Chapter 3: Airframe Structures — General Concepts

Designing for strength: limit and ultimate loads

The certification code for large aeroplanes, CS-25 (FAR Part 25 in the USA), sets the strength every transport airframe must have. Loads are defined at two levels:

• Limit load — the maximum load expected in service: from manoeuvres, gusts, landing and ground handling, cabin pressurisation and so on. The structure must carry limit loads without detrimental permanent deformation, and any deformation at limit load must not interfere with safe operation.
• Ultimate load — limit load multiplied by the factor of safety, normally 1.5. The structure must carry ultimate load without failure for at least 3 seconds (CS 25.303 and 25.305), demonstrated by static tests and analysis.

For a large aeroplane the positive manoeuvring limit load factor is normally +2.5 g (the formula in CS 25.337 gives at least 2.5 and need not exceed 3.8) and the negative limit is −1.0 g. Gust loads, landing loads, ditching, emergency-landing inertia loads (for example 9 g forward on seat and cargo restraints) and pressurisation (with additional factors) are all specified too. On top of static strength, the structure must survive the repeated loads of service — the subject of fatigue and damage tolerance below.

Stress, strain and the six ways a structure is loaded

Stress is the internal force per unit area that resists an external load (N/m² or psi). Strain is the resulting deformation divided by the original dimension — a ratio with no units. Within the elastic range, stress is proportional to strain (Hooke's law; the ratio is Young's modulus) and the part springs back when unloaded. Beyond the yield point it deforms permanently, and at the ultimate tensile strength it breaks.

Fatigue

Fatigue is failure under loads that are repeated many times, each well below the static strength. Every flight is one pressurisation cycle for the fuselage and one ground–air–ground cycle for the wing, and gusts add millions of smaller cycles. Cracks start at stress concentrations — holes, sharp corners, scratches, corrosion pits and fretting at joints — and grow a little with every cycle. The de Havilland Comet accidents of 1954, traced to fatigue cracking from the corner of a cut-out in the pressure cabin, changed structural design forever: rounded window corners, crack stoppers, multiple load paths and full-scale fatigue testing all followed.

Primary, secondary and tertiary structure

Manufacturers also identify Principal Structural Elements (PSEs) — the parts that contribute most to carrying flight, ground and pressurisation loads and whose failure could be catastrophic — and the Structural Significant Items (SSIs) that the maintenance programme inspects. The class decides how damage is handled: the Structural Repair Manual (SRM) divides damage into allowable damage (no repair needed, within stated limits), repairable damage (an SRM repair applies) and damage that needs replacement or a specially approved repair.

Safe-life, fail-safe and damage tolerance

• Safe-life — the part is designed and tested to last a fixed life (cycles, hours or landings) without developing a dangerous crack, then it is retired whatever its condition. Used where cracks cannot be tolerated or found in time, typically landing-gear components and some engine parts. Life-limited parts must be tracked with their back-to-birth records.
• Fail-safe — the structure has multiple load paths: if one member fails, the remaining structure carries the required loads until the failure is found. Examples are split spar caps, back-up fittings, crack-stopper straps and multi-piece skin panels.
• Damage tolerance — the modern approach required for transport aeroplanes by CS 25.571. Damage from fatigue, corrosion or accidents is assumed to exist; testing and analysis show it grows slowly and predictably, and that the structure keeps enough residual strength until it is found by scheduled inspections. Safe-life is only used where damage tolerance is impractical.

Damage tolerance links design to maintenance. The inspection programme (including Supplemental Structural Inspection Documents, the corrosion prevention and control programme, CPCP, and checks for widespread fatigue damage up to the aircraft's limit of validity) tells you where to look, with what method and how often. Skipping a task, or using a less sensitive NDT method than specified, removes the safety margin the design depends on.

Finding your way: stations, zones and access panels

• Fuselage station (FS) or body station (BS) — distance aft of a datum (station 0), usually ahead of the nose so that every station is positive.
• Water line (WL) — height above a horizontal datum, usually below the aircraft or at a reference such as the cabin floor.
• Buttock line (BL) — distance left (LBL) or right (RBL) of the vertical centre plane (BL 0).
• Wing station (WS), horizontal and vertical stabiliser stations (HSS, VSS), nacelle stations, and aileron and flap stations measured along their own reference lines.
• Units are inches on Boeing types and millimetres on Airbus types; Airbus also locates fuselage structure by frame (FR) and stringer (STGR) numbers.

Access and inspection panels are identified within their zone. A typical scheme is the three-digit sub-zone number followed by letters giving the panel's sequence and position (for example top, bottom, left or right), as in 571AB on an Airbus wing. Panels marked as structural (stressed panels) must have all their fasteners fitted, and some must be installed before the aircraft is jacked or pressurised — follow the AMM.

Electrical bonding

Bonding connects every conducting part of the airframe into one electrically continuous, low-resistance whole. It:

• keeps all parts at the same potential, so no sparks can jump between them — vital near fuel and its vapour;
• provides the current return path for single-wire electrical systems, through the structure;
• lets lightning current flow through the structure without arcing across joints, hinges or pipes;
• reduces radio interference and susceptibility to HIRF, and protects people from shock.

Hinged surfaces, engines on anti-vibration mounts, pipes, equipment racks on shock mounts and panels are linked by bonding jumpers (braided straps). The FAA AMT handbook gives typical limits: each bond or ground connection should normally not exceed 0.003 Ω; isolated parts that can build up static (larger than about 3 square inches) need a static bond of less than about 1 Ω; and a static discharger's mount should be within 0.1 Ω of the airframe. Always use the figures in the aircraft's own manuals. Bonds are measured with a low-resistance ohmmeter (milliohmmeter / bonding meter) after the joint is made and before the area is refinished; contact faces are cleaned to bare metal (anodising is an insulator) and then re-protected. Jumpers must be short, not soldered, and must not restrict control movement.

