Xiaoqian Sun

• Doctor of Philosophy
• Associate Professor at Beihang University

Flight delay is one of the most severe problems faced by the aviation industry, and its excessive impact on air transport operations and society has attracted wide attention by researchers. Due to the structural nature of air transportation and the intrinsic dependencies between system components, many studies on flight delays have used concepts fr...

Flight delay is a fundamental problem present in the global aviation system. Delay-inducing disruptions are caused by various reasons, including increased global connectivity / dependency, weather phenomena, and limited infrastructure resources. Given the excessive amount of time and money lost due to delays in aircraft operations, the prediction o...

The aviation industry faces various challenges in meeting long-term sustainability goals amidst surging demand for air travel and growing environmental concerns of the general public. The year 2050 is set as an ambitious goal for net zero emissions, a substantial reduction in carbon dioxide emissions per passenger kilometer flown, major improvement...

The choice of hub locations is fundamental in various transportation and communication systems, enabling transshipment, sorting, and consolidation functions. Recent research has highlighted the benefits of stratification when searching for hub facilities, taking into account the fact that the demand at each site belongs to different strata, e.g., d...

Large Language Models (LLMs), based on their highly developed ability to comprehend and generate human-like text, promise to revolutionize all aspects of society. These LLMs facilitate complex language understanding, translation, content generation, and problem-solving, enabled by vast historical data processing and fine-tuning. Throughout the past...

Air transport management research, concerned with all facets of aviation operations, policies, and strategies, is an essential element of making our aviation system more sustainable and preparing it for the challenges inherent to the present and future. Based on a data-driven categorization of almost 2,000 papers published on the subject, we discus...

The rapid development of civil aviation over the past decades has not only led to an increasing competition among airlines, but also the rise of irregularities, with challenges concerning the improvement of regulations and schedules under the maximization of profitability. Consequently, the airline scheduling optimization problem has received signi...

Disruptions to airline schedules precipitate flight delays/cancellations and significant losses for airline operations. The goal of the integrated airline recovery problem is to develop an operational tool that provides the airline with an instant and cost-effective solution concerning aircraft, crew members and passengers in face of the emerging d...

Hub Location Problem (HLP) is an important research problem in air transportation, telecommunication and postal service system. Plenty of techniques have been proposed to deal with HLP, including exact methods and meta-heuristics. This study is the first to propose deep-learning based framework evaluated on Uncapacitated Single Allocation p-Hub Med...

The COVID-19 pandemic has hit the airline industry hard, leading to heterogeneous epidemiological situations across markets, irregular flight bans, and increasing operational hurdles. Such a melange of irregularities has presented significant challenges to the airline industry, which typically relies on long-term planning. Given the growing risk of...

Since the outbreak of the COVID-19 pandemic early in the year 2020, the aviation system has been under extreme pressure for more than two years, with many stakeholders anticipating the moment of recovery. In Summer 2022, eventually, many airlines reached profit margins again. Several airlines even reported to outperform pre-pandemic indicators, inc...

In Summer 2022, after a lean COVID-19 spell of almost three years, many airlines reported profits and some airlines even outperformed their pre-pandemic records. In context of the perceived recovery, it is interesting to understand how different markets have gone through the pandemic challenges. In this study, we perform a spatial and temporal diss...

Benchmarking different aspects of air transport, and especially airport operations, is a research topic both important and complex, as proposed solutions are usually limited by data availability and underlying hypotheses. We here propose the use of two well-known statistical physics concepts, the Hurst exponent and the irreversibility, to measure t...

For nearly three years with the COVID-19 pandemic, China has implemented a set of strict policies to control the flux of potential virus carriers in cross-border flights: The so-called Circuit Breaker mechanism. In this study, we review the evolution of this mechanism - a rather unique experiment in the global aviation system - from a data-driven p...

Hub location problems are widely analyzed in fields of logistic and transportation industry for cost reduction. In this paper, a novel algorithm framework based on machine learning is proposed to improve solution quality of hub location problems for large-scale instances. First, a deep-learning based probabilistic hub-ranker (DLHr) is developed to...

Targeting at improving door-to-door accessibility, efficiency and reducing environmental impact, recent decades have witnessed vigorous development of multimodal transport. Coupled through passenger transfer, the failure of one mode, however, is likely to cause cascading disruptions to the complete system. Therefore, to maintain safe and efficient...

