Spare parts shortages on the three Air Force systems GAO reviewed have undermined the performance of assigned missions and the economy and efficiency of maintenance activities. Specifically, the Air Force did not meet its mission-capable goals for the E-3 or C-5 aircrafts during fiscal years 1996-2000, nor did it have enough F-100-220 engines to meet peacetime and wartime goals during that period. These shortages may also affect personnel retention. GAO recently reported that the lack of parts and materials to successfully complete daily job requirements was one of six major factors causing job dissatisfaction among military personnel. Item managers at the maintenance facilities often indicated that spare parts shortages were caused by the inventory management system underestimating the need for spare parts and by delays in the Air Force's repair process as a result of the consolidation of repair facilities. Other reasons included difficulties with producing or repairing parts, reliability of spare parts, and contracting issues. The Air Force and the Defense Logistics Agency have planned or begun many initiatives to alleviate shortages of the spare parts for the three systems GAO reviewed.