This document reports on the rationale and development process undertaken to construct a tool for assessing the impact of change in automated Air Traffic Management (ATM) systems on mental workload. The tool has been named Assessment of the Impact on Mental workload (AIM)'. Findings and conclusions from a pilot validation study carried out to examine the preliminary validity of AIM are also provided. This deliverable has been developed as part of the Solutions for Human-Automation Partnerships in European ATM (SHAPE)' Project conducted within the Human Factors Sub-Programme (HSP) of the EATM Human Resources Programme (HRS) by the Human Factors Team of the EUROCONTROL Human Factors Management Business Division (DAS/HUM) under the supervision of the EATM Human Resources Team (HRT).