This paper presents a simple methodology for measuring the impact on flight inefficiency of future Remotely-Piloted Aircraft Systems (RPAS) operations. State-of-the-art inefficiency metrics have been selected and adapted to this particular case in which the analysed traffic is not real but simulated. Then, three different simulation exercises encompassing a total of eight simulation runs have been performed in which an in active Air Traffic Controller (ATCo) was involved. Preliminary results show that the use of these metrics is acceptable to measure the impact of RPAS operations on the flight inefficiency of surrounding traffic and lays the foundation to extend the simulation exercises to get statistically significant outcomes.