Airframe Construction and Structural Components
Airframe construction and structural components tests structural types, load paths, materials and the inspection consequences of damage.
Structural types
- Truss: steel tube or wood members in tension and compression; the skin is non-structural.
- Monocoque: the skin carries all the load; efficient but weak at cut-outs.
- Semi-monocoque: skin carries shear and tension, stringers stiffen and carry axial load, frames and bulkheads hold shape and prevent buckling.
- In a wing, spars carry bending, ribs transfer load and hold aerofoil shape, and the skin carries torsion and shear. Primary structure includes spars, wing attachments and engine mounts; secondary structure includes flaps and doors; tertiary structure is cosmetic.
Fatigue, corrosion and composites
Repeated stress concentrates at rivet holes and cut-outs, and cracks grow in tension zones. Fail-safe design allows load paths to be shed, while safe-life parts are retired by hours. Corrosion includes galvanic action between dissimilar metals, exfoliation in clad alloy and stress corrosion in highly loaded areas. Composites can delaminate or absorb moisture beneath an apparently undamaged surface. New Zealand operators inspect under Part 43, the maintenance programme, airworthiness directives and supplemental inspection documents.
Worked example
In a semi-monocoque wing under positive g, the upper spar cap is in compression and the lower cap in tension. A crack in the lower cap grows under tension and directly reduces residual strength; a buckle or dent in the upper cap reduces compression stability. Both are defects, but the tension crack is the more immediate fail-safe concern, and any repair needs approved data.
Traps
- Calling a semi-monocoque structure a monocoque.
- Treating corrosion as cosmetic because it looks superficial.
- Missing composite damage below the surface.
- Assuming any rivet-hole crack can simply be stop-drilled without approval.