Principal Structural Elements
PSE’s are those elements of primary structure which contribute significantly to carrying flight, ground, and pressurization loads, and whose failure could result in catastrophic failure of the airplane.
Engineering design and damage evaluation – repair criteria for aircraft structures are location dependent depending on whether the structure is considered and classified as either Primary, Secondary or as a PSE (Principal Structural Element). Areas classified as PSE’s must be designed (by regulations) with a level of Damage Tolerance that maintains residual strength, fail safe ability, limits damage growth rates and resists a catastrophic failure due to the effect of manufacturing defects or typical damage scenarios.
Typically most aircraft structural repair manuals contain guidance through charts or diagrams that define what parts of the aircraft are classified as Principal Structural Elements – PSE’s, Primary or Secondary structure. This information is typically in ATA chapter 51.
Additionally, virtually all repairs (other than straight parts replacement) on transport aircraft to Principal Structural Elements or Primary structures are classified as Major Repairs thus necessitating the use of Approved rather than Accepted data to accomplish the repair. This rule should be considered the minimum requirement since some regulatory organizations are more restrictive than the others. For example, EASA regulations (Commission Regulation European Community [EC ] 2042/2003 Annex I Part M require “approved” data for both minor and major repairs. This is is in contrast to the FAA that requires “approved” data for only major repairs and “acceptable” data for minor repairs.
Examples of Principal Structural Elements typically include:
(1) Wing and empennage.
(a) Control surfaces, slats, flaps, and their mechanical systems and attachments (hinges, tracks, and fittings);
(b) Integrally stiffened plates;
(c) Primary fittings;
(d) Principal splices;
(e) Skin or reinforcement around cutouts or discontinuities;
(f) Skin-stringer combinations;
(g) Spar caps; and
(h) Spar webs.
(a) Circumferential frames and adjacent skin;
(b) Door frames;
(c) Pilot-window posts;
(d) Pressure bulkheads;
(e) Skin and any single frame or stiffener element around a cutout;
(f) Skin or skin splices, or both, under circumferential loads;
(g) Skin or skin splices, or both, under fore and aft loads;
(h) Skin around a cutout;
(i) Skin and stiffener combinations under fore and aft loads;
(j) Door skins, frames, and latches; and
(k) Window frames.
(3) Landing gear and their attachments.
(4) Engine mounts.