## 7 Conclusions  
For this study, we designed three types of U-shaped area operation procedures for complex airport apron systems and constructed arrival and departure traffic operation rules and mathematical models for each operation procedure. To evaluate the performance of the operation on the apron, we proposed a U-shaped area operation performance evaluation index system and evaluation methods from three dimensions: safety, efficiency, and environmental protection, which should be taken into account in airport surface operation. A case study of the Wuhan Tianhe Airport was carried out to quantitatively assess the operation procedures proposed, including simulation evaluation and comprehensive verification analysis of apron U-shaped area operation under different operation procedures and flight flows. The main conclusions of this paper are as follows. First, the designed global-shared operation procedure can fully release the operation potential of the complex airport U-shaped area and increase the apron operation capacity by 14.8% and 5.0% compared to the exclusive and partition-shared operation procedures, respectively. When the hourly flight volume is lower than 30 aircraft/h, i.e., the capacity and traffic flow ratio corresponding to  
Figure 14: Number of Conflicts on the Surface Network under Three Aprron Operation Procedures. (a) Exclusive; (b) Partition-shared; (c) Global-shared. all three operation procedures is less than 60%, the difference in the operation effect of the three procedures is small; when the hourly flight volume increases from 30 to 50 aircraft/h, the advantages of the global-shared operation procedures are more prominent, which can effectively reduce the conflict probability, conflict resolution time, flight fuel consumption, and emission and improve the efficiency of apron resource utilization. Compared with the exclusive and partition-shared operation procedures, the global-shared operation procedure could reduce the probability of aircraft conflict by 32.2% and 11.8%, respectively and the average conflict resolution time per flight by 16.1 s and 10.6 s, respectively and could increase the resource utilization rate of each U-zone by 66% and 25%, respectively. The average daily fuel consumption of a single aircraft could be reduced by 3.6 kg and 2.4 kg, respectively, and the average daily carbon emission of a single aircraft could be reduced by 16.7 kg and 11.0 kg, respectively, translating into respective reductions of 1.2 tons and 0.8 tons of carbon emission in a whole year, which has a significant effect on effectively promoting fuel saving and emission reduction of the airport. In the process of apron control, airports and ATC departments should not overly pursue restrictive and strict operational rules in the design of U-shaped area operation procedures and ignore the efficiency of apron resource utilization and aircraft operation, but should achieve a comprehensive trade-off between capacity, efficiency, and environmental protection and other performance indices as far as possible under the premise of ensuring operational safety. Future studies should research the optimized design of apron operation procedures combined with apron control handover procedures for the three subsystems of runways, taxiways, and aprons on the surface and draw on the results of other fields to further improve the refinement of the operational assessment of the aprons and the surface as a whole and optimize the surface operation efficiency. In our future work, the proposed procedures will be further validated from the point of view of airport controllers' workloads as well as flexibly adapted to the traffic flow by analyzing the traffic flow characteristics of different airports at different times of the day. Y.L. : Methodology, Validation, Writing-original draft. M.H. : Methodology, Review, Editing. J.Y. : Methodology, Review, Editing. J.S. : Data curation, Conceptualization, Writing-original draft. S.W. : Data curation, Conceptualization, Software. Z.Z. : Data curation, Investigation.