The following definitions apply to this sub-chapter:

(1) "Aircraft ready for flight" means aircraft owned or acquired through rental or lease (but not interchange) which are in the possession of the taxpayer and are available for service on the taxpayer routes.

(2) "Cost of aircraft by type" means the average original cost or value of aircraft by type, which are ready for flight.

(3) "Departures" means for purposes of these rules all takeoffs, whether they are regularly scheduled or charter flights, that occur during revenue service.

(4) "Revenue service" means the use of aircraft ready for flight for the production of revenue.