of Energy laboratory operated by a subsidiary of Lockheed Martin Corp. With main facilities in Albuquerque and Livermore, Calif., Sandia has broad-based research and development programs contributing to national security, energy and environmental technologies, and economic competitiveness.