In recent years, the integration of large fixed-wing unmanned aerial vehicles (fixed-wing UAV) into joint airspace alongside manned civil aviation aircraft has become a critical research topic. This study focuses on establishing a quantitative methodology for assessing flight safety risks and safety separation for fixed-wing UAV operations within the same airspace as conventional airliners, ultimately leading to a design scheme for predetermined track line protection areas. The goal is to enhance the development of intelligent low-altitude airspace and smart logistics technologies.
1. Current Status and Problem Description
The development of fixed-wing UAV technology exhibits several key characteristics. First, industrial development is increasingly large-scale. Since 2023, under the guidance of the Civil Aviation Administration of China (CAAC), the China Academy of Civil Aviation Science and Technology has formed the “Civil Aviation Drone Industry Technology Innovation Strategic Alliance” and organized a series of industry forums. These forums cover topics such as International Civil Aviation Organization (ICAO) standards, safety mechanisms, and products from various fixed-wing UAV manufacturers. Second, basic research on flight safety has become richer. Researchers like Hao Zhipeng proposed an operational risk certification method that includes evaluation indices and calculation processes. However, current evaluations are limited to small rotary-wing UAVs, indicating a need for broader adaptability. Third, the systematic correlation among research outcomes has strengthened. For instance, Zhang Honghai studied capacity under stable flight conditions, enabling efficient operation in low-altitude high-density airspace. Liu Jie introduced a trajectory planning method focusing on flight mission time windows.
Considering these achievements and ICAO’s Instrument Flight Procedure Panel (IFPP) discussions on collision risk modeling (CRM), this work targets “smart low-altitude” and “cooperative operation” features. We aim to solve the safety risk and safety separation quantification for fixed-wing UAV and manned aircraft flying in integrated joint airspace, forming a protection area analysis and delineation method for predetermined track lines.
2. Technical Route
Based on the “Interim Regulations on Unmanned Aerial Vehicle Flight Management” and previous research accumulation, we identified three key technical challenges and 12 implementation steps, shown in the table below.
| Key Problem | Solution | Research Outcome |
|---|---|---|
| Precise trajectory prediction for fixed-wing UAV | Trajectory data feature analysis, preprocessing, 4D trajectory model establishment using clustering and spatiotemporal series | High-accuracy 4D trajectory prediction model for fixed-wing UAV |
| Flight conflict risk points | Flight performance analysis per phase, conflict template design based on ICAO ACP | Conflict template with spatiotemporal characteristics |
| Integrated flight safety separation | Risk evaluation via collision risk simulation, safety separation calculation | Quantified safety separation reference values for fixed-wing UAV |
The specific technical roadmap includes three major parts:
- Flight Trajectory Processing: Preprocess historical flight data of fixed-wing UAV, including spatial and temporal features. Build a 4D trajectory prediction model that matches the phase-specific flight performance of fixed-wing UAV. The model provides high-precision and stable spatial-temporal predictions. Then, combine actual navigation performance (ANP) characteristics to form trajectory configuration for each phase.
- Safety Risk Design: Introduce the aircraft collision probability (ACP) concept from ICAO for approach, departure, and precision approach phases. Establish a conflict relation model by processing airspace environment data, compute lateral safety risk values for a predetermined flight path, and evaluate reliability.
- Safety Separation Study: Using current civil aircraft flight procedures and protection areas at the study airports, combine fixed-wing UAV lateral safety risk values to construct a density calculation model meeting ACP standards, obtaining spatiotemporal safety separation values.
3. Research Results
3.1 Establishment of a Spatiotemporal 4D Trajectory Prediction Method for Fixed-Wing UAV
Currently, 4D trajectory prediction for civil aircraft is mature. For fixed-wing UAV, three differences exist: (1) Navigation technology: fixed-wing UAV uses satellite and ground augmentation systems, with smaller data volumes, easier transmission, and lower positioning errors. (2) Flight altitude: typically 3,000–6,000 m, where low-altitude long-range flight is vulnerable to terrain and obstacles, posing data link failure risks. (3) Human factors: fixed-wing UAV operates with reduced pilot involvement compared to manned aircraft, requiring adjustments in flight technical tolerance and positioning accuracy.
Our simulation used ADS-B data for clustering and preprocessing (eliminating outliers). We established a multi-layer trajectory data processing model based on flight phases, utilizing trajectory deviation and dynamic optimization algorithms. Each spatial point was described with time and space features. Taking the approach segment as an example, the predicted vs. actual values are shown in Figure 1 (the figure is referenced as a hyperlink below).
Note: The hyperlink above represents the actual experimental comparison graph, which is embedded in the text.