There exists a tremendous number of research surveys on various aspects of UAV logistics, mobility and monitoring tasks in the literature. These surveys have been published in distinct venues, often having a significant overlap in goals and key findings. In this study, we provide a meta review across nearly 100 extant UAV surveys and overview paper...

The COVID-19 pandemic can be considered an unparalleled disruption to the aviation industry in the last century. Starting with an at-that-time inconceivable reduction in the number of flights from March 2020 to May 2020, the aviation industry has been trying to navigate through and out of the crisis. This process is accompanied with a significant n...

The COVID-19 pandemic has hit the transportation sector hard; particularly air transportation, as a major mode of long-distance transportation, has been affected tremendously. Since the dawn of COVID-19, politicians and policy makers have discussed the idea of introducing travel bubbles between countries (or counties), to allow for a continued exch...

The impact of the COVID-19 pandemic is unprecedented in airline history, with irregular flight bans, the inability for accurate demand estimation, several turns in the epidemiological evolution, and a wide range of downstream effects on all aviation stakeholders. While most airlines have increasingly entered a recovery stage, compared to the utmost...

This paper proposes variable neighborhood search (VNS) heuristics to solve hub network design problems with profits, which are uncapacitated hub location problems with incomplete hub networks. These problems seek to locate hub facilities, design hub networks, and assign spokes to hubs to maximize total profits. Six problems consisting of multiple a...

The percolation threshold is the critical point at which a giant connected component emerges for the first time in the percolation process. Its theoretical estimation has recently gathered much attention, especially for real networks with complicated structures. However, the theoretical methods so far mainly provide lower bounds of the percolation...

Throughout the past decades, network dismantling gained an increasing interest by the research community, given tremendous importance of robust socio-technical systems. The problem of optimally dismantling a given network is provably NP-hard. Accordingly, existing studies on network dismantling resort to various heuristics, including the use of nod...

Our study provides an experimental benchmark for state-of-the-art solution algorithms with hub location problems. Such problems are fundamental optimization problems in location science with widespread application areas, such as transportation, telecommunications, economics, and geography. Given they combine aspects of facility location and quadrat...

The devastating impact of COVID-19 on aviation is unprecedented and undoubted in the recent sci-entific literature, with many studies having dissected different facets of COVID-19-induced changes to the industry. A few studies have stepped further and highlighted that the COVID-19 pandemic could have positive long-term impacts on aviation. Given th...

Aviation has been hit hard by COVID-19, with passengers stranded in remote destinations, airlines filing for bankruptcy, and uncertain demand scenarios for the future. Travel bubbles are discussed as one possible solution, meaning countries which have successfully constrained the spread of COVID-19 gradually increase their mutual international flig...

COVID-19 has hit our society hard, with more than 242 million cases reported worldwide and more than 4.9 million directly related fatalities. The role of Africa throughout the pandemic has been puzzling, since the African continent seems to have gone through the pandemic better than other continents; clearly better than predicted by the public duri...

Most real-world complex systems are extremely vulnerable to targeted attacks, making their immunization an important yet challenging task. One of the most effective attack strategies is targeting articulation points specifically. In this study, we first propose a generalized definition of network robustness. Then we address the problem of strengthe...

On-demand air mobility services, often called air taxis, are on the way to revolutionize our urban/regional transportation sector by lifting transportation to the third dimension and thus possibly contribute to solving the congestion-induced transportation deadlock many metropolitan regions face today. Although existing research mainly focuses on t...

Citation: Rosenow, J.; Chen, G.; Fricke, H.; Sun, X.; Wang, Y. Impact of Chinese and European Airspace Constraints on Trajectory Optimization. Aerospace 2021, 8, 338. Abstract: Air traffic trajectory optimization is a complex, multidimensional and non-linear optimization problem and requires a firm focus on the essential criteria. The criteria cove...

It is well-known that real-world systems, modeled as complex networks, are mostly robust against random failures but susceptible to targeted attacks. In this study, we propose a novel perspective to solve the network dismantling problem. Instead of designing an effective attack from scratch, we show how knowledge extracted from random failures in t...

