The consideration of spares availability for the operational phase rarely impacts system design during the development phase. However, the development and operational scenarios drive maintenance, support and test equipment, and sparing requirements. Operational Availability (Ao) is impacted by prime and support and test equipment spares availability, while the need for spares is a direct result of component reliability. Quality of design and manufacture are part of that reliability picture.
Once the system is designed to minimize failures, there are several approaches used to ensure supply support objectives are met:
(1) The spares, or replacement items, have at least the same quality and reliability as contained in the original equipment.
(2) The spares stock at both the organizational and intermediate maintenance levels is based on the results of analyses as to the relative mission critically for each replaceable item.
Considerable progress has been made in recent years toward acquisition of spares with satisfactory quality and reliability. This has been done by properly including in the spares acquisition specifications and drawings the risk reduction techniques and quality manufacturing standards used for the prime hardware. For example, the tailored incoming inspection and environmental stress screening provisions determined during the system FSD should be used, as appropriate, for spares acquisition.
Another factor that can be significant relative to spares quality is degradation due to storage or the dormant reliability factor. Many materials and devices degrade during storage life. The cost-effective approach of buying all spares needed until phase out while a production line is running is no longer valid for these cases. Studies must be made to determine how much storage time (degradation) is tolerable.
Having sufficient spares at each level of maintenance while minimizing inventory costs is one of the most difficult challenges in supply support. The issues of which items to spare and the quantity to spare are always complicated by limited funds and changing environments. To handle this complexity, the Services have or are developing modeling techniques to assist in highlighting spares requirements based on operational demands in peacetime and under war plan taskings. The integrated efforts of war planners, users, designers, manufacturers, and commodity managers are necessary to provide the requisite operational availability with constrained funds.