very few modifications would make the cut if the only considerations were economic. If the payback period is long, measures providing only modest cost savings—say, through fuel savings—can become economically attractive. Unfortunately, the length of time a particular MDS will remain in the inventory from today is not predictable with much confidence.

The Air Force has a planned retirement schedule for all MDSs based on a number of important factors, only one of which is the estimated airframe life in hours. Other factors include such things as the vintage of the component technology, which affects reliability, the availability of spares, the viability of the vendor base, and other factors that in turn affect not only the estimates of future support costs but also, in some cases, the operational suitability of the aircraft for modern conflict. Aircraft of the class examined by this study typically have long airframe lives, about 30,000-50,000 hours. Considering that a typical utilization rate for these aircraft might be ~500 hours per year, they could be around for 60 to 100 years, if and only if estimated airframe life were the only consideration, which it is not. The other considerations typically lead to the much earlier planned retirement of a particular MDS. These plans become very volatile, however, as budgetary pressures preclude replacement of the MDS with more modern airframes.

Table 2-1 shows the inventory of various MDSs and illustrates the above points by estimating the remaining life assuming 30,000 hours airframe life for those aircraft with very stressful missions and