What are the common signs of metal fatigue in aircraft structures?

The common signs of metal fatigue in aircraft structures primarily include cracks, discoloration, and surface deformation because these indicators directly reflect the material's response to repeated stress and strain. Metal fatigue occurs when a material is subjected to cyclic loading, which can lead to microscopic cracks that grow over time.

Cracks are one of the most critical signs, as they can compromise the structural integrity of the aircraft. Discoloration may indicate oxidation or other chemical changes within the material, often as a result of environmental exposure or stress. Surface deformation, which can manifest as warping or bulging, suggests that the structure has undergone significant mechanical stress that could worsen over time if not addressed.

In contrast, increased fuel consumption, engine overheating, and poor cabin pressure are not direct indicators of metal fatigue. These issues may arise from factors such as engine performance, aerodynamics, or environmental conditions, but they do not directly correlate with the fatigue process in the structural materials of the aircraft. Understanding these signs is vital for maintenance and inspection practices to ensure aircraft safety and longevity.