Static dischargers (wicks) on the trailing edges of wings and tail release precipitation static gradually, at a point away from the antennas, to cut radio interference. They are not lightning protection, but missing wicks may limit dispatch under the MEL.

Lightning-strike protection

An airliner is struck by lightning roughly once a year in service. The strike usually attaches at an extremity, travels through the airframe and exits from another extremity. Surfaces are divided into lightning zones:

• Metal structure conducts the current away harmlessly provided the bonding is intact.
• Composites are far less conductive (carbon fibre about 1 000 times more resistive than aluminium, the resin about a million times). Exposed composite parts therefore have a conductive outer layer — expanded copper or aluminium foil/mesh, flame-sprayed metal or conductive paint — tied into the bonding system.
• Radomes carry diverter strips that lead the strike round the radar antenna to the fuselage.
• Fuel tanks must not spark: thick skins, sealed and bonded fasteners and cap seals, bonded pipes and careful fuel-quantity wiring.

After a reported strike the AMM's conditional lightning-strike inspection is carried out: find the entry and exit points (pitted or burned skin and fasteners, discoloured paint), check composite parts for delamination, inspect bonding jumpers, static dischargers and antennas, and — on some types — check the standby compass. Damage is assessed against the SRM.

Drains, ventilation and system installation provisions

• Drains — water from condensation, rain and spills collects at the lowest points. Drain holes in stringers, frames and skins, and pressure-operated drain valves in the fuselage lower lobe (open on the ground, closed when the cabin is pressurised), let it escape. Blocked drains mean trapped water, corrosion and ice that can jam controls, so keeping drain paths clear is a routine task.
• Ventilation — compartments that can collect flammable vapour or that hold heat sources (fuel-system areas, battery bays, the area around the APU) are ventilated, and the airflow is arranged to prevent condensation. Drain masts carry fluid leakage clear of the aircraft.
• System installation provisions — the structure is designed with attachment points, brackets, clips and reinforced holes for wiring looms, pipes and equipment, with routing that keeps redundant systems apart. Holes for systems in frames and webs are flanged or reinforced with doublers. You may not drill or add attachments to structure without approved data.

How the airframe is built

Old aircraft used a truss of tubes with a non-structural covering. A pure monocoque (an eggshell) relies on the skin alone and is too heavy at airliner size. Nearly every transport aeroplane uses semi-monocoque, stressed-skin construction: the skin carries loads together with a framework of longitudinal and transverse members.

• Wings are built around a wing box of front and rear spars, ribs and stiffened upper and lower skin panels, which also forms the integral fuel tanks. Large skin panels are often machined from solid with integral stiffeners.
• Empennage surfaces use the same spar–rib–skin principle on a smaller scale, increasingly in carbon-fibre composite.
• Attachments — the wing box joins the fuselage centre wing box through heavy fittings and frames; the tail attaches to reinforced frames and the aft pressure bulkhead area; engines hang from pylons fixed to the wing by fittings that often include fuse pins, designed to shear in an extreme overload so the engine breaks away cleanly without rupturing the fuel tank.

Joining the structure: riveting, bolting and bonding

• Riveting — solid aluminium-alloy rivets (access to both sides) for most skin and stringer joints; blind rivets where only one side can be reached; high-strength Hi-Lok/Hi-Lite pins with collars whose hex breaks off at the correct torque, and swaged lockbolts. Interference-fit fasteners improve fatigue life. Fasteners are often installed wet with sealant for corrosion protection.
• Bolting — high-strength steel or titanium bolts at highly loaded and removable joints such as wing-to-fuselage, gear and engine attachments, with the correct torque and locking.
• Adhesive bonding — metal-to-metal bonding (doublers, stiffened panels) and sandwich panels (aluminium or Nomex honeycomb core between skins); composite parts are co-cured or bonded. Bonded joints spread load smoothly and have no fastener holes to start cracks, but need very clean, controlled surface preparation.

Protecting the surface: corrosion protection

Dissimilar metals (and carbon fibre against aluminium) are kept apart by sealant, primer or barrier tape to prevent galvanic corrosion.

Cleaning the airframe

Dirt holds moisture and salts against the structure and hides cracks and leaks. Use only cleaning agents approved in the AMM for each material — wrong alkaline or acidic cleaners attack aluminium, cadmium plating and transparencies, and some solvents damage composites and sealants. Before washing, protect pitot and static ports, AOA vanes, air intakes, wheels and brakes; avoid high-pressure water on bearings, seals and hinges; rinse thoroughly; and re-lubricate after washing as the AMM requires. Contaminated surfaces must be cleaned before any inspection or NDT.

Airframe symmetry: alignment and symmetry checks

After a heavy or overweight landing, severe turbulence, a major structural repair or an accident, the airframe's alignment may be checked against the manufacturer's figures:

1. Level the aircraft on jacks — using a spirit level or inclinometer on the specified levelling points, or a plumb bob over a levelling grid plate.
2. Dihedral — measured with an inclinometer or dihedral board at stated wing stations.
3. Incidence of wings and tailplane — measured with an incidence board or inclinometer at rigging points.
4. Fin verticality — checked with a plumb bob or inclinometer.
5. Symmetry — steel-tape measurements from datum points on the fuselage centreline to matching points on each wing tip and tailplane tip; left and right must agree within the stated tolerance.

Large modern aircraft are increasingly surveyed with theodolites or laser trackers, but the principle is unchanged: compare left with right and with the manufacturer's datum figures.