Which category of certified aircraft operator cannot be approved for return to service after a major repair or alteration?

A 14 CFR Part 121 operator cannot be approved for return to service after a major repair or alteration. This regulation governs the operation of scheduled air carriers and outlines stringent safety requirements and operational standards that must be adhered to, as these operators hold a higher responsibility for public transportation of passengers and cargo.

Return to service after a major repair or alteration for Part 121 operators must follow very specific procedures, including the need for a higher level of oversight and documentation. This is due to the complexity and size of the aircraft typically operated under Part 121, as well as the potential risk associated with their operations. Furthermore, Part 121 operators must rely on an FAA-designated airworthiness representative or utilize an approved Maintenance Program, ensuring compliance with rigorous standards and maintaining safety.

In contrast, operators under 14 CFR Parts 125, 133, and 145 have different regulatory requirements. Part 125 applies to commuter and on-demand operations, Part 133 relates to rotorcraft for hire, and Part 145 refers to repair stations. While these categories have their own specific obligations, they don't face the same return to service constraints as Part 121 operators due to different operational contexts and regulatory structures.