Compound Operation Modes

In many practical applications, a system operates under a combination of regular operation and unsupported mission modes. It is usually difficult to formulate one analytical criterion for spare parts calculations in such situations. Instead, the usual practice is to perform several calculations for each operation mode, and then combine the results in the appropriate manner.

Example 1 An oceangoing vessel has a total mission time of 10 years. During this time, it sails for periods of two months, during which no spares supply is possible. In this case, the spares stocks computation is done in three steps:

1.	Calculate sail-away kits for two months’ unsupported mission.

2.	Calculate base spare parts stocks for 10 years’ regular operation.

3.	Overall spares stock is the sum of the quantities computed in steps (a) and (b).

Example 2 During wartime, a weapon system is used frequently and enjoys short repair turnaround times. The same weapon system, during peacetime, is used only occasionally and experiences long repair turnaround times. Spare parts planning for such a system can be accomplished as follows:

1.	Calculate spare parts quantities for peacetime operation.

2.	Calculate spare parts quantities for wartime operation.

3.	Total spares stock is the maximum of the quantities computed in steps (a) and (b).