 The investigation also identified deficiencies in the assessment of aircraft designs and their certification and in the way aircraft were inspected.The Boeing 707 320/420 series had an enlarged horizontal stabiliser (tailplane) assembly compared to earlier 707 aircraft, and in the redesign the increased loads on the tailplane structure had been taken by replacing some of the aluminium skinning with stainless steel.In addition, the spar attachment fittings had been redesigned, making them both stronger and stiffer.