FAR 1 defines a major repair as a repair that, if not performed correctly, can significantly impact the weight, balance, structural strength, performance, operation of the power plant, flight characteristics or other qualities affecting airworthiness. This type of repair must be done in accordance with accepted practices and cannot be done without extensive review and testing. The Federal Aviation Administration (FAA) allows for major modifications to certified aircraft through two main regulatory channels: Form 337 or a Supplemental Type Certificate (STC). During the inspection of major repairs or alterations, the owner of an aircraft must also ensure that they are compatible with any previous repairs and modifications that have been made to the aircraft.
Minor repairs cover everything else, except for major repairs specified in the aircraft maintenance manual, as well as any repair work affecting weight and balance, structural strength, performance, operation of the power plant and flight characteristics. The manufacturer can only rebuild or modify the aircraft unless it is also certified as a repair station. A major alteration is an alteration that is not listed in the specifications of the aircraft, the aircraft engine or the propeller. It is the responsibility of the owner of an aircraft to determine airworthiness by inspecting repairs or alterations to verify that they comply with approved data and ensure that the aircraft is able to operate safely.
When performing a major repair or alteration on an aircraft, it is important to obtain approved data from the FAA or a future purchaser of the aircraft. The owner must also provide a maintenance authorization signed by an authorized repair station representative and enter all relevant information.