Which type of maintenance involves significant repairs and upgrades?

The type of maintenance that involves significant repairs and upgrades is rework. Rework refers to the process of repairing or modifying equipment or systems that do not meet the required standards or specifications. This type of maintenance often involves extensive efforts to diagnose issues, disassemble equipment, and make necessary replacements or upgrades to bring the system back to optimal working condition.

Rework encompasses everything from enhancing existing systems to ensuring compliance with updated standards, making it a vital process within aviation maintenance where safety and efficiency are paramount. This contrasts sharply with the other maintenance types, which may involve routine checks, preventive measures, or less intensive repairs that do not necessitate comprehensive overhauls or upgrades.