Is a Supplemental Type Certificate (STC) a major or minor alteration? This is one of those questions that can easily disappear down the aviation rabbit hole. However, with some basic knowledge on aircraft alterations and certifications, it can also number among the easiest to answer.
A majority (pun intended) of people believe all STCs are major alterations simply due to its name. Another group believes that if a part comes with STC paperwork, it always requires completing a Form 337.
Fortunately for us, from a maintenance point of view, STCs and aircraft alterations are as close to an apple-to-orange comparison as we have in aviation.