14 CFR § 65.81

A certificated mechanic may perform or supervise the maintenance, preventive maintenance or alteration of an aircraft or appliance, or a part thereof, for which that person is rated (but excluding major repairs to, and major alterations of, propellers, and any repair to, or alteration of, instruments), and may perform additional duties in accordance with §§ 65.85, 65.87, and 65.95. However, a certificated mechanic may not supervise the maintenance, preventive maintenance, or alteration of, or approve for return to service, any aircraft or appliance, or part thereof, for which that person is rated unless that person has satisfactorily performed the work concerned at an earlier date. If that person has not so performed that work at an earlier date, that person may show the ability to do it by performing it to the satisfaction of the Administrator or under the direct supervision of a certificated and appropriately rated mechanic, or a certificated repairman, who has had previous experience in the specific operation concerned.

A certificated mechanic may not exercise the privileges of that person's certificate and rating unless that person understands the current instructions of the manufacturer, and the maintenance manuals, for the specific operation concerned.