Aircraft cruise phase altitude optimization considering contrail avoidance

Abstract

Contrails have been suggested as one of the main contributors to aviation-induced climate impact in recent years. To reduce the climate impact of contrails, mitigation policies such as taxation will be necessary in the future to incentivize jet aircraft operators to reduce contrail production. Contrails form in regions of the atmosphere with the right ambient conditions and they can be avoided by flying around these regions; this research investigates one such contrail avoidance strategy that uses flight level optimization to minimize contrail formation. A cruise phase flight profile system model was developed in this research that optimizes for environmental objectives such as contrails, CO₂, and NOx, alongside traditional objectives such as fuelburn and flight time. Using this system model and 11 different aircraft types on 12 weather days, a preliminary study was done to determine the price range of contrail taxation that would incentivize airlines to operationally avoid contrails. Result suggests a price range of 0.12$/NM to 1.13$/NM on contrail tax would effectively incentivize contrail avoidance. Furthermore, since operating costs differ depending on the type of aircraft, a single price on contrail tax may incentivize contrail avoidance on a small aircraft, but not larger ones. To account for this difference, a method of assigning contrail tax to different aircraft types is introduced using the aircraft maximum takeoff weight. Assuming airlines are incentivized to fly contrail avoidance strategies, the climate impact of the flight profiles was evaluated for 287 flights along 12 O-D pairs for the 24 hour day of April 12, 2010. Under various assumptions of contrail radiative forcing and time horizon of climate impact evaluation, the flight level optimization reduced the average climate impact per flight by as much as 39.1% from a baseline of wind-optimal flight at optimal cruise altitude. In comparison, a complementary lateral optimization method reduced 13.3% from the same baseline. Furthermore, flight level optimization shows to be more fuel efficient by reducing the climate impact of contrails by as much as 94% from the baseline, compared to 60% using the lateral approach. In terms of the CO₂ emission from the additional fuelburn, the climate impact of lateral method was 4 times higher than the flight level approach. Lastly, result shows that designing for long-term environmental objectives is more energy efficient (reduction in climate impact per additional kilogram of fuel used) than short-term, which suggest reducing CO₂ emission is favored over contrail avoidance in designing for climate impact optimal flight profiles.