A Fleet Replacement Squadron (FRS), is a unit of the United States Navy or Marine Corps that trains Naval Aviators, Naval Flight Officers (NFOs) and enlisted Naval Aircrewmen on the specific front-line aircraft they have been assigned to fly. Students, referred to as Replacement Pilots, Replacement Flight Officers or Replacement Aircrew are either newly winged aviators (Category I), aviators transitioning from one type aircraft to another (Category II), or aviators returning to the cockpit after a period of non-flying (Category III). After completing the training regimen, graduates are assigned to fleet squadrons. Additionally, FRSs are responsible for training aircraft mechanics, providing replacement aircraft for fleet squadron attrition, and standardizing maintenance and aircraft operations.
The U.S. Navy and U.S Marine Corps sometimes share FRSs. For example, up until the recent establishment of an FRS for the Navy's new CMV-22 Osprey aircraft USN personnel were trained by the USMC's VMMT-204, and all USMC F-35C pilots and maintenance personnel are trained by the USN's VFA-125. In the past, USN H-53 students were once trained at the USMC H-53 FRS and F/A-18C/D students of both service were trained in either the USMC F/A-18C/D FRS or by one of the USN's F/A-18C/D FRSs. An FRS which trains students of both services will be staffed with instructors of both services as well.
From 1958 to 1970 the FRSs for carrier based aircraft were organized under Readiness Carrier Air Group FOUR (RCVG-4) or Readiness Antisubmarine Carrier Air Group FIFTY (RCVSG-50) in the Atlantic Fleet and Readiness Carrier Air Group TWELVE (RCVG-12) or Readiness Antisubmarine Carrier Air Group FIFTY ONE (RCVSG-51) in the Pacific Fleet. These Readiness Carrier Air Groups were colloquially called "Readiness Air Groups" or "RAGs" for short. In 1963 RCVG-4 and RCVG-12 were redesignated Readiness Carrier Air Wings RCVW-4 and RCVW-12 which had no effect on the use of the term "RAG" and in 1970 they and the remaining RCVSG-50 and 51 were disestablished which also had no effect on the use of the term "RAG" and to this day Fleet Replacement Squadrons are still referred to as "RAGs".
Note: The parenthetical (1st) or (2nd) appended to some squadron designations in the tables below are not a part of that squadron's designation. They are added to indicate that the designation was used more than one time during the history of U.S. Naval Aviation and to specify which use of the designation is indicated.