Spare part A spare part , spare , service part , repair part , or replacement part , is an interchangeable part that is kept in an inventory and used for the repair or replacement of failed parts . Spare parts are an important feature of logistics management and supply chain management , often comprising dedicated spare parts management systems . Capital spares are spare parts although acknowledged to have a long life or a small chance of failure would cause shutdown of equipment for a prolonged period because of the long delivery of their replacement . Spare parts are an outgrowth of the industrial development of interchangeable parts and mass production . Classification . In logistics , spare parts can be broadly classified into two groups , repairable s and consumable s . Economically , there is a tradeoff between the cost of ordering a replacement part and the cost of repairing a failed part . When the cost of repair becomes a significant percentage of the cost of replacement , it becomes economically favorable to simply order a replacement part . In such cases , the part is said to be " beyond economic repair " ( BER ) , and the percentage associated with this threshold is known as the BER rate . Analysis of economic tradeoffs is formally evaluated using Level of Repair Analysis ( LORA ) . Repairable . Repairable parts are parts that are deemed worthy of repair , usually by virtue of economic consideration of their repair cost . Rather than bear the cost of completely replacing a finished product , repairables typically are designed to enable more affordable maintenance by being more modular . This allows components to be more easily removed , repaired , and replaced , enabling cheaper replacement . Spare parts that are needed to support condemnation of repairable parts are known as replenishment spares . A rotable pool is a pool of repairable spare parts inventory set aside to allow for multiple repairs to be accomplished simultaneously . This can be used to minimize stockout conditions for repairable items . Consumable . Parts that are not repairable , are considered consumable parts . Consumable parts are usually scrapped , or " condemned " , when they are found to have failed . Since no attempt at repair is made , for a fixed MTBF , replacement rates for consumption of consumables are higher than an equivalent item treated as a repairable part . Because of this , consumables tend to be lower cost items . Because consumables are lower cost and higher volume , economies of scale can be found by ordering in large lot sizes , a so - called Economic order quantity . Repair cycle . From the perspective of logistics , a model of the life cycle of parts in a supply chain can be developed . This model , called the repair cycle , consists of functioning parts in use by equipment operators , and the entire sequence of suppliers or repair providers that replenish functional part inventories , either by production or repair , when they have failed . Ultimately , this sequence ends with the manufacturer . This type of model allows demands on a supply system to ultimately be traced to their operational reliability , allowing for analysis of the dynamics of the supply system , in particular , spare parts . Inventory management . Cannibalization . When stockout conditions occur , cannibalization can result . This is the practice of removing parts or subsystems necessary for repair from another similar device , rather than from inventory . The source system is usually crippled as a result , if only temporarily , in order to allow the recipient device to function properly again . As a result operational availability is impaired . Commercial . Industrialization has seen the widespread growth of commercial manufacturing enterprises , such as the automotive industry , and later , the computer industry . The resulting complex systems have evolved modular support infrastructures , with the reliance on auto parts in the automotive industry , and replaceable computer modules known as FRU s . Military . Military operations are significantly affected by logistics operations . The system availability , also known as mission capable rate , of weapon system s and the ability to effect the repair of damaged equipment are significant contributors to the success of military operations . Systems that are in a mission - incapable ( MICAP ) status due lack of spare parts are said to be " awaiting parts " ( AWP ) , also known as not mission capable due to supply ( NMCS ) . Because of this sensitivity to logistics , militaries have sought to make their logistics operations as effective as possible , focusing effort on operations research and optimal maintenance . Maintenance has been simplified by the introduction of interchangeable modules known as line - replaceable unit s ( LRUs ) . LRUs make it possible to quickly replace an unserviceable ( failed ) part with a serviceable ( working ) replacement . This makes it relatively straightforward to repair complex military hardware , at the expense of having a ready supply of spare parts . The cost of having serviceable parts available in inventory can be tremendous , as items that are prone to failure may be demanded frequently from inventory , requiring significant inventory levels to avoid depletion . For military programs , the cost of spare inventory can be a significant portion of acquisition cost . In recent years , the United States Department of Defense ( DoD ) has advocated the use of performance - based logistics ( PBL ) contracts to manage costs for support of weapon systems .