Definition
A weakening of a metal part caused by the gradual buildup of damage from repeated stress cycles over time. Each individual stress event may be well below the strength limit of the material, but the damage from each cycle adds together until tiny cracks form and eventually grow into a failure.
Plain English
Damage to metal that builds up slowly from being flexed or stressed over and over again. Any single bend is harmless, but the wear adds up, and after enough cycles the part can crack or break.
Context Anchor
Seen in aviation human factors, personal minimums, crew scheduling, and fitness-for-flight decisions.
Derivation
‘Cumulative’ comes from the Latin cumulare, meaning ‘to heap up.’ ‘Fatigue’ comes from the French fatiguer, meaning ‘to tire.’ Together they describe damage that piles up as the metal is ‘tired out’ by repeated loading — a useful image, because the part really does grow weaker with use, even when nothing dramatic has happened to it.
Why Pilots Care
It degrades judgment, reaction time, and decision-making even when no single duty period exceeded limits, raising the risk of operational errors.
Analogy
Bend a paperclip once and nothing happens. Bend it back and forth a few dozen times and it snaps — not because the last bend was harder, but because the damage from every bend added up.
Intuition Check
Do not think of cumulative fatigue as just “being tired today.” It means tiredness has been building because the body and mind have not fully recovered between days or duty periods.
Example Sentence 1
Helicopter rotor blades are retired at a set number of flight hours to prevent failure from cumulative fatigue.
Example Sentence 2
Regulations cap total flight hours in a 30-day period to reduce the chance of cumulative fatigue in commercial operations.