Definition
The gradual lengthening or deepening of a crack in an aircraft structural component over time, typically driven by repeated stress cycles such as flight loads, pressurization, vibration, or thermal changes. A small crack that initially seems insignificant can grow with each cycle until the affected part can no longer carry its design load.
Plain English
A crack in metal or composite parts of the airplane that slowly gets bigger every time the aircraft is flown or stressed. Each flight makes it a little worse until, eventually, the part can fail.
Context Anchor
Seen during preflight or maintenance inspection of wing surfaces, tail surfaces, skin panels, brackets, and attachment areas.
Derivation
Crack comes from Old English cracian, meaning to make a sharp sound or split. Progression comes from Latin progressio, meaning a moving forward step by step. Together the term describes a split that advances, step by step, rather than appearing all at once.
Why Pilots Care
Unchecked crack progression can reduce structural strength until a critical component fails in flight.
Intuition Check
Do not read “progression” as a good or normal kind of progress here. In this context, crack progression means damage is increasing.
Example Sentence 1
During preflight, the pilot found a hairline crack near a wing rivet and grounded the airplane, knowing crack progression could turn it into a structural failure.
Example Sentence 2
Regular inspections track crack progression so the tail spar can be replaced before it becomes unsafe.