 This had the unforeseen effect of changing the way the tailplane structure handled gust loads, the stiffer fittings being no longer able to help in absorbing and transferring the stresses caused by gusts and other normal aerodynamic loads, the flexure (i.e., the bending loads) of the left and right horizontal stabilizers instead having to be carried by the stabiliser spars entirely by themselves.This led over time to fatigue cracking in the right horizontal stabiliser's rear spar, which, due to the concealed (internal) nature of the tailplane construction, was not noticed by maintenance engineers.The 707 had been designed to a 'fail-safe' philosophy, and failure of the tailplane rear spar had been calculated to be insufficient to cause the loss of the aircraft, the remaining front spar being sufficiently strong to enable the aircraft to land safely, the damage then being expected to be repaired before the aircraft was re-flown.