What type of modifications generally require a Supplemental Type Certificate?

A Supplemental Type Certificate (STC) is required for changes that significantly modify the type design of an aircraft. Such modifications can include alterations that affect the aircraft's performance, safety, or operational characteristics. These changes are critical because they may impact how the aircraft operates under various conditions, including handling, aerodynamic performance, and compliance with safety regulations.

In contrast, minor cosmetic changes do not usually affect the performance or safety of the aircraft, and thus do not necessitate an STC. Routine maintenance checks are part of regular upkeep and are prescribed by regulations to ensure that the aircraft remains airworthy without requiring a change in type design. Upgrades to navigation systems may or may not require an STC depending on the extent and nature of the changes; however, not all upgrades alter the fundamental performance or design of the aircraft.

Thus, modifications that impact the aircraft’s overall performance or structural integrity are indeed the type of changes that necessitate obtaining a Supplemental Type Certificate.