## 3 Holding Area Conceptual Design and Turning Performance Analysis  
### Holding Area Conceptual Design for Low-Altitude Environments  
For UAM vehicles to fly safely in a low-altitude environment with many obstacles, UAM flight routes should be designed to ensure obstacle clearance and avoid noise-sensitive areas. Therefore, in this study, we aim to provide a conceptual design of the UAM holding area, which can be operated safely and efficiently in a low-altitude environment. The characteristics of the UAM holding areas conceptually designed through this study are different from those of holding areas for current aircraft. In the holding procedure for conventional aircraft, the '1 min leg' procedure is usually performed, in which each leg is flown for 1 min. Alternatively, the'mile leg' procedure is performed, in which the aircraft flies for a set distance on each leg. Since the holding area for conventional aircraft is established at high altitudes, there is no risk of obstacles. If the 1 min leg procedure is followed, the length and shape of the holding area are not uniform due to the influence of wind. Therefore, when applied to the low-altitude environments in which UAM vehicles are operating, it may not be possible to secure a sufficient area to establish such holding areas due to the complex obstacles. Additionally, safety from obstacles cannot be guaranteed when UAM vehicles are performing holding procedures [6]. In order for UAM vehicles to safely operate in a low-altitude urban environment with many complex obstacles, UAM holding areas should be designed and established based on the vehicle performance and regional characteristics. Additionally, if the vehicle deviates from the flight route while performing the holding procedure, there is a high risk of encroaching on a prohibited area. Therefore, the vehicle operated must meet the required operational performance. In addition, UAM holding areas should be designed so that UAM vehicles can fly each leg based on distance, not time. This is because UAM vehicles can safely operate in a limited space through UAM holding areas. Additionally, the speed of UAM vehicles should be defined so that they can fly at a relatively low speed, and an appropriate bank angle and rate of turn standards should be established so that the vehicle does not deviate from the holding area [6]. Assumptions have been established for the conceptual design of UAM holding areas. The established assumptions are as follows and can be modified for a detailed UAM holding area design [6]:  
* The UAM holding area serves as a link between UAM flight routes in opposite directions. * The difference in altitude of the UAM flight routes is 1000 ft.
* Each UAM flight route is 100 m wide. (Apply RUNP-50) [7]. * The turning standard of UAM vehicles is the same right turn as that of conventional aircraft. Through these considerations and assumptions, concepts of holding areas have been designed as shown in Figure 4. There are three conceptually designed holding areas, and they can be established differently depending on the regional characteristics. They are designed to be able to perform holding procedures at low altitudes, as well as to perform turning procedures in order to divert or fly over another flight route [6]. The holding area in concept 1 can be established in an area where each UAM flight route is set in the lateral direction or in a narrow area to avoid a prohibited area. Concept 1 has excellent space efficiency, but due to its complex design, it has the disadvantage of requiring precise situational awareness and the separation of UAM vehicles during the cruise, climb, and descent. Concept 1 is suitable for the UAM corridor of the Han River [6]. A holding area such as concept 2 can be established in areas where sufficient space is secured with few obstacles. Since the cruise flight route (enroute) is set in the longitudinal direction, there is an advantage that the turning section can be sufficiently secured.