14 CFR § 121.363

Each certificate holder is primarily responsible for—

The airworthiness of its aircraft, including airframes, aircraft engines, propellers, appliances, and parts thereof; and

The performance of the maintenance, preventive maintenance, and alteration of its aircraft, including airframes, aircraft engines, propellers, appliances, emergency equipment, and parts thereof, in accordance with its manual and the regulations of this chapter.

A certificate holder may make arrangements with another person for the performance of any maintenance, preventive maintenance, or alterations. However, this does not relieve the certificate holder of the responsibility specified in paragraph (a) of this section.