The European Organisation for the Safety of Air Navigation (EUROCONTROL) ASM Improvement Initiative aims to deliver concrete ASM improvements in 2009 and 2011. One of the objectives is to implement new procedures to support the optimisation of airspace usage for both civil and military airspace users. This Preliminary Safety Case (PSC) Report provides some of the evidence to support the top-level claim that the activation/deactivation of airspace via application of ASM/ATFCM Procedure 3 contributes to the achievement of an acceptable level of safety in the operating environment within which it is implemented.