Structural Parts Classification
Definitions
Principal Structural Element (PSE)
A PSE is an element that contributes significantly to the carrying of flight, ground, or pressurization loads, and whose integrity is essential in maintaining the overall structural integrity of the airplane (Ref. FAA AC 23-13A)
Primary Structure.
Primary structure is that structure which carries flight, ground, or pressurization loads, and whose failure would reduce the structural integrity of the airplane (Ref. FAA AC 23-13A)
Safe-Life.
The safe-life of a structure is that number of events, such as flights, landings, or flight hours, during which there is a low probability that the strength will degrade below its design ultimate value due to fatigue cracking (Ref. FAA AC 23-13A)
Fail-Safe.
Fail-safe is the attribute of the structure that permits it to retain its required residual strength for a period of unrepaired use after the failure or partial failure of a principal structural element (Ref. FAA AC 23-13A)
Secondary structure
Secondary structure is that structure which carries only air or inertial loads generated on or within the secondary structure (Ref FAA AC25.571-lA).