In this study, the flight parameters affecting fuel economy, flight cost and $CO^2$ reduction values in Cessna 172S aircraft with hybrid-electric propulsion system were optimized using the Grey Relational Analysis method. For this purpose, the flight data obtained as a result of the design for five different flight times (A) and five different cruise altitudes (B) were compared with conventional values and the optimum flight time and cruise altitude were determined. Flight missions were designed using the L25 (5^2) orthogonal index in five series for two different parameters. While the mission sequence with optimum fuel, cost savings and $CO^2$ reduction was achieved as A3B3 for 120 min of flight time and 2400 m cruising altitude, the sequence with the lowest operating performance systems was assigned as A5B1 for 180 min of flight time and 1200 m cruising altitude. The results show that the data obtained by the grey relational analysis method are in agreement with the data obtained by the conceptual analysis.