Airframe: Structures, Sheet Metal, and Systems
Structures and sheet metal
Every airframe structural question traces back to the five stresses - tension, compression, shear, bending, and torsion - and to the idea that failures start where stress concentrates: cutouts, sharp corners, abrupt thickness changes, and mismatched rivet patterns. Know the difference between a lap joint, a butt joint with a doubler, and a splice; know why a repair must transfer load through the same path as the original structure.
Rivet layout is a reliable question source. FAA guidance calls for a minimum edge distance of about 2 rivet diameters (2D) and a rivet pitch of roughly 3D in a typical repair, with rows staggered and the manufactured head on the outside. Traps: answers given in sixteenths when the stem gives eighths, forgetting that edge distance is measured from the center of the rivet, and using the wrong rivet alloy or head type for the sheet being repaired.
Worked example
A repair uses 1/8-in rivets. Minimum edge distance = 2 × 0.125 = 0.25 in (1/4 in). Typical pitch = 3 × 0.125 = 0.375 in (3/8 in). If a proposed layout shows a rivet 3/16 in from the edge, it fails: 3/16 < 1/4.
Systems and the usual traps
Hydraulics: pressure is set by the system relief valve and is the same throughout a static system, while force = pressure × piston area, so a larger piston area gives more force, not more pressure. Rigging: control cable tension changes with temperature - use the rigging chart and the specified temperature rather than "snug." Corrosion: dissimilar metals plus an electrolyte give galvanic attack; the fix is separation, sealing, and drainage. Fabric, wood, and welding questions usually hinge on inspection access and moisture/rot control rather than obscure technique.