Airframe Structures, General Concepts
Structural classification, safe-life/fail-safe/damage-tolerance, stressed-skin construction and load types.
Airframe structures (general), summary notes
- 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.
- Ultimate load
- ultimate = limit load × 1.5 (safety factor)
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.
General structures concept map
Airframe Structures, General
General structures quiz
Airframe Structures, General, quiz
1. Primary structure is defined as structure whose failure:
Would be catastrophic (carries flight/ground loads)Has no effectOnly affects appearance2. A 'safe-life' component is:
Retired at a set life before a crack is likelyLeft in service until it cracksNever inspected3. A 'fail-safe' structure provides:
An alternate load path if one member failsNo redundancyA single load path only4. 'Damage tolerance' relies on:
Inspection catching cracks before they become criticalNever inspecting the structure, since the design is proved to be free of defects at buildInstant failure of the member as soon as a crack starts, so that the fault cannot be missed5. Semi-monocoque construction uses:
Skin plus frames, stringers and longerons sharing loadSkin carrying all load aloneA tubular frame with fabric only6. Ultimate load equals limit load multiplied by:
1.51.03.07. Which is NOT one of the five basic structural loads?
MagnetismTensionShear8. Limit load is the:
Maximum load expected in serviceLoad that causes failureZero-load condition