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An STC introduces an alteration that was not covered by the airplane's
original type certificate. Installation of an STCed part or system must be
accomplished in accordance with the STC. This may be a major or minor
alteration, but as I understand it, it would in any case need to be signed
off by an A&P, and possibly an AI. In many cases a new weight and balance
would need to be calculated and recorded.

An IA must sign off the 337, which is required of any major alteration. An STC
provides authority to make the major alteration. A minor alteration requires
neither a 337 nor an STC.