# Hub Airport End-Around Taxiway Construction Planning Development: A Review
## 4 The Construction Principles and Key Points of End-Around Taxiway Layout
### Limiting Requirements in End-Around Taxivany Design
#### 4.3.2 REAT, Runway End-Around Taxiway Mode
The Type A surface originates from the published endpoint of the takeoff area and has a climb gradientof 1.2% [82]. It initially has a width of 180 m and gradually expands at a rate of 25% on both sides until reaching a fixed width of 1800 m [86]. In practice, the design of taxiways does not take into account the constraints of the Type A surface. This is mainly due to its stringent requirements, which result in longer distances between the EATs and the runway. Aircraft operating on taxiways act as moving obstacles and can provide emergency avoidance in the case of an engine failure, which is a significant difference compared to fixed obstacles. As shown in Figure 13, the limitations set by Appendix 14 surfaces, the Obstacle Identification Surface (OIS), and the Type A surface were computed incrementally along the runway extension line and in the vertical runway direction. The calculations were based on the assumption that the runway elevation is equal to the taxiway elevation. It is clear that the requirements of the Type A surface are excessively strict. As a result, the practical scenarios do not take the Type A surface requirements into consideration. Step 1: Calculate the length requirements in the direction of the runway centerline extension. As shown in Table 6, the primary constraint for the REAT along the extended centerline of the runway is the takeoff climb surface, which is aligned with the approach and inner approach surfaces. This constraint starts 60 m beyond the runway end and mandates a climb gradient of 2%. Step 2: Determine the position of the REAT along the extended centerline direction of the runway, taking into account the 2% climb gradient constraint determined in the previous step. Based on this position, calculate the restricted widths of the takeoff climb surface, OIS, and Type A surface in the vertical direction of the runway. Make a comparison between these widths. The width layout of REAT should consider the impact of the OIS to ensure that the aircraft's tail remains below the OIS. If necessary, measures such as load reduction or avoidance should be taken in case of penetration. However, it is important to note that the limitations of the takeoff climb surface should never be exceeded. Table 7 shows the comparison results. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**CODE**} & \multirow{2}{*}{**Aircraft Type**} & **Vertical Tail Height** & **Takeoff Climb Surface** & **OIS** & **Type A Surface** \\ \hline \multirow{2}{*}{C} & A320 & 12.08 & 664 & 283.2 & 1066.7 \\  & B737 & 12.7 & 695 & 308 & 1118.3 \\ \hline \multirow{2}{*}{D} & B757 & 13.74 & 747 & 349.6 & 1205.0 \\  & B767 & 16.13 & 866.5 & 445.2 & 1404.2 \\ \hline \multirow{2}{*}{E} & B747 & 19.6 & 1040 & 584 & 1693.3 \\  & A340 & 17.93 & 956.5 & 517.2 & 1554.2 \\ \cline{1-1}  & A330 & 18.3 & 975 & 532 & 1585.0 \\ \hline F & A380 & 24.27 & 1273.5 & 770.8 & 2082.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Distance from the runway end to the REAT (meters). Figure 13: The comparison of OIS, take-off climb surface, and type A surface in the vertical direction. Other Requirements  
The REAT is calculated based on the takeoff climb surface and is positioned beyond 660 m from the runway end. If the runway is equipped with an ILS guidance system, this position is already located outside the critical and sensitive areas of the Localizer (LOC) and Glide Path (GP). However, if adjustments are made, such as lowering the REAT height, there is a possibility that the REAT system might enter into these sensitive areas. Nevertheless, during REAT operation, when aircraft are taking off, the LOC and GP signals are not active. Therefore, similar to the BAT model, it is sufficient to assess the potential impact of this situation. If the REAT is located beyond 960 m, there is no need to extensively consider its impact on the approach lighting system. However, if the REAT is located within the protected area, similar to the BAT system, the lights should be installed on the shoulder instead of the pavement.