2.1 UAM

To operate UAM, three components are essential: aircraft, vertiport, and corridor (Fig. 1). The aircraft are electric-powered vertical take-off and landing aircraft, similar to helipads, known as eVTOL. The term for the operating and service concept is vertiport, which can be categorized into Vertihub, Vertistop, Vertistation, or Vertistop depending on the number, size, and functionality of the aircraft they accommodate. The flight path for the aircraft is referred to as skyway, corridor, or trajectory. This paper will use the term UAM route.

Fig. 1. UAM, (a) eVTOL (HYUNDAI), (b) Vertiport (NASA), (c) Corridor (Ministry of Land, Infrastructure and Transport)

The definition of a UAM route according to the Korean Urban Air Mobility (KUAM) Operation Concept 1.0 is ‘an airspace operated separately for safe UAM operations, through which UAM aircraft travel to their destinations’ (Fig. 2). UAM operates a different system from existing manned or unmanned aerial vehicles. Regional risks and flight standards vary according to the flight route, so it is necessary to fly strictly managed routes rather than freely flying. The following considerations should be taken into account in selecting and designing UAM routes, as described in the operational concept.

Fig. 2. Corridor (Source: Ministry of Land, Infrastructure and Transport)

First, reflecting the requirements of the local community and existing stakeholders. Second, meeting the public conditions of local environments, noise, safety, and security. Third, minimizing the impact on Air Traffic Management and Unmanned Aircraft System Traffic Management operations. Fourth, considering the overlap of existing and new corridors and minimizing the impact on flight safety.

Corridors must be designed taking into account the above considerations. Additionally, they are designed for the purpose of traveling to airports and other urban centers, and existing helicopter routes can also be utilized. The flight altitude for UAM is set at approximately 450±150 m above ground level (KUAM Concept of Operations 1.0).

2.2 Research Area

The study area is Gangneung City in Gangwon Special Self-Governing Province, which has been selected by the Ministry of Land, Infrastructure and Transport as a future Mobility as a Service Station (MaaS Station) site (Fig. 3). A future Mobility as a Service Station integrates MaaS, a comprehensive mobility service software offering route optimization, reservation, and payment solutions, with a physical transportation hub called a Station, providing access to various modes of transport such as buses, taxis, rails, and PMs. Gangneung Station, located in the East Coast traffic and tourism hub, is set to enhance the convenience of public transportation for tourists by establishing a transfer center. Furthermore, in conjunction with the World Congress on Intelligent Transportation Systems (ITS) in 2026, there are plans to operate a UAM pilot service in Gangneung as an eco-friendly mobility autonomous sharing city centered around Gangneung Station.

Fig. 3. Gangneung MaaS Station (Source: Gangneung City)

2.3 Safe Distance between UAM Aircraft and Building

This study assumes a scenario where a UAM aircraft collides with a building. Equation (1) was used to determine the safety distance required to prepare for a collision between a UAM aircraft and a building.

In Equation (1), $R$ is the safety distance, $H$ is the building height, and $k$ is the proportionality constant which must consider factors such as the size and speed of the aircraft, flight stability, weather conditions, and the density of buildings. However, research on formulas that consider these factors is very lacking at this stage. Therefore, this study proposes assuming proportionality constants based on building height as shown in Table 1. Research on collisions between UAVs and buildings ^{(Pang et al., 2022)} indicated that while low-rise buildings, with their high density, presented a greater collision risk, high-rise buildings, with their lower density, had a lower risk of collision. These findings were reflected in the selection of values for the proportionality constant $k$.

Fig. 4. Number of Buildings by Height in Gangneung

Figure 4 is prepared by extracting building height data for downtown Gangneung, utilizing the GIS Building Integrated Information dataset from V-World. Analysis shows that buildings ranging from 5 to 10 m are most common in Gangneung, a medium-sized city where high-rise buildings over 20 m are scarce. Reflecting the regional characteristics of Gangneung City, proportionality constants were assigned higher values for buildings between 5 to 10 m and 10 to 15 m.

For regulations applicable to UAM flights which are still under research, it is necessary to refer to regulations related to helicopters and UAVs, which share similarities with UAM. According to the regulations of the FAA (Federal Aviation Administration), ICAO (International Civil Aviation Organization), and our Ministry of Land, Infrastructure and Transport (MOLIT), there are regulations concerning the safety distance from structures (Table 2). FAA regulations on helicopter flights (14 CFR Part 91.119) require that aircraft maintain a distance of about 150 m or more from any structure during flight.

Looking at UAV regulations, there are only restrictions on flight altitude and distance from buildings (Table 3). Considering these regulations, it is impossible to select segments where UAM could fly near Gangneung Station. Therefore, this study conducted research by applying safety distances based on building height, based on Equation (1).

2.4 Risk Radius in Case of Aircraft Crash

This paper conducts research based on the specifications of Hyundai Motor Company’s UAM aircraft, S-A1, which is expected to be introduced in Gangneung UAM. In this study, the risk radius in case of a UAM aircraft crash was calculated using a free-fall model (equation of motion considering gravity and air resistance) that accounts for drag used during the aircraft crash ^{(la Cour-Harbo, 2019)}. The equation is as follows:

(2)

In equation (2), $m$ represents the aircraft weight, $\vec{v}$ represents the velocity vector, and $\vec{g}$ represents the gravitational acceleration vector.

$c$ is the formula used in fluid dynamics to calculate the resistance force when an object moves through air. $\rho_{air}$ is air density, $A$ is the frontal cross-sectional area of the aircraft, and $C_{d}$ is the drag coefficient. $\rho_{air}$ can vary with temperature and pressure, and $C_{d}$ is a dimensionless coefficient determined by the object's shape, the roughness of its surface, and the state of the fluid's flow. Equation (3) sets out Hyundai's aircraft specifications for determining the risk radius of an aircraft crash as shown in Table 4. The study was conducted assuming the aircraft's weight, frontal area, and maximum speed. The flight altitude was set at the maximum flight altitude of 600 m prescribed in the UAM operation concept document.

Using Equation (2) and the UAM specifications from Table 4, the risk radius in case of a UAM crash is approximately 330 m, and the kinetic energy is approximately $4.1\times 10^{6}$ J. Since the exact crash location of a malfunctioning aircraft can be difficult to predict and influenced by various variables, it is assumed that it crashes within the accident risk radius.

In addition to the formula mentioned above, several regulations were checked and used for the study. Applying the 1-1 rule used to set the ground risk buffer in JARUS SORA when analyzing the impact of increased accident risk radius during UAM crash, it is impossible for UAM to fly in the downtown area of Gangneung. The 1-1 rule requires that the aircraft must maintain a ground risk buffer equal to the flying altitude away from populated areas when flying at a certain altitude. With the highest operational altitude of K-UAM being 600 m, this increases the accident risk radius to 600 m ^{(Kim et al., 2022)}. With the accident risk radius increasing to 600 m, UAM cannot operate at Gangneung Station located in the central downtown area. Therefore, in this study, the risk radius during UAM vehicle crash was set at 330 m and the research was conducted.