Structural classification, safe-life/fail-safe/damage-tolerance, stressed-skin construction and load types.
Notes
Structures (general) — summary notes
Free
Main ideas
Structure is classified by load importance: PRIMARY (failure catastrophic — spars, frames), SECONDARY (lesser loads) and TERTIARY (fairings, little load).
Three design philosophies: SAFE-LIFE (retire the part at a set life before a crack is likely), FAIL-SAFE (an alternate load path carries the load if one member fails) and DAMAGE TOLERANCE (scheduled inspection finds slow-growing cracks before they become critical).
Modern aircraft use stressed-skin construction — monocoque (skin carries all load) and, usually, semi-monocoque (skin plus frames, stringers and longerons share load).
Members react five basic loads — tension, compression, shear, bending and torsion — and the airframe is designed to a limit load (max expected) and an ultimate load (limit × 1.5).
Zone and station numbering, drains/vents, bonding and lightning-strike protection support system installation and maintenance access.
⚠ Exam trap: know the philosophies apart — safe-life RETIRES the part, fail-safe adds an ALTERNATE LOAD PATH, damage tolerance relies on INSPECTION; ultimate load = limit load × 1.5.
Key formulas
Ultimate load
ultimate = limit load × 1.5
Solved examples
How does a damage-tolerant design keep a cracked structure safe until the next inspection?
The structure is designed so that any crack that starts (at a fastener hole, say) grows slowly and predictably. The inspection interval is set short enough that a crack will be detected — by NDT or visual inspection — while it is still well below the length at which the remaining structure could fail. Detected damage is then repaired or the part replaced.