Flight efficiency is of great concern in the Air Traffic Management (ATM) community since today's ATM inefficiencies affect both airspace users (AUs) and Air Navigation Service Providers (ANSPs). Each actor has their own vision of flight efficiency: whereas airlines are concerned mainly with aspects that impact their business strategy (fuel consumption, schedule adherence and cost), ANSPs consider other aspects such as sector capacity, Air Traffic Controller (ATC) interventions, emissions and noise. Capturing both visions in new Key Performance Indicators (KPIs) is important to take new steps towards more sustainable air traffic operations. Centro de Referencia I+D+i ATM (CRIDA), Boeing Research & Technology Europe (BR&TE), Centre for Applied Data Analytics (CeADAR) and Flightradar24 (FR24) joined together, with the expert assessment of Iberia, Air Europa, KLM and Turkish Airlines as members of the AURORA's Airspace Users Group, to conduct this research under the AURORA project (Grant 699340) supported by SESAR Joint Undertaking under European Union's Horizon 2020 research and innovation program.