The simulation covered approximately 16 minutes of flight duration, descending from 19,426 ft to 6,883 ft over nearly 140 km. The predicted trajectory matched the actual path with high accuracy, confirming model reliability.
3.2 Conflict Calculation Model for Fixed-Wing UAV Flight Performance
Because civil aircraft have well-established CRM methods and flight procedure design standards, their predetermined track lines are reliable. For fixed-wing UAV, we developed a conflict model based on ANP features and spatiotemporal relations. Using the concept of a three-dimensional protection volume, we established relative position expressions between two fixed-wing UAVs relative to the predetermined track line intersection point O:
$$S_{12}^{i,n} = f\left(\varepsilon, x_i, y_i, z_i\right)$$
$$C_{12}^{i,n} = g\left(\varepsilon, x_i, y_i, z_i\right)$$
where \(i\) denotes the UAV index, \(\varepsilon\) is coordinate tolerance. Introducing time \(t\), the distance between two fixed-wing UAVs is given by:
$$D_{12}(t) = \sqrt{(x_1 – x_2)^2 + (y_1 – y_2)^2 + (z_1 – z_2)^2}$$
Further, the intersection angles \(\alpha\) and \(\beta\) are defined:
$$D_{12}(t, \Delta t) = \sqrt{S_{12x}^2 + S_{12y}^2 + S_{12z}^2}$$
Considering spatiotemporal series, we defined distances along the predetermined track:
$$S_{12}(t,\Delta t) = \int_{t}^{t+\Delta t} \dot{S}_{12}(\tau) d\tau$$
The actual operation variable \(t\) yields a coupled relationship between \(S_{12}(t,\Delta t)\) and \(S’_{12}(t,\Delta t)\). This results in a conflict template with spatiotemporal features suitable for fixed-wing UAV.
3.3 Safety Separation Reference Values for Integrated Manned-Unmanned Operations
Joint airspace integration must consider density, aircraft types, and existing control strategies. ICAO has not yet issued guidance for such operations, and national aviation authorities lack implementation plans. To address this, we used modified Y-5, AT 200, and Class C civil aircraft data at small/medium airports. The experiment airport structure is illustrated in the following table.
| Waypoint | Coordinates (relative) | Altitude (ft) |
|---|---|---|
| ARP | (0,0) | 0 |
| A1 | (10,5) | 6000 |
| A2 | (25,15) | 8000 |
| A3 | (40,25) | 10000 |
| D1 | (12,8) | 7000 |
| D2 | (30,20) | 9000 |
Using civil aircraft CRM results, the protection area radius was computed as \(r_{CRM} = 5.937\) km. For integrated flight, the safety separation radius for fixed-wing UAV was reduced to \(r_{safe} = 4.875\) km, based on our conflict model and ACP thresholds. The following table summarizes the comparison.
| Scenario | Protection Radius (km) | Protected Area (km²) |
|---|---|---|
| Civil aircraft CRM | 5.937 | 110.7 |
| Integrated flight for fixed-wing UAV | 4.875 | 74.7 |
The reduction in protection radius indicates that fixed-wing UAV can operate in closer proximity to manned aircraft while maintaining acceptable safety levels, thereby increasing airspace capacity. The uncolored area in the protection diagram represents potential predetermined track planning space for fixed-wing UAV, offering greater flexibility.
4. Conclusion
This study established a precise 4D trajectory prediction model for large fixed-wing UAV and validated its accuracy using historical data. Following civil aircraft CRM principles, we developed a spatiotemporal conflict template tailored to fixed-wing UAV navigation performance, consistent with ICAO ANP and ACP standards. By combining these results, we produced safety separation reference values and protection area delineation for fixed-wing UAV in integrated flight at small/medium airports. The quantified separation values (4.875 km) provide a basis for low-altitude high-density airspace planning. Future work will extend to different fixed-wing UAV types and payload requirements, and simulate non-independent operation airspace to support capacity assessment in integrated scenarios.
Key contributions include:
- High-accuracy 4D trajectory prediction for fixed-wing UAV using spatiotemporal series.
- Conflict template incorporating fixed-wing UAV ANP characteristics.
- Quantified safety separation for fixed-wing UAV in joint airspace.
- Protection area design for predetermined track lines, aiding air traffic control optimization.
The results directly promote the development of smart low-altitude airspace and intelligent logistics technology, with immediate applicability in future civil-military drone integration.
Acknowledgment
This research was supported by the Civil Aviation Administration of China Safety Capacity Building Project (2022237) and the Civil Aviation Administration Education Talent Project (No.14002600100019J104).