Aircraft Fatigue and Corrosion: Causes, Detection, and Prevention
Aircraft structures are continuously exposed to environmental factors and operational stresses that can lead to degradation over time. Two primary forms of degradation are corrosion, a chemical deterioration of materials, and fatigue, a weakening caused by repeated loading. Understanding the causes, signs, and prevention of these phenomena is essential for maintaining airworthiness.
Corrosion
Corrosion is the deterioration of a metal due to a chemical or electrochemical reaction with its environment. The rate and severity depend on the metal type and environmental conditions.
Indicators of Corrosion
- Aluminium and Magnesium: Corrosion deposits typically appear as a white or grey powder. Small white spots may indicate pitting, which can be very structurally damaging to an airframe.
- Ferrous Metals: Corrosion varies from red to dark reddish-brown stains (rust).
- Paintwork: Discolouration of paint can also be an indicator.
- Visual Signs: Paint wear, bulges, and impact dents are often cosmetic but can hide or lead to localized stress.
Factors Affecting Corrosion Rate
Five main factors influence the speed of deterioration in atmospherically exposed metals:
- Temperature: Generally, a 10°C increase in temperature doubles the corrosion rate.
- Humidity: Has a direct effect; higher humidity leads to a higher corrosion rate.
- Rainfall: Increases humidity, accelerating corrosion.
- Pollution (Sulphur Dioxide): Accelerates corrosion damage.
- Air Salinity: High over oceans at low altitude. Salt build-up mixed with water creates electrolytes that aggressively attack aluminium and magnesium.
Corrosive Substances
Certain substances are inherently corrosive to aircraft materials:
- Mercury: Highly corrosive to aluminium.
- Battery Acid & Toilet Waste: Highly corrosive mixtures.
- Water: Causes oxidation and rust.
- Hydraulic Fluids: Can be corrosive to paintwork, sealing compounds, and rubber materials (though designed to be non-corrosive to metals). They should not be mixed and are skin/eye irritants.
Stress Corrosion
Stress corrosion is the result of the combined effect of sustained tensile stresses and a corrosive environment.
- It is particularly characteristic of aluminium, copper, and high-strength alloy steels.
- It can cause premature failure and visible cracking.
- Cracking and buckling (indicating compression) are signs of continuous stress exceeding yield strength, often resulting from events like hard landings.
Prevention
Operators must follow a constant cycle of cleaning, inspection, and lubrication to prevent corrosion.
- Regular Washing: Essential to remove acidic contaminants and salt deposits, especially when operating in marine atmospheres or warm, humid climates.
- Washing the aircraft (or at least sensitive areas like engines and landing gear) with fresh water helps prevent corrosion significantly.
Fatigue
Fatigue is the weakening of a material caused by repeated (cyclic) stresses or loadings, which can lead to failure at a load much lower than the material's static strength.
- Cyclic Loads: Examples include the pressurization cycle (fuselage expansion/contraction) and landing impacts.
- Life Limit: An aircraft that performs more cycles (e.g., short flights) will reach its fatigue life limit sooner than one flying fewer, longer sectors. For example, an aircraft doing six 30-minute flights per day accumulates fatigue faster than one doing fewer long flights.
- Detección: Fatigue cracks are typically detected visually during scheduled maintenance. Systems like HUMS (Health and Usage Monitoring Systems) monitor mechanical health but are not the primary method for detecting airframe fatigue cracks.
- Effect of Ageing: Ageing leads to an increase in scratches, dents, and dirt on the airframe surface, which increases skin friction drag by disturbing laminar airflow.