Modifications and repairs to an aircraft, engine, or propeller constitute any alteration to the officially approved type design. This alteration can manifest physically through the installation of a new system or the replacement of a part with a new part number. However, it may also be confined to a revision of documentation.

Repairs, on the other hand, are the outcomes of design activities aimed at rectifying damage and/or restoring the aircraft/engine/propeller to an airworthy condition. This encompasses the evaluation of damage to determine whether continued operation is permissible without the necessity for physical repair.

In both cases, whether dealing with changes or repairs, obtaining approval from the Civil Aviation Authority (CAA) is mandatory. The method of approval is contingent on the classification of the specific change or repair and the availability of Design Organization Approval (DOA) privileges for use in the approval process.