The increase in airline passenger traffic, coupled with the continued economic viability of older airplanes, has resulted in their continued use. As aircraft age, structural degradation presents a risk to air safety in the form of fatigue and corrosion. The safety threat associated with operating airplanes beyond their design life expectancy is difficult to quantify. However, accidents have occurred which have been associated with structural degradation due to fatigue and/or corrosion. To counteract this threat, the FAA has established the National Aging Aircraft Research Program. The goal of the program is to assure the continued structural airworthiness of the U.S. commercial fleet of in-service and future airplanes beyond their economic design lives.