What typically causes minor dents or holes in structural parts?

Minor dents or holes in structural parts are typically caused by collision or impact. This occurs when the aircraft, or its components, experience physical interactions that can result in localized damage. Examples include striking objects on the runway, ground handling incidents, or bird strikes. Such impacts can create visible damage that may not compromise the overall integrity of the structure but can be significant enough to warrant inspection and repair.

While corrosion over time, manufacturing defects, and improper maintenance procedures can lead to issues within structural components, they do not typically manifest as minor dents or holes. Corrosion results in material degradation rather than immediate surface damage, manufacturing defects involve flaws from the production process that may lead to structural weaknesses but not necessarily identifiable dents, and improper maintenance can lead to issues or failures but is less likely to produce minor surface damage directly. Thus, collision or impact is the primary cause of the visible physical alterations described in the question.