# Hub Airport End-Around Taxiway Construction Planning Development: A Review
## 4 The Construction Principles and Key Points of End-Around Taxiway Layout
### Principles and Steps of Taxiway Design Optimization  
It is recommended to carry out the planning and design work of taxiways according to the following steps and principles.  
1. Land supply situation  
The construction of EATs requires additional land use, and new airports need to reserve space for the construction of these taxiways in master plans. However, expansion efforts lacking original allowances for EATs have constrained land resources [69]. Therefore, only BAT (Back-Around Taxiway) can be planned within the limited space. In some cases, aircraft may even adopt short-runway takeoff procedures, using short connecting taxiways as BATs. This practice has been implemented for many years at several airports in the United States. In the master plan of Asian airports, EATs have been included, and land has been reserved in recent years. This undoubtedly promotes the implementation of EATs in the future. Developing multiple EAT design options based on different aircraft types and operational characteristics. The choice of aircraft types (tail height) is the primary determinant in the selection of EAT layout. In the designing of hub airports, accurate predictions of air traffic volume and the proportion of aircraft types are crucial factors for taxiway planning. Taking DFW as an example, when designing the EAT, D-class aircraft, such as B767 and B757, were selected as the main design types to meet the operational requirements of mainstream aircraft. However, with the gradual retirement of D-class aircraft worldwide, short-haul flights are predominantly served by C-class aircraft like B737, A320, and C919, accounting for more than 80% of operations at domestic hub airports. Long-haul flights are mainly operated by popular E-class wide-body aircraft, such as B747 and A330, while F-class aircraft have a minimal presence and are not suitable for consideration [73; 74]. In 2023, the wide-body aircraft proportion at PEK and SHA is estimated to be around 30%. These airports serve as major international hubs and handle a significant number of intercontinental or long-haul flights. Most domestic hub airports have a widebody aircraft proportion of less than 20%. According to the Airbus Global Market Forecast 2023-2042, the demand for widebody aircraft is projected to be 8220, accounting for 20% of the market, while single-aisle aircraft are forecasted to make up 80% with 32,630 units. Except for a few international hubs, the widebody aircraft proportion at most airports ranges from 10% to 20% [74]. Airport managers often prefer an EAT that can accommodate all types of aircraft. However, the tail height of widebody aircraft can cause excessive taxiing distances, resulting in increased taxiing time and fuel consumption. Therefore, it is recommended to carefully select the appropriate aircraft type based on the forecast. Additionally, it is advisable to compare different EAT layouts for various aircraft types (such as C or E), considering factors such as the total length of taxi routes, taxiing time, and overall fuel consumption throughout the entire life cycle of the design. The relative positioning of taxiways, runways, and terminals, as well as the layout of EATs (such as N-shaped or M-shaped configurations), should be determined based on the overall airport master plan. Comparative analysis of multiple solutions in terms of technical and economic aspects over the entire life cycle.  
The construction of EAT requires a significant amount of land and incurs substantial engineering costs. The height difference between the runway and the EAT can reduce the taxiing distance but results in a large volume of earthwork. Therefore, it is necessary to compare different EAT layouts with varying elevations and different aircraft types and flows based on indicators such as taxiing time, fuel consumption, and engineering costs over the entire life cycle.