Definition
Progressive structural weakening of metal caused by repeated cycles of stress, even when each individual stress is well below the metal's breaking strength. Over time, microscopic cracks form and grow with each load cycle, eventually leading to sudden failure of the part.
Plain English
Metal that is bent, flexed, or loaded over and over again gets tired. Each cycle does a tiny bit of damage that adds up. Eventually the part can break, even though no single load was ever heavy enough to break it on its own.
Context Anchor
Seen in weight and balance discussions when explaining why excessive weight, hard landings, turbulence, or repeated high loads can shorten the life of an aircraft structure.
Derivation
From Latin 'fatigare', meaning 'to tire out'. The same idea as a person becoming tired from repeated effort: the metal isn't overloaded once, it's worn down by being worked again and again.
Why Pilots Care
Unchecked metallic fatigue can cause sudden structural failure in flight-critical components such as wings and fuselage.
Analogy
Bend a paperclip back and forth. The first bend does nothing visible. After enough bends it snaps. No single bend was strong enough to break it; the repetition was.
Grounding Statement
Each repeated load can leave a tiny amount of damage in the metal until a crack begins or grows.
Intuition Check
Metallic fatigue does not mean the aircraft is simply old or worn out. It means metal has been weakened by repeated loads over time.
Example Sentence 1
Repeatedly flying the aircraft over its maximum gross weight contributes to metallic fatigue in the wing spars and landing gear.
Example Sentence 2
Higher operating weights accelerate metallic fatigue in the wing structure over the aircraft's service life.