The paper examines the horizontal efficiency of aircraft approaches in the lower airspace. We propose two measures for horizontal efficiency and study the determinants, notably air traffic control's choice variables, of both efficiency metrics using robust (MM) and Tobit regression analyses. Our metrics indicate an average deviation from the optimum efficiency of 21.6 % and 19.6 %, respectively. We calculate that these inefficiencies generate approximately 165,088 tons of CO2 emissions and fuel costs of around € 23.8 million per annum. Improving the efficiency of approaches is thus a means to reduce aviation's externalities and lower its negative impact on the climate and noise. The metrics are significantly affected by the volume of flights, aircraft weight, weather threats, and the decision variables of air traffic control, which are runway change, runway choice and route choice. This is evidence that air traffic control can contribute to horizontal efficiency and reduce externalities.