Structures, General Concepts
Structural classification, safe-life/fail-safe/damage-tolerance, stressed-skin construction and load types.
Structures (general), summary notes
- 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.
- Ultimate load
- ultimate = limit load × 1.5
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.
Stress, strain & the tensile test
General structures concept map
Structures, General
General structures quiz
Structures, General Concepts, quiz
1. Primary structure is structure whose failure:
Would be catastrophicHas no effectOnly affects appearance2. A 'safe-life' component is:
Retired at a set lifeLeft until it cracksNever inspected3. A 'fail-safe' structure provides:
An alternate load path if one member failsNo redundancyA single load path4. 'Damage tolerance' relies on:
Inspection finding cracks before they are criticalNever inspectingInstant failure5. Semi-monocoque construction uses:
Skin plus frames, stringers and longeronsSkin aloneA wooden truss6. Ultimate load = limit load ×:
1.51.03.07. Which is NOT a basic structural load?
MagnetismShearTorsion8. Limit load is the:
Maximum load expected in serviceFailure loadZero-load case