The inefficiency of approaches to large airports causes additional fuel consumption and emissions. Before mitigating interventions can be designed, the level and determinants of the inefficiency must be known. We study the approach efficiency at four large European airports to understand the efficiency problem and identify its determinants. This study thereby extends the research on the latter in several ways. We compare vertical and horizontal approach profiles and the vertical and horizontal efficiency measures on ‘flows’ (direction-runway links), discuss correlation and complementarities of both measures, and explore the heterogeneity of horizontal and vertical inefficiency concerning hour of day and flow used. The efficiency is significantly reduced by increased approach volumes, weather hazards and higher aircraft weight, while horizontal efficiency declines due to higher departure traffic. Additionally, runway and flow choice are relevant determinants that indicate the influence of air traffic control on efficiency. The heterogeneity analysis reveals that the efficiency-enhancing potential of the flow choice varies across the day. In this way, our study contributes to a broader understanding of approaches, adds to the knowledge required for airport-wide efficiency improvements and the reduction of the environmental impact of aviation.