The critical engineering principles concerning metal fatigue learned from the Comet 1 program were applied directly to the design of the Boeing 707 (1957) and all subsequent jet airliners. For example, detailed routine inspection processes were introduced, in addition to thorough visual inspections of the outer skin, mandatory structural sampling was routinely conducted by operators; the need to inspect areas not easily viewable by the naked eye led to the introduction of widespread radiography examination in aviation; this also had the advantage of detecting cracks and flaws too small to be seen otherwise. Another visibly noticeable legacy of the Comet disasters is the oval windows on every jet airliner; the metal fatigue cracks that destroyed the Comets were initiated by the small radius corners on the Comet 1's almost square windows. The Comet fuselage was redesigned and the Comet 4 (1958) went on to become a successful airliner, pioneering the first transatlantic jet service, but the program never really recovered from these disasters and was overtaken by the Boeing 707.