The owner or operator is responsible for maintaining the aircraft in an airworthy condition.
The primary responsibility for maintaining an aircraft in an airworthy condition falls on the owner or operator of the aircraft. Certain inspections must be performed on the aircraft, and the owner must maintain the airworthiness of the aircraft during the time between required inspections by having any defects corrected. Airworthy means the aircraft conforms to its type design and is in a condition for safe operation.
14 CFR 3.5 Statements about products, parts, appliances and materials.
14 CFR 91.403 General (Maintenance)
FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge pg. 9-8