Metal fatigue is a reasonably well understood phenomenon, that is managed by the aircraft inspection cycle. The actual crack appearance rate diverging from the

Metal fatigue is a reasonably well understood phenomenon, that is managed by the aircraft inspection cycle. The actual crack appearance rate diverging from the modelled appearance rate at design time is a potential worry and indicates an issue somewhere along the line. How bad that issue actually is requires more investigation. It's a complex situation because metal fatigue is a statistical process, so you don't know if the cracks you've found are the best case or worst case.