small fatigue cracks extending from adjacent rivet holes in a longitudinal lap joint in the fuselage. The accident focused international attention on the problems of operating an aging commercial fleet.

In 1990 approximately 46 percent of the U.S. commercial air transport fleet was over 15 years old, and 26 percent was over 20 years old. If current usage and replacement trends continue, the number of aircraft over 20 years old will double by the year 2000. Currently some 3,200 aircraft are affected by FAA Airworthiness Directives that concern operation and maintenance of the aging fleet. The review of experience with aging aircraft has caused an increase in the emphasis on stress corrosion, corrosion, fatigue, and MSD issues. This experience has caused, in turn, the selection of new aircraft alloys with better constituent chemistry control or changes in heat treatment tempers. Also, it has stimulated the development of new organic finishes that significantly retard corrosion, as well as the implementation of design practices to vastly improve corrosion resistance.

The FAA and NASA developed a cooperative research effort aimed at providing a technological basis for ensuring the continued safe operation of the aging commercial aircraft fleet. Each agency developed a program consistent with its mission. The FAA's National Aging Aircraft Research Program addresses the aging aircraft structural safety concerns and provides certification authorities and operators with the tools to meet those concerns. NASA's Airframe Structural Integrity Program is focused on developing advanced integrated technologies to economically inspect for damage and to analytically predict the residual strength of older airplanes. Together these programs form the technological basis for a cooperative effort with U.S. industry to address the critical aging aircraft issues.

MSD is a form of widespread fatigue damage that is characterized by small cracks emanating from structural details such as fastener holes (Sampath, 1993). If cracks emanate from adjacent fastener holes, they have the potential to link up and lead to unexpected catastrophic failures as described in the previous section. Also, even without link-up, multiple-site cracks can severely degrade the capability of the structure to withstand major damage from other discrete sources as is described later in this section.

In the past, the standard industry practice was to visually inspect the airframe for damage. Various levels of inspections ranging from daily walk-around inspections to detailed tear-down inspections were performed. Instrumented nondestructive evaluation (NDE) methods such as eddy current probes were used only to inspect local regions of the structure where previous cracking problems had occurred. While these inspection methods were labor intensive and highly subjective, they were acceptable because the airframe was designed to survive a two-bay skin crack with a severed frame or stiffener. This design criterion was established to enable the airplane to tolerate major discrete source damage (i.e., such as might be encountered as a result of an engine structural failure) as well as large cracks resulting from the link up of smaller fatigue cracks or the unstable propagation of manufacturing flaws or other service-induced damage. Such damage is large enough that it should be easily detected, and the operator does not need to search for small cracks to ensure the structural integrity of the airframe. However, this "fail-safe" philosophy assumed that the structure adjacent to the major damage (e.g., the two-bay crack) was free of MSD. Design residual strength requirements were based on this assumption. However, the existence of very small cracks (e.g., a few hundredths of an inch or tenths of a millimeter in length) in the adjacent structure can severely degrade this residual strength and thus jeopardize the safety of the airplane as it did in the Aloha Airlines incident. Therefore, inspection of aging aircraft has become much more onerous than for newer aircraft because safety is vitally dependent on the detection of the very small cracks associated with this onset of MSD. This represents a major challenge to the inspection and aircraft industries.

The principal technical needs are (1) to develop and verify advanced NDE technology that can reliably and economically detect disbonds, small MSD fatigue cracks, and corrosion and characterize their effect on the residual strength; and (2) to develop and verify advanced fracture mechanics and structural analysis methodology to predict fatigue crack growth and residual strength of airframe structures to determine in-service inspection thresholds and repeat intervals, quantitatively evaluate inspection findings, and design and certify structural repairs. NDE methods related to MSD are described in chapter 8, and fracture mechanics and structural analysis methods are described in chapter 6.

Corrosion of aging aircraft has been described as an insidious problem (Marceau, 1989). While other aging mechanisms, such as wear and fatigue, are somewhat predictable and can be addressed by the airline maintenance programs to preclude major structural problems, corrosion—especially in its localized forms—is very difficult to predict and detect. Factors that influence the extent of corrosion on aircraft are materials selection, design, component processing and finishing, operational environments, and maintenance programs.

It is anticipated that airplanes manufactured today will experience fewer corrosion problems than those in the current aged fleet because of significant design and corrosion protection improvements that have been implemented and because of operators' increased awareness of the role of these improvements in preventive maintenance. Clearly, maintenance

Front Matter (R1-R14)

Executive Summary (1-4)

1 Requirements and Drivers (5-10)

2 Structural Concepts (11-25)

3 Metallic Materials and Processes (26-35)

4 Polymetric Composite Materials and Processes (36-44)

5 Environmentally Compliant Materials and Processes (45-48)

6 Methodologies for Assessment of Structural Performance (49-56)

7 Aircraft Maintenance and Repair (57-66)

8 Nondestructive Evaluation (67-70)

9 Committee Findings (71-76)

References (77-82)

Appendix: Biographical Sketches of Committee Members (83-84)