The carriage of ACAS is mandatory in European airspace, and has been demonstrated to reduce the risk of midair collision through the resolution advisories (RAs) that it issues. The FARADS programme is assessing the feasibility of downlinking RAs to controllers. Part of the assessment is an investigation of whether downlinking RAs will further reduce the risk of midair collision. The current study has developed a PC based ‘contingency tree’ tool which, when supplied with accurate event probabilities, will enabled any safety benefit to be evaluated.