The environmental impact of aviation is a well-known issue scientists all around the world are desperate to tackle. The population that can afford air travel is growing and this demand is likely to double within the next 20 years. This increasing demand comes at a hefty price: the emissions of greenhouse gases cause climate change and local air pollution. One of the main research topics at the moment to try to stagnate these consequences of air travel is the electrification of aircraft. Although all-electric aircraft might sound like a good idea for smaller aircraft, the current electric component technology level is still insufficient to propel larger aircraft. Currently more researchers are shifting their focus towards hybrid-electric propulsion systems. The objective of this study was to add to the scientific knowledge about the performance of hybrid-electric aircraft by assessing changes in the mission of a hybrid-electric A320neo. For this purpose the aircraft performance tool MASS (Mission, Aircraft and Systems Simulation) has been used, which has been developed at NLR (Royal Netherlands Aerospace Centre). This tool assesses parallel hybrid-electric aircraft for a given mission. This mission model had to be redesigned to be more efficient at automating the mission variation process. The main mission variables that this study focuses on are: cruise altitude, Take-off and climb velocities, range and hybridization factor The study shows that designing a new mission for hybrid-electric aircraft does have an impact on the overall fuel consumption and emissions. However, the gains are very small for EIS2030 battery technology forecasts with 3.10% fuel reduction compared to the reference aircraft. Mission assessment is therefore a tool that has the potential to increase the performance of hybrid-electric aircraft, but it needs to work alongside other measures, such as hydrogen technology in the future.