Airframe Structure, Loads and Structural Limitations
Loads and the structural ideal
An airframe is a load path. Every question about structure reduces to asking which member carries which load and what limits that member. Wings carry bending (spar caps in tension and compression), shear (webs), and torsion (closed torsion box formed by skin and ribs). The fuselage is a beam carrying bending from tail loads and pressurisation hoop stress; the skin-stringer-frame combination is semi-monocoque, with stringers stiffening skin against buckling and frames maintaining shape and limiting crack propagation.
Limit and ultimate load
Certification uses limit load (the maximum expected in service) and ultimate load (limit × 1.5). At limit load no permanent deformation is permitted; at ultimate load the structure must withstand for three seconds without failure. The manoeuvring speed VA is the speed at which full deflection of flight controls produces limit load; below VA the wing stalls before it overstresses, above VA the structure may fail first. VNE is a never-exceed limitation, not a design strength boundary. Gust load increases with speed and is a common trap: turbulence penetration speed VB is below VA.
Fatigue, fail-safe and damage tolerance
Fail-safe design ensures a failed element leaves a residual load path; damage-tolerant design uses inspection to find cracks before they become critical. Safe-life components (landing gear, some rotating parts) are retired by cycle count regardless of condition.
Worked example
If limit load is 40 kN, ultimate is 60 kN. A question stating 'the wing must not fail below 40 kN' is describing ultimate load only if it says 1.5 × limit; read carefully whether the figure is limit or ultimate. Rule of thumb: at ultimate load, deformation is allowed; at limit load, none.
Traps
Confusing VA with VNE, forgetting the 1.5 factor, and assuming composite structures behave like aluminium (they show little yielding before failure).