This paper is meant to give guidelines on how to perform supply base rationalization at KLM Engineering & Maintenance. A methodology is written to help KLM Engineering & Maintenance to outsource the management of expendables to first tier suppliers. The desired level of outsourcing is determined by using the essentiality model of expendables. The essentiality of expendables identifies the strategic importance of the activity, which determines the preferred level of outsourcing. Outsourcing the management of expendables will create a leaner supply base. By using the essentiality model it is found that 35% of the expendables can be outsourced using some degree of hierarchical steering, 48% can be outsourcing using market steering and 17% do not need to be outsourced.