# 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 aircraft operating on the Runway End-Around Taxiway taxis in front of the departing aircraft. Therefore, careful consideration of the influence of aircraft operating on the REAT on the departing aircraft is essential. 1. Appendix 14 Obstacle Limitation Surfaces The Precision Approach Path Obstacle Limitation Surfaces and the Basic Instrument Landing System (ILS) surfaces specified in Annex 14--Aerodromes of the International Civil Aviation Organization (ICAO) [70] are consistent. They primarily include the Conical Surface, Inner Horizontal Surface, Approach Surface, Inner Approach Surface, Transitional Surface, Inner Transitional Surface, Balked Landing Surface, and Takeoff Climb Surface. EATs should be located outside the Basic ILS surfaces [67]. REAT is primarily used in front of the departing aircraft. The take-off climb surface serves as the primary limitation surface for restricting factors. In the direction of the runway extension centerline, the dominant limiting factors include a limitation surface that begins 60 m from the runway end with a gradient of 2%. The vertical restrictions consist of a lateral separation rate of 12.5% and a starting width range of 180 m [72; 73]. Furthermore, the operation of REAT must not interfere with the operation of an adjacent runway. The approach for addressing this issue is similar to the SAT mode and will not be discussed in detail in this article. 2. Flight Procedures and Aircraft Performance Restriction In the International Civil Aviation Organization (ICAO) DOC 8168 [75] regulations, the primary factors that affect EAT are the procedures near the vicinity of the runway end, including approach procedures, departure procedures, the basic ILS surface, and the Obstacle Assessment Surface (OAS). Additionally, considerations are given to the Visual Segment Surface (VSS) and the Aerodrome Obstruction Chart-ICAO Type A (Operating Limitation) [82; 83]. The operation of REAT does not require consideration of approach-related procedures. It only needs to consider the restrictions of Obstacle Identification Surface (OIS) and the Aerodrome Obstruction Chart-ICAO Type A (Operating Limitation) in the departure procedure. (1) Obstacle Identification Surface (OIS): The purpose of the OIS is to determine whether there is a potential collision risk between obstacles within the protected area and departing aircraft. The OIS begins at the Departure End of Runway (DER), starting at a height of 5 m and gradually sloping upward at a gradient of 2.5%. It has an initial width of 300 m and expands towards both sides of the runway at a rate of 15%. Obstacles must maintain a lower height than the OIS [84; 85]. If obstacles within the protected area do not penetrate the OIS, it does not affect instrument departure aircraft. The aircraft can climb following the published procedure gradient of 3.3% (2.5% + 0.8%) [82]. However, when obstacles penetrate the OIS plane, a flight path must be specified in the departure procedure or the departure climb gradient must be modified to ensure enough vertical clearance for the aircraft to safely fly over the obstacles. If this condition cannot be met, the aircraft should reduce its load. Obstacles below 60 m can be visually avoided. The REAT design should strive to ensure that aircraft operating on it remain below the OIS. However, certain modified airports may encounter challenges in meeting this criterion due to terrain limitations. In such situations, the requirements for taxiway operations can be fulfilled by making adjustments to the Procedure Design Gradient (PDG), reducing the payload of specific aircraft models, or employing visual obstacle avoidance techniques. (2) Aerodrome Obstruction Chart-ICAO Type A (Operating Limitation): The purpose of the Type A surface is to guarantee that aircraft can successfully clear obstacles within the designated takeoff path in the event of an engine failure. If this requirement cannot be met, the takeoff weight should be reduced by decreasing the effective payload.