Design Philosophy, Loads and Maintenance

Stress, strain and loads

Hang an aeroplane's weight on its wings, or its wings' lift on its fuselage, and every part in between must pass the force along. Two quantities describe that carrying:

Stress — the internal force per unit area inside a structural part as a result of external loads: the same total force through a slender fitting means more stress than through a thick one.

Strain — the deformation caused by the action of stress on a material: stressed parts stretch, compress or twist, usually by amounts far too small to see.

The two always travel together, and they are present both when the aircraft is parked and when it is manoeuvring: on the ground the structure carries its own weight, its fuel and its payload through the landing gear, and in flight every gust and every turn multiplies the loads the same parts must pass along.

The loads themselves come in three kinds, and a pilot has a hand in the size of each:

Static loads — loads that are constant or change slowly: the weight of structure, fuel and payload in a parked aircraft, or the steady 1g lift of cruise. The pilot's influence is in the loading itself: how heavily the aircraft is loaded sets the static baseline every other load adds to.

Dynamic loads — loads applied suddenly and briefly: a gust, a hard landing, an abrupt control input. These can peak far above the static level, and the pilot's influence is direct: the speed flown in turbulence (the gust envelope of the Principles of Flight course puts numbers on this), the landing technique and the smoothness of control inputs decide their magnitude.

Cyclic loads — loads applied and removed over and over: a pressurisation cycle every flight, a landing every sector, a control load in every turn. No single application threatens the structure; the repetition is what matters, because cyclic loads drive the fatigue this lesson returns to below. The pilot's influence is in how many cycles the aircraft flies and how severe each one is made.

Whatever produces a load, the member carrying it feels it in one of five basic forms:

Tension — the member is pulled from both ends and stretches: a bracing cable, or a bolt clamping two fittings together.

Compression — the member is pushed from both ends and squeezes: a strut holding two surfaces apart, or a shock strut at touchdown.

Shear — parallel forces try to slide one face of the member past the other: the working load of a rivet holding two overlapping sheets of skin.

Torsion — a twisting load turns one end of the member against the other: a shaft driven at one end against resistance at the other.

Bending — a beam loaded across its length bows, which stretches one side in tension and squeezes the other in compression: a wing carrying lift, or a floor beam carrying freight.

Real structures rarely feel just one form. Bending, as the list says, is tension and compression at once, and a single firm landing puts compression into the gear struts, bending into the wings, shear into thousands of rivets and torsion into the fuselage. Which members carry which of these loads, surface by surface, is the next lesson's subject; the point here is that every load on the aircraft ends up as stress in some part, whether the part shows it or not.

Designing for failure

Designers do not pretend structures last forever. For each part they choose a philosophy for how failure will be kept away from the aircraft, and the three structural design philosophies are exam vocabulary:

Safe life — the part is certified for a fixed life in flying hours, cycles or calendar time, set with a generous margin so that a crack is not expected even to appear before the life expires. At the limit the part is retired no matter how healthy it looks. This is the philosophy for parts whose failure cannot be tolerated and whose interior cannot practically be inspected, landing-gear members being the classic case.
Fail-safe (multiple load paths) — the load is shared among several members, so that when one fails the remaining members carry the load, at a reduced margin, until the failure is found at inspection and repaired. The structure tolerates the failure; the inspection programme guarantees the tolerance is temporary.
Damage-tolerant — the modern refinement: the design accepts that cracks will exist, keeps stress levels and materials such that cracks grow slowly and predictably, and schedules inspections so that any crack is caught in the long interval after it becomes detectable and before it becomes critical.

Certification

None of these choices is left to a manufacturer's taste. Every aircraft is certified against specifications determined by the competent authority, and those certification specifications cover aspects such as the design, the material quality and the build quality of the aircraft: a type that has not demonstrated compliance does not fly. The competent authority for European aviation is EASA, and for aeroplanes two of its rulebooks are named by the syllabus:

CS-23 — the certification specification for Normal, Utility, Aerobatic and Commuter Aeroplanes: the light singles and twins of training and touring;

CS-25 — the certification specification for Large Aeroplanes: the transport jets and turboprops of airline flying.

CS is short for certification specification. The distinction matters beyond the exam: much of what the coming lessons describe as standard equipment or required strength is a requirement of these specifications, and later lessons quote them wherever a rule explains a design (the two discharges required of an engine fire-extinguishing system, for example).

