Module 11A.2–11A.3 — Airframe structures: concepts, construction, fuselage, doors, wings, stabilisers, nacelles

Levels

11.2 (ATA 51 general concepts and strength requirements) is level 2 for A1 and B1.1; construction methods 11.2(c) are level 1 for A1, level 2 for B1.1. 11.3 (fuselage, doors, windows, wings, stabilisers, control surfaces, nacelles) is level 1 for A1 and level 2 for B1.1; doors safety devices and cargo loading are level 1 A1 / level 2 B1.1.

11.2(a) General concepts

Zonal and station identification

• Zones (ATA iSpec 2200 convention): major zones by hundreds — 100 lower fuselage, 200 upper fuselage, 300 empennage, 400 powerplants and nacelles, 500 left wing, 600 right wing, 700 landing gear and doors, 800 passenger and cargo doors. Sub-zones and zones refine location (e.g. 211, 212). Access panels are numbered by zone (e.g. 511AB).
• Station lines give positions in inches (or mm) from a reference datum:

Reference	Measures
Fuselage station (FS / STA)	Distance aft from the datum (often ahead of the nose)
Water line (WL)	Height above a reference plane below the fuselage
Buttock line (BL)	Distance left or right of the aircraft centreline
Wing station (WS)	Distance outboard from the centreline (or wing reference)
Nacelle / stabiliser stations	Local equivalents

Electrical bonding

Bonding connects all metal parts electrically to give a low-resistance path so that:

• static charges equalise (no sparking, radio interference reduced);
• lightning current can flow through the structure without arcing;
• a good earth return is available for electrical systems;
• the risk of shock from faulty equipment is reduced.

Bonding jumpers bridge moving parts (control surfaces, shock struts), pipes and equipment racks. Resistance limits are very small (typically a few milliohms) and are measured with a bonding tester (milliohmmeter) per the AMM.

Lightning strike protection

• Lightning typically attaches at extremities (nose/radome, wing tips, tail, engine nacelles) and exits at another extremity.
• Protection: conductive structure, diverter strips on radomes, expanded metal foil/mesh or conductive layers in composite skins, bonded fasteners and fuel tank sealing to prevent sparks inside tanks, static dischargers (wicks) on trailing edges to bleed precipitation static and reduce radio interference.
• After a strike: lightning-strike inspection per AMM (entry/exit burns, pitting, damaged bonding, compasses/avionics checks).

11.2(b) Airworthiness requirements for structural strength

Structural classification

Class	Definition	Example
Primary	Carries flight, ground or pressurisation loads; failure could be catastrophic	Spars, main frames, longerons, wing skins, landing gear attachments
Secondary	Carries some load; failure would affect the aircraft but not cause immediate loss	Fairings with some load, some control surface parts
Tertiary	Little or no load; failure has no effect on safety	Non-structural fairings, interior trim brackets

Design philosophies

Philosophy	Principle
Safe-life	Component is retired after a fixed life (hours, cycles) before fatigue cracks can form — e.g. landing gear legs
Fail-safe	Multiple load paths/crack stoppers so that if one member fails, others carry the load until the failure is found — redundant structure
Damage tolerance	Structure can sustain damage (cracks, corrosion, accidental damage) and still carry required loads, with an inspection programme that detects damage before it reaches critical size — used for most modern transport structure

Stresses

Stress	Description
Tension	Pulling apart (wing lower skin in flight)
Compression	Pushing together (wing upper skin in flight)
Shear	Layers sliding over each other (rivets, spar webs)
Bending	Combination of tension and compression across a section
Torsion	Twisting (wing torsion box, shafts)
Hoop stress	Circumferential tension in a pressurised cylinder: σ = p r / t — twice the longitudinal stress
Fatigue	Failure under repeated cyclic loads below the static strength; pressurisation cycles are a major source
Strain	Deformation per unit length caused by stress

Drains, ventilation and system installation provisions

• Drain holes and valves at low points prevent water collecting (corrosion, freezing that can jam controls). Fuselage bilge areas have drain valves that close when pressurised.
• Ventilation removes fumes and moisture (e.g. fuel vapour areas, battery compartments).
• System installation provisions: brackets, clamps and lightening holes with grommets; segregation of wiring from fluid lines; clearances from moving parts and structure.