The majority of procedures and rules in aviation are written in blood. We design and build the safest aircraft possible, given the best available knowledge and skill. Inevitably, an unforeseen circumstance will cause a catastrophic wing failure. Every failure and accident must be learned from to prevent a repeated incident.
A perfectly designed wing is still no better than the materials it is made of. Aviation went through a steep learning curve with wood, fabric and steel construction. This learning curve started over when alloys of aluminum became the standard material. Impurities in the alloy, inter-granular corrosion, cracks and heat treating errors cause failures. Because development never stops, we will never run out of lessons to learn. As wings made of composite materials get older, more lessons are on the horizon. De-lamination of the composite plies are still difficult to detect and very dangerous.
Pure aluminum cannot corrode, but it's not strong enough to be used anywhere in a wing. Only alloys of aluminum can be used and corrosion is a constant threat. Several types of corrosion attack metals. All of them are a threat to structural integrity.
Metal fatigue and crack propagation are perhaps the biggest danger. The internal structural components, spars and ribs, must be incredibly strong yet light and flexible. An in flight failure of a spar rarely has a good outcome. Thankfully, careful inspections normally detect cracks before they cause a failure. A wing must be allowed to twist and bend during the stresses encountered in flight. The wing tips of a B 52 flex up and down twelve feet during routine flights. Vibration and flutter cause reductions in strength. All wings have a limit on the number of takeoff and landing cycles because of these risks.
Aircraft wings are flying fuel tanks. The electrical system adds an extreme risk of explosion. A full fuel tank is actually very safe. As the tanks empty, vapor space in the tank can become a very dangerous and explosive environment. TWA Flight 800 was lost because of ignition of the flammable fuel/air mixture in the center wing tank. This hazard is greatly reduced by systems which replace air with nitrogen.
Overweight or hard landings can potentially overstress a wing. Severe turbulence and excessive maneuvers in flight can cause excessive wing loading and jeopardize safety. Pilots must keep maintenance personnel informed of these occurrences.
Even a bird strike can cause major damage. A Bombardier Dash 8 was almost lost when a goose tore through the leading edge. Electrical and hydraulic systems in the wing were completely destroyed. Only pilot skill and system redundancy saved the aircraft.
Wings contain many moving parts. Control mechanisms, control cables, electrical and hydraulic systems can all fail and cause loss of an aircraft. Ailerons, flaps and spoilers can cause loss of control when they fail.
The fields of design, materials, fabrication and maintenance must all work hand in hand in order to have a safe wing. Metal fatigue and cracks, impacts, corrosion, vapor ignition, stress, deficient design and faulty maintenance are all sources of problems. An oversight in any of these areas can cause a catastrophic wing failure. Service Bulletins and Airworthiness Directives apply new knowledge and information to existing aircraft. All previous experience is carried forward into the new designs. This process enables us to learn from every failure and use this knowledge to build and maintain safer aircraft. No loss of aircraft or life should be in vain.