14 CFR § 121.125

Each certificate holder conducting supplemental operations must show that it has—

An approved flight following system established in accordance with subpart U of this part and adequate for the proper monitoring of each flight, considering the operations to be conducted; and

Flight following centers located at those points necessary—

To ensure the proper monitoring of the progress of each flight with respect to its departure at the point of origin and arrival at its destination, including intermediate stops and diversions therefrom, and maintenance or mechanical delays encountered at those points or stops; and

To ensure that the pilot in command is provided with all information necessary for the safety of the flight.

A certificate holder conducting supplemental operations may arrange to have flight following facilities provided by persons other than its employees, but in such a case the certificate holder continues to be primarily responsible for operational control of each flight.

A flight following system need not provide for in-flight monitoring by a flight following center.

The certificate holder's operations specifications specify the flight following system it is authorized to use and the location of the centers.