Airframe Structures and Sheet Metal Work
Core concepts
The airframe is a semi-monocoque structure: skin carries shear and some tension, stringers and longerons stiffen it, frames and bulkheads maintain shape and carry concentrated loads, and the wing spar caps carry bending. Candidates must know the difference between primary, secondary and tertiary structure, and which repairs require an approved data source (SRM, AD, or engineering disposition). Understand sheet-metal fundamentals: 2024-T3 versus 7075-T6, bend allowance and setback, bend radius versus thickness, grain direction, and the purpose of relief holes, fillets and stop-drilling cracks. Know rivet types (solid, blind, Hi-Lok, lockbolt), edge distance rules (typically 2D minimum), rivet pitch, and the difference between lap, butt, flush and joggle joints. Understand corrosion protection: alodine conversion coating, primer, sealant, and the need for interference-fit fasteners in critical joints.
Traps in questions
Questions often test edge distance and rivet spacing measured from the hole center, not the edge of the rivet head. Another trap is confusing the direction of bend allowance (added to flat pattern) with setback (removed from layout). Candidates also confuse stop-drilling (temporary crack arrest, not a repair) with a permanent repair. Watch for questions about countersinking versus dimpling: dimpling is for thin sheet, countersinking for thicker material or where dimpling is impractical. Also remember that heat-treated aluminum alloys lose strength if bent without the correct temper condition.
Worked example / rule of thumb
Rule of thumb: minimum edge distance for a rivet is 2 times the rivet diameter (2D). If a rivet is 4.0 mm diameter, minimum edge distance is 8.0 mm from hole center to material edge. For a 3/32-inch rivet (about 2.4 mm), minimum edge distance is about 4.8 mm. When checking a drawing, measure from hole center, and confirm the drawing's note if a tighter value is approved by the SRM.