Civil aircraft trajectory analyses - impact of engine degradation on fuel burn and emissions

Abstract

Commercial aviation and air traffic is still expected to grow by 4-5% annually in the future and thus the effect of aircraft operation on the environment and its consequences for the climate change is a major concern for all parties involved in the aviation industry. One important aspect of aircraft engine operation is the performance degradation of such engines over their lifetime while another aspect involves the aircraft flight trajectory itself. Therefore, the first aim of this work is to evaluate and quantify the effect of engine performance degradation on the overall aircraft flight mission and hence quantify the impact on the environment with regards to the following two objectives: fuel burned and NOxemissions. The second part of this study then aims at identifying the potential for optimised aircraft flight trajectories with respect to those two objectives. A typical two-spool high bypass ratio turbofan engine in three thrust variants (low, medium and high) and a typical narrow body single-aisle aircraft similar to the A320 series were modelled as a basis for this study. In addition, an existing emissions predictions model has been adapted for the three engine variants. Detailed parametric and off-design analyses were carried out to define and validate the performance of the aircraft, engine and emissions models. The obtained results from a short and medium range flight missions study showed that engine degradation and engine take-off thrust reduction significantly affect total mission fuel burn and total mission NOx emissions (including take-off) generated. A 2% degradation of compressor, combustor and turbine component parameters caused an increase in total mission fuel burn of up to 5.3% and an increase in NOx emissions of up to 5.9% depending on the particular mission and aircraft. However, take-off thrust reduction led to a decrease in NOx emissions of up to 41% at the expense of an increase in take-off distance of up to 12%. Subsequently, a basic multi-disciplinary aircraft trajectory optimisation framework was developed and employed to analyse short and medium range flight trajectories using one aircraft and engine configuration. Two different optimisation case studies were performed: (1) fuel burned vs. flight time and (2) fuel burned vs. NOx emitted. The results from a short range flight mission suggested a trade-off between fuel burned versus flight time and showed a fuel burn reduction of 3.0% or a reduction in flight time of 6.7% when compared to a “non-optimised” trajectory. Whereas the optimisation of fuel burn versus NOx emissions revealed those objectives to be non- conflicting. The medium range mission showed similar results with fuel burn reductions of 1.8% or flight time reductions of 7.7% when compared to a “non- optimised” trajectory. Accordingly, non-conflicting solutions for fuel burn versus NOx emissions have been achieved. Based on the assumptions introduced for the trajectory optimisation analyses, the identified optimised trajectories represent possible solutions with the potential to reduce the environmental impact. In order to increase the simulation quality in the future and to provide more comprehensive results, a refinement and extension of the framework also with additional models taking into account engine life, noise, weather or operational procedures, is required. This will then also allow the assessment of the implications for airline operators in terms of Direct Operating Costs (DOC). In addition, the degree of optimisation could be improved by increasing the number and type of optimisation variables.