Aeroplane Airframe Structures and Systems
Load paths in a semi-monocoque airframe
The skin is a stressed member: it carries shear and, in a pressurised fuselage, hoop and longitudinal tension. Stringers run fore-aft and carry bending; frames (rings) run circumferentially, hold the cross-section shape and stabilise the stringers against buckling; bulkheads pick up concentrated loads such as wing attachment, engine mounts and pressure bulkheads. A wing is normally a box of front and rear spars, ribs and skin, with the upper skin in compression and the lower skin in tension in flight.
Materials and protection
2024-T3 aluminium is common for skin because of its fatigue resistance. 7075-T6 is stronger and used for spars, fittings and extrusions, but is more notch-sensitive and less corrosion-resistant. Composite structure fails differently: look for delamination, disbond, crushed core and barely visible impact damage, and restore lightning-protection mesh and bonding after repair. Joints need the correct fastener type, hole fit, interference or sealant, and wet assembly in fuel and pressurised areas.
Worked example
A 24-year-old lap joint on a pressurised fuselage shows white powder and lifting skin edges: exfoliation corrosion. Reasoning: strip and clean the area, then measure material loss against the SRM allowable limits. If within limits, treat per the SRM, restore sealant and protective finish and record it in the log; if material loss exceeds limits, an OEM repair scheme is required. Inspect adjacent fastener rows, because exfoliation follows the grain structure along the joint.
Traps
Calling frames "stringers"; using 7075 where 2024 is specified for fatigue; ignoring rebalancing or re-protection after repair; missing corrosion hidden under sealant or insulation blankets.