In 1998, the combined commercial aircraft fleet operated by U.S. airlines was over 7,000 airplanes. At the start of the jet age (1950s to 1960s), little or no attention was paid to corrosion and corrosion control. One of the concerns is the continued aging of the airplanes beyond the 20-year design life. Only the most recent designs (e.g. Boeing 777 and late version 737) have incorporated significant improvements in corrosion prevention and control in design and manufacturing. The total annual direct cost of corrosion to the U.S. aircraft industry is estimated at $2.2 billion, which includes the cost of design and manufacturing ($0.2 billion), corrosion maintenance ($1.7 billion), and downtime ($0.3 billion).
The annual (1996) corrosion cost to the U.S. aircraft industry is estimated at $2.225 billion, which includes the cost of design and manufacturing at $0.225 billion, corrosion maintenance at $1.7 billion and downtime due to corrosion at $0.3 billion. With the availability of new corrosion resistant materials and an increased awareness of the importance of corrosion to the integrity and operation of jet aircraft, the current design service life of 20 years has been extended to 40 years without jeopardizing the structural integrity and significantly increasing the cost of operation.
One of the major concerns of the aircraft and airline industry is the continued aging of several types of aircraft beyond the 20-year design service life. This aging of the fleet has been the subject of considerable attention by industry and government for many years, and has resulted in increased maintenance efforts of the aging aircraft. Due to the competitive nature of the airline industry however, corrosion maintenance is often not performed adequately. This also may have been due to the lack of understanding of the corrosion process and the inability to predict the nucleation and growth behavior of corrosion in airframe components. Hence, corrosion has not been incorporated in the damage tolerance assessments, where, instead, a find and fix approach has generally prevailed. This approach leads to extensive corrosion of both structural and non-structural components, significantly increasing the cost of maintenance. This may in the near future have a significant impact on the availability or down time of the aging aircraft, further increasing the corrosion-related costs. Finally, as airframes continue to age and are kept in service, corrosion will increasingly affect the structural integrity of these airframes.
While it is upon the airframe manufacturers to mitigate corrosion, the operators must have a corrosion control program in place throughout the life of the airplane. The find and fix approach must be complemented by an approach based on understanding of the corrosion process and the ability to predict and monitor its behavior. Corrosion prediction models must be developed so that a cost-effective corrosion integrity program can be developed. Moreover, there is a need for improved inspection and monitoring techniques to expand the capabilities to detect and monitor corrosion and cracks beginning at an early stage.