Aircraft Structures and Airframe Systems
Aircraft Structures and Airframe Systems tests how load paths, structural idealisation and damage tolerance are applied to the airframe. The exam expects you to identify primary structure, explain semi-monocoque behaviour and calculate simple stresses.
Load paths and construction
The skin carries pressure and shear; stringers and frames stiffen it and redistribute concentrated loads. A wing box acts as a beam: spars take bending and shear, ribs maintain section shape and transfer fuel and torsion loads. Primary structure failure is catastrophic; secondary structure failure may be tolerated but still affects airworthiness.
Static and damage tolerance
Fail-safe design requires load paths to remain after a member fails; safe-life parts are retired at a calculated life. Damage tolerance uses inspection to find cracks before they grow to critical size. Pressurisation gives hoop stress twice the longitudinal stress for a thin cylinder.
Worked example
Cabin differential pressure 55 kPa, fuselage radius 1.5 m, skin thickness 2 mm. Hoop stress = p r / t = 55,000 × 1.5 / 0.002 = 41.25 MPa. Longitudinal stress = p r / (2t) = 20.6 MPa. The hoop value governs a longitudinal crack; a circumferential crack is driven by the longitudinal stress, so repair schemes must restore both paths.
Traps
Using cabin absolute pressure instead of differential overstresses the calculation; mixing radius and diameter halves or doubles the answer. Do not treat a cut-out as harmless: doublers and fail-safe straps restore load path. Fatigue is not the same as static strength.