Airframe Structures — General Concepts

Structural classification, safe-life/fail-safe/damage-tolerance, stressed-skin construction and load types.

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Main ideas

Structure is classified by importance: PRIMARY structure carries flight/ground loads and its failure is catastrophic (spars, main frames); SECONDARY structure carries lesser loads; TERTIARY carries little load (fairings).
Design philosophies: SAFE-LIFE (retire the part at a set life before any crack is likely), FAIL-SAFE (an alternate load path carries the load if one member fails), and DAMAGE TOLERANCE (structure tolerates cracks that scheduled inspection will find before they become critical).
Modern aircraft use stressed-skin construction — monocoque (skin carries all load) and, more usually, semi-monocoque (skin plus frames, stringers and longerons share the load).
Members react five basic loads: tension, compression, shear, bending and torsion; the whole airframe is designed to a limit load (max expected) and an ultimate load (limit × 1.5 safety factor).
Zone and station numbering locate structure and systems; drains, vents, bonding and lightning-strike protection are built in.
⚠ Exam trap: know the three philosophies apart — safe-life RETIRES the part, fail-safe provides an ALTERNATE LOAD PATH, damage tolerance relies on INSPECTION to catch damage; ultimate load = limit load × 1.5.

Key formulas

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What is the difference between a fail-safe and a damage-tolerant structure?
A fail-safe structure has built-in redundancy: if one load path (e.g. one of several fasteners or a tear strap) fails, a parallel path carries the load until the failure is found. A damage-tolerant structure is designed and inspected so that any crack grows slowly enough to be detected at scheduled inspections before it reaches critical length. Modern designs combine both ideas.

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