critical parts - have an essential importance in production continuity; value of the part is usually very high, and losses that its lack in stock can bring is very serious; this part is often required one-off and in a few units, due to the nature of repair, that is why so important is decision about the size of an order; part is usually a kind of protection, which at the time of failure have to be in stock, because their lack causes high financial penalties as a result of production line stoppage;