The product categories are Structures, Avionics, Instruments, Engines, Propellers and Components. All product ratings fall under one or another of these categories, which are arranged to be mutually exclusive.The process categories are NDT and Welding and these are the only two processes appearing in the list of specialized work.
In the product categories, AMOs are rated by the types of product they are authorized to maintain, and the permitted scope of work is indicated by limitations. The maximum scope of work for a product rating is "all work other than specialized NDT and welding." Process AMOs are rated according to the processes they may perform, (i.e. an NDT AMO may have ratings for magnetic particle inspection and radiography).
Process AMOs may perform work on any type of aircraft or product. The distinction between the product and process privileges means that, for example, while a propeller AMO may be authorized to perform overhauls, if the overhaul includes NDT tasks, the AMO must also hold an NDT rating, or it will be obliged to sub-contract the NDT tasks to an AMO that does.
In addition to the organizational benefits they share with Aircraft AMOs, specialized AMOs also ensure the additional level of skill needed to perform specialized work, which is required regardless if the aircraft is commercial or private. Specialized AMOs may also perform non-specialized work within the bounds of their ratings, just as Aircraft AMOs may work on private aircraft. A large organization may hold ratings in several different categories, whereby the distinctions are transparent to the staff in everyday operation. However, they can quickly become important if enforcement action is contemplated.
To return to the example of bringing a small piston-powered aircraft into commercial air service, if the last engine overhaul was done by a freelance AME and not by an engine AMO, the misunderstanding comes about because until recently, the overhaul of small piston engines was on the list of specialized work. The work has now been de-classified to non-specialized. Most people understand that these engines can now be overhauled by freelance AMEs (provided, of course, they are to be installed in private aircraft). What is not so widely appreciated, is that the change in classification also means that, even if they are to be installed in a commercial aircraft, no specialized AMO is needed. The engines can instead be overhauled by an Aircraft AMO rated for the aircraft type, provided it has applicable procedures in its approved manual.
In the case cited, the aircraft is being acquired with the engine installed. The engine complied with all the applicable requirements at the time of installation. No special action is required, because the wording of the regulations makes clear that the need for an AMO depends on the circumstances existing at the time the work is done. The requirements are not retroactive. Thus, if an Aircraft AMO performs a non-specialized task, and that task is subsequently classified as specialized work, we do not have to have the task repeated by a specialized AMO. Similarly, if non-specialized work is performed on a private aircraft by an AME, and the aircraft subsequently enters commercial service, the work does not have to be repeated by an AMO of any kind.
So, when is a specialized AMO needed? Hopefully, this discussion helped to explain some of the scenarios found in the various AMOs and will serve as a guide when planning and scheduling maintenance in the future.