Method of slowing the propagation of cracks in a fail safe structure and fail safe frame, especially for fuselage

A failure mitigating mainframe configured to be connected to a fuselage skin of an aircraft and to slow a spread of cracks in the mainframe, comprising:

• profiled longitudinal structural components assembled back to back by fasteners, the longitudinal structural components including first sides that are longitudinally aligned with one another and bonded to the fuselage skin, and second the other sides that are longitudinally aligned with one another and spaced a distance away from the first sides, the second sides defining an external face of the longitudinal structural components;
• and
• a structural element made of composite and coupled with fasteners to the external face of the longitudinal structural components so as to provide the mainframe in a break repairing configuration in which a load borne by a cracked longitudinal structural component is transferred locally into the structural element made of composite and is not redistributed throughout all other components of the mainframe, thereby avoiding acceleration of any other cracks in other components of the mainframe.