Fatigue

Fatigue is failure by repetition: a part loaded and unloaded, cycle after cycle, develops microscopic cracks at points of stress concentration, and each further cycle grows them a little, until a crack reaches the length at which the remaining material fails suddenly, at a load the intact part once carried with ease. The stress of any single cycle is nowhere near the material's static strength; the count is what does the damage. Fatigue is therefore what limits the useful life of an aircraft structure, and that life is counted in cycles, not just hours: a flight is one pressurisation of the fuselage and one landing on the gear whether it lasted forty minutes or thirteen hours.

Five factors decide how quickly fatigue develops:

Corrosion — corroded material is weaker, and its pits are ready-made stress concentrations, so cracks start earlier and grow faster. Fatigue and corrosion feed each other, which is why they are inspected for together.

Number of cycles — fatigue counts applications of load. A short-haul airframe flying eight sectors a day ages far faster, in fatigue terms, than a long-haul airframe of the same calendar age.

Type of flight manoeuvres — aggressive manoeuvring, training circuits and gusty low-level work all mean larger and more frequent load swings, and each swing is a fatigue cycle.

Stress level — the higher the stress amplitude of each cycle, the fewer the cycles a part survives, and the drop is steep: the relationship is strongly nonlinear. This is why overstressing an airframe once leaves damage out of all proportion to the single event.

Level and quality of maintenance — fatigue management assumes inspections happen and find what they look for. Missed inspections, poor repairs or undetected damage let cracks grow unwatched through the very window the design counted on.

Corrosion

Corrosion is the other ageing process: chemical attack, mostly by moisture and salt on aluminium alloys and steel, that converts strong metal into weak oxide. Its effects are thinning, pitting and weakening of structure, and its pits, as the fatigue list said, are where cracks like to start. During the pre-flight inspection corrosion shows itself visually as:

surface pitting and roughness — small dark-rimmed craters in the metal;
white or grey powdery deposits — the oxide left by corroding aluminium;
bubbling, blistering or flaking paint — corrosion lifting the finish from beneath;
stains and streaks around fasteners and skin joints — corrosion products carried out by moisture.

The risk of corrosion rises in particular operating environments: salt-laden marine air, hot and humid tropical climates, and industrially polluted atmospheres. The countermeasures are unglamorous and effective: wash and rinse the aircraft regularly (especially after operating near salt water), keep drain holes clear so moisture cannot pool inside the structure, hangar the aircraft where possible, and repair damaged paint and protective coatings promptly, because the coating is the barrier the chemistry must break first.

The environment is not the only source of attack: the aircraft carries highly corrosive fluids on board as part of its own systems and equipment. Battery electrolyte, lavatory fluids and waste, and some hydraulic fluids all attack structure wherever they leak or spill, which is why a stained battery bay or a streak below a toilet service panel is a maintenance item, not a cosmetic one.

The pre-flight inspection

A walkaround cannot see stress, only its footprints, so it concentrates on the areas where stress concentrates:

the landing gear and its attachment points — the members that take the touchdown loads;
the wing roots — where the wings' bending loads are handed to the fuselage;
engine mounts and pylons — thrust, weight and vibration all carried in one fitting;
control-surface hinges and fittings — small parts carrying full aerodynamic loads;
skin panels and rivet lines around doors, windows and repairs — edges and holes are stress concentrations by nature.

Even at those places, the visual cues of any deformation are limited: wrinkled or buckled skin, popped or missing rivet heads, the dark trail of a loose rivet working in its hole (mechanics call such a rivet a smoking rivet), cracked paint along a rivet line, or doors and panels that no longer sit flush. By the time deformation is visible from the outside, the damage inside is usually well ahead of it. Anything found is therefore a reason to call maintenance, not to look harder and go: the deeper checks use methods beyond eyesight.

Maintenance methods

That deeper work runs to schedule, and three scheduling methods decide when a part is worked on:

Hard-time (fixed-time) maintenance — the part is removed, overhauled or replaced at a fixed interval of hours, cycles or calendar time, regardless of its apparent condition: the scheduling mirror of the safe-life philosophy.
On-condition maintenance — the part stays in service as long as scheduled inspections and tests show it still meets its standards; its measured condition, not the calendar, decides its removal. A brake worn to its wear indicator comes off the aircraft this way.
Condition monitoring — no scheduled removal at all: the part runs while its parameters (oil analysis, vibration, performance trends) are watched, and the data decides when action is needed. Because nothing is scheduled, the method is only allowed where a failure would announce itself and could be tolerated until the trend catches it: neither hidden nor immediately hazardous. It suits fleets, where the statistics are large enough to trust.