14 CFR § 121.709

No certificate holder may operate an aircraft after maintenance, preventive maintenance or alterations are performed on the aircraft unless the certificate holder, or the person with whom the certificate holder arranges for the performance of the maintenance, preventive maintenance, or alterations, prepares or causes to be prepared—

An airworthiness release; or

An appropriate entry in the aircraft log.

The airworthiness release or log entry required by paragraph (a) of this section must—

Be prepared in accordance with the procedures set forth in the certificate holder's manual;

Include a certification that—

The work was performed in accordance with the requirements of the certificate holder's manual;

All items required to be inspected were inspected by an authorized person who determined that the work was satisfactorily completed;

No known condition exists that would make the airplane unairworthy; and

So far as the work performed is concerned, the aircraft is in condition for safe operation; and

Be signed by an authorized certificated mechanic or repairman except that a certificated repairman may sign the release or entry only for the work for which he is employed and certificated.

Notwithstanding paragraph (b)(3) of this section, after maintenance, preventive maintenance, or alterations performed by a repair station that is located outside the United States, the airworthiness release or log entry required by paragraph (a) of this section may be signed by a person authorized by that repair station.

When an airworthiness release form is prepared the certificate holder must give a copy to the pilot in command and must keep a record thereof for at least 2 months.

Instead of restating each of the conditions of the certification required by paragraph (b) of this section, the air carrier may state in its manual that the signature of an authorized certificated mechanic or repairman constitutes that certification.