Some concern was expressed by the Air Force about problems of logistics support for the JT3D engines.
Fluctuations in refueling demand as well as in demand to support general-purpose forces might have dictated the time frame during which re-engining was feasible.
Congress focused the JT3D re-engining program on the KC-135s in the Air National Guard and the Air Force Reserve. The Guard and Reserve, however, did not maintain backup aircraft in their inventory. Thus, unless JT3D re-engining continued, there would have been no backup KC-135E aircraft.
CBO examined three approaches for increasing the Air Force’s tanker capability:
Continue the current CFM56 re-engining program at the maximum rate of six per month, for a total of 334 additional re-engined KC-135R aircraft.
Continue the CFM56 re-engining program at a reduced maximum rate of four per month, for a total of 334 re-engined KC-135R aircraft.
Combine the JT3D and CFM56 re-engining programs at a maximum rate of six per month, for a total of 334 additional re-engined aircraft—166 KC-135Es and 168 KC-135Rs.
The combined JT3D/CFM56 approach offered more capability in the near to mid term (through 1992). Initially, this approach also cost significantly less ($4.3 billion over 4 years as opposed to $7.1 and $7.4 billion for the 6 and 4 per month CFM56 approaches, respectively). Also, having the CFM56 and JT3D programs ongoing could offer some competitive pressure to keep costs down. However, in the long run, the combined alternative would provide about 50 fewer KC-135A equivalents than either approach involving the pure CFM56. Also, the age of the JT3D engines and variability among them might have made them more difficult and costly to maintain.
The pure CFM56 re-engining approaches also offered advantages. The CFM56 was a brand-new engine, making it inherently more capable. Moreover, in the long run, the CFM56 might have cost no more than the combined JT3D/CFM56 approach. Air Force estimates of LLCs suggested that, over 20 years, the cost would have been about the same. Finally, the CFM56 was quiet and met the noise and emissions standards that applied to nonmilitary aircraft.