Since blade nicks repair considered as a major repair, I would like to ask,
1- Who is authorized to repair engine blade nicks? 2- Could a mechanic with powerplant rating do the repair? 3- Could a sheet metal specialist do the repair ?
From 14 CFR 65.85, emphasis mine:
Except as provided in paragraph (b) of this section, a certificated mechanic with a powerplant rating may approve and return to service a powerplant or propeller or any related part or appliance, after he has performed, supervised, or inspected its maintenance or alteration (excluding major repairs and major alterations). In addition, he may perform the 100-hour inspection required by part 91 of this chapter on a powerplant or propeller, or any part thereof, and approve and return it to service.
So a mechanic with just a powerplant rating cannot approve the airplane for return to service after a major repair. This requires an IA. 14 CFR 65.95:
(a) The holder of an inspection authorization may—
(1) Inspect and approve for return to service any aircraft or related part or appliance (except any aircraft maintained in accordance with a continuous airworthiness program under part 121 of this chapter) after a major repair or major alteration to it in accordance with part 43 [New] of this chapter, if the work was done in accordance with technical data approved by the Administrator
The IA does not need to perform the repair themselves. Anyone working under the proper supervision can do the actual repair, from 14 CFR 43.3(d):
A person working under the supervision of a holder of a mechanic or repairman certificate may perform the maintenance, preventive maintenance, and alterations that his supervisor is authorized to perform, if the supervisor personally observes the work being done to the extent necessary to ensure that it is being done properly and if the supervisor is readily available, in person, for consultation. However, this paragraph does not authorize the performance of any inspection required by Part 91 or Part 125 of this chapter or any inspection performed after a major repair or alteration.
The IA needs to inspect the repair afterwards and make sure it conforms to requirements.
The manufacturer is also permitted to perform the repairs, per 14 CFR 43.3(j).