In the last decades, the environmental impact of aviation has been an emerging topic of discussion worldwide. With an expected CO2 emission increase of 21% by 2040 for air traffic, there is an urgency to reduce the negative impact on the environment. On top of that, the number of passengers is expected to double within the next 20 years. This challenge requires intervention from governments, airlines and passengers. Creating a more reliable flight planning would reduce the fuel consumption and costs for airlines. However, for airlines with a hub-and-spoke network, the schedule design relies on maximizing the connecting passengers. A more reliable planning with embedded buffers could lead to a lower offering of connecting flights. The goal of this research is to design a novel optimization model for balancing reliability and connectivity by buffer scheduling in the flight schedule, and to evaluate the impact on environmental sustainability. This research provides a Proof of Concept with a case of KLM Royal Dutch Airlines. The optimization model presents a Pareto optimal front that makes the trade-off between reliability and connectivity explicit. Main findings of this research are that (i) the model is suitable for supporting decision-making on a basic level, (ii) the stochastic variance of the model should be limited and (iii) the flight schedule presented by the model reduces the CO2 emissions significantly. Further research is required on how to optimize schedules with a higher level of complexity for the methodological elements and on the business side.