The Pratt & Whitney Aircraft JT9D engine has recently entered commercial service after an intensive program of development and flight testing. The engine design represents a significant advance over current day engines in terms of achieving increased performance with reduced mechanical complexity, excellent thrust to weight ratio, improved maintenance capability through modular construction and outstanding capability for condition monitoring. This improvement has in the main resulted from the selection of an arrangement where each of two rotors is simply supported by two main bearings rather than the six or more in use in current engines. Experience to date has confirmed that the JT9D engine design has outstanding qualifications for successful commercial operation. Those difficulties encountered to date have been quickly resolved by means of fixes which were available as a result of the continuing JT9D improvement program. The operating record of the engine has been accordingly good, and there is every reason to believe that its rugged design will provide the basis for a highly successful career.