Fatigue accumulates through repeated loading
Pressurization cycles, takeoffs, landings and engine operation repeatedly stress structures and rotating parts. Microscopic cracks can grow even when each individual load is well within the material’s static strength.
Some failures are too critical to manage only by inspection
Engine disks and other highly loaded parts can have mandatory retirement lives because their failure consequences are severe. Once the approved life is reached, the part is removed regardless of appearance.
Other structures use damage-tolerance programs
Designers assume small defects may exist and establish inspection intervals that should detect growth before it becomes dangerous. Non-destructive testing can find cracks that are invisible to the eye.
Cycles can matter more than hours
A short-haul aircraft may pressurize and land several times each day, accumulating fatigue cycles quickly. A long-haul jet can fly many more hours with fewer takeoff-and-landing cycles.
Environmental exposure creates separate aging mechanisms
Corrosion, moisture, heat and chemical contamination can degrade components independently of fatigue. Calendar limits may therefore apply even to lightly used aircraft.
Limits can change as evidence improves
Fleet findings, laboratory testing or accident investigations can lead regulators and manufacturers to shorten intervals, mandate inspections or redesign parts. Continuing airworthiness is based on new information as well as original certification.
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