So, here the FAA tells us that it is the alterations to the main structure of the aircraft that make an alteration important. Unless you are modifying or attaching something to a primary structure, it will have no appreciable effect on the structural strength of the aircraft. In the case of balloons, carrying out small fabric repairs to the envelopes (as defined in and in accordance with the balloon manufacturer's instructions) does not require repair or replacement of the charging tape. Xxxi) Repair of fabric covers involving an area greater than that required to repair two adjacent ribs.
A Form 337, Major Repair and Alteration (Fuselage, Power Plant, Propeller, or Apparatus) is the documentation required for one-off repairs or modifications to an individual fuselage, engine, or propeller.