A Trajectory Optimization Based Analysis of the 3Di Flight Efficiency Metric

As a means of measuring progress towards fuel and emissions reduction targets, the United Kingdom Air Navigation Service Provider NATS developed the 3Di flight efficiency metric. In principle the 3Di score of a flight is calculated by comparing a flown trajectory to a theoretical fuel/CO2 optimum trajectory. In response to a Eurocontrol review of the metric, the 3Di score has been analysed using a trajectory optimization method based on optimal control. The results suggest that further development of the metric is required to make it sensitive to vertical flight inefficiencies not related to periods of level flight. The results also show that the BADA trajectories used in the 3Di score to define the optimum fuel efficient operation of the aircraft are not optimal fuel efficient trajectories. Additionally, the results show that fuel inefficiencies introduced by flight planning restrictions need to be accounted for in any vertical flight inefficiency metric.