# 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
Balked landing surface: For code 3 and a runways, the balked landing surface commences 1800 m from the threshold at a 3.33% gradient. Its precipitous slope obviates constraints on end-around taxiways [70]. Along the extended runway centerline, the primary limitations include the 2% climb gradient surface (approach and inner approach surfaces) originating 60 m beyond the runway threshold. Vertically, the restrictions are a 15% divergence and 280 m initial width for the approach surface [70]. 2. Flight Procedures and Aircraft Performance Restriction  
The SAT involves the complete arrival and departure procedures. The departure constraints, such as the OIS and Type A surfaces, are identical to the EAT mode and will not be reiterated. The arrival constraints of VSS, OAS, and CRM procedures will be discussed. 1. The Visual Segment Surface (VSS)  
The Visual Segment Surface originates 60 m from the runway threshold, with an initial width of 300 m, 15% divergence on both sides, and a length extending to the point where the VSS surface reaches the Obstacle Clearance Height (OCH) altitude. Vertically, the initial elevation of the VSS equals the runway elevation, with an upward slope equal to the glide path angle minus 1.12 degrees, generally 3 degrees. Figure 14 demonstrates the vertical direction of the VSS. 2. The Obstacle Assessment Surface (OAS)/Collision Risk Model (CRM)  
The Obstacle Assessment Surface (OAS) proposed in ICAO Doc 8168 \"Procedures for Air Navigation Services--Aircraft Operations\" states that if the OAS is penetrated, the OCA/H must be raised, thus increasing the Aerodrome operating minima. The Collision Risk Model ensures that the collision risk between aircraft on precision approach and surrounding obstacles is below 1 \(\times\) 10\({}^{-7}\)[75]. Since the OAS is above the basic ILS, the ILS is evaluated first. If obstacles penetrate ILS, OAS is applied. OAS comprises simplified surfaces derived from CRM's calculated 1 \(\times\) 10\({}^{-7}\) collision probability. CRM serves as an alternative when obstacle density below OAS is excessive [88]. Therefore, for EATs meeting basic ILS requirements, the OAS/CRM serves only as a checkpoint surface. In cases where the end-around taxiing aircraft penetrates the ILS surface, the OAS surface needs re-evaluation, and methods like adjusting flight procedures should be applied to eliminate obstacle impact without compromising flight safety. Figure 14: The vertical direction of the VSS. In summary, the Annex 14 limiting surfaces and the VSS are calculated separately in the extended runway centerline direction and perpendicular to the runway [70]. The calculations are based on the assumption that the runway elevation is equal to the taxiway elevation. Step 1: Calculate the length requirements in the direction of the runway centerline extension, as shown in Table 8.  
Along the extended runway centerline, the primary limitations of the SAT include the 2% climb gradient surface (approach and inner approach surfaces) originating 60 m beyond the runway threshold. Step 2: Based on step 1, calculate the restricted widths of the approach surface, VSS, in the vertical direction of the runway. Table 9 makes a comparison between these widths. Vertically to the runway, besides the approach surface, the relationship between end-around taxiing aircraft and the VSS surface must be considered. If unavoidable VSS penetration occurs, methods like adjusting flight procedures should be implemented. 3. Other Requirements  
Unlike REAT, SAT operations need to account for the approach lighting system. Neither the end-around taxiway nor the aircraft should obstruct the approach lights. The taxiway has to be placed beyond the 960 m protection area from the threshold. However, this leads to excessive taxiing distance and land use. Generally, displacing the threshold inward shifts the approach lights and reduces the taxiing distance. The threshold is displaced no more than 300 m to minimize impact on pilot landing operations. Airports like SHA and KWE have implemented threshold displacement.