An experimental program of work was undertaken to assess the life improvement gained through utilizing cold expansion technology to enhance the resistance of cracking at fatigue critical locations (FCLs) in an aging military trainer aircraft. The FCLs considered included a countersunk hole and nutplate on two different wing spars. The overall goal of the experimental work was to provide a comparative assessment of the influence of the major variables examined, which included cold expansion method and process, hole size, countersink size, nutplate geometry, flight spectrum, and joint configurations. The results of this work clearly illustrated the inherent sensitivity of life improvement factors to the myriad of variables involved. Nevertheless, the foregoing experimental work clearly demonstrated the valuable life improvement that is provided by cold expansion for nearly all cases examined.