Understanding and predicting mode choice behavior in urban areas is an ongoing challenge, with several factors identified in past studies, e.g. built-environment, household statistics, trip properties, and many models being developed, e.g., regression and nested logit models. Existing research studies are predominantly designed around stated prefer...

While COVID-19 has devastating effects on aviation, several recent studies have highlighted the potential of the pandemic-induced break for rethinking air transportation, hopefully orchestrating changes towards the construction of a more pandemic-resilient aviation system. Here, pandemic-resilient means that aviation stakeholders can sustain the im...

The air transportation industry has undergone unprecedented changes throughout the COVID-19 pandemic, as measured in terms of flight cancellations, aircraft retirements, airline bailouts, and disconnection of worldwide communities. In this study, we performed a cross-comparison of the impact COVID-19 had on three aviation centers of the world—the U...

The advent of COVID-19 is a sensible reminder of the vulnerability of our society to pandemics. We need to be better prepared for finding ways to stem such outbreaks. Except from social distancing and wearing face masks, restricting the movement of people is one important measure necessary to control the spread. Such decisions on the lock-down/redu...

After more than a year with COVID-19, it becomes increasingly clear that certain variants of concern have the potential to be game changers, determining the future of our aviation. These variants pose significant health threats and possibly undermine ongoing vaccination efforts. Recent research showed that flight bans on the initial SARS-CoV-2 outb...

This study investigates an airline integrated robust scheduling problem, subsuming three steps: schedule design, fleet assignment, and aircraft routing, while considering the effects of propagated delays. We propose a mixed integer programming model incorporating propagated delays and flight re-timing decisions; the model is extended towards a seco...

Air transportation systems are an important part of the critical infrastructure in our connected world. Accordingly, a better understanding and improvements in the resilience of the overall air transportation system are essential to the well-functioning of our society and overall sustainability of human beings. In the literature, network science is...

COVID-19 has been a major setback for air transportation; many airlines had to request for bailouts and the international flights connectivity is only restarting slowly. Accordingly, many aviation stakeholders put hopes into the ongoing process of vaccination, with the expectation that a high degree of vaccination will push the envelope for a retur...

Well-designed multi-modal transportation networks are crucial for our connected world. For instance, the excessive construction of railway tracks in China, at speeds up to 350 km/h, makes it necessary to consider the interaction of rail with air transportation for network design. In this study, we propose a model for an air/rail multi-modal, multip...

Despite China's massive infrastructure expansions in airports and high-speed railway (HSR), accessibility to these infrastructures from the passenger's perspective has yet been clearly analyzed and compared. The main objectives of this study are: (1) to evaluate accessibility by travel time between a given population grid cell and the nearest airpo...

This paper aims to analyze and understand the impact of the corona virus disease (COVID-19) on aviation and also the role aviation played in the spread of COVID-19, by reviewing the recent scientific literature. We have collected 110 papers on the subject published in the year 2020 and grouped them according to their major application domain, leadi...

COVID-19 is one of the most impactful pandemics in recent history, not only in terms of direct casualties but also regarding socio-economic impact. The goal of our study is to investigate the degree of synchronization between the number of confirmed cases in specific countries, on one hand, and how/at which stage these countries adapted their air t...

The idea and development of on-demand air mobility (ODAM) services are revolutionizing our urban/regional transportation sector by exploring the third dimension: vertical airspace. The fundamental concept of on-demand air taxi operations is not new, but advances in aircraft design and battery/engine technology plus massive problems with congestion...

Throughout the past years, researchers increasingly study the resilience of transportation systems through the lens of complex networks. This model simplification has helped to identify bottlenecks for all kinds of systems, e.g., subway, railway, and road networks. Nevertheless, for large networks, with ten thousand and more nodes, standard complex...

Solving the vast majority of hub location problems is NP-hard, implying that optimally solving large-scale instances (with hundreds of nodes) with exact solution techniques is extremely difficult. While heuristics have been developed which scale up to hundreds of nodes for specific problem types, these techniques do not scale up for further larger...

The current outbreak of COVID-19 is an unprecedented event in air transportation. This is probably the first time that global aviation contributed to the planet-wide spread of a pandemic, with casualties in over two hundred countries. As of August 23rd, 2020, the number of infected cases has topped 23 million, reportedly relating to more than 800,0...

During planning and execution of their flights, airlines face complex decision-making processes: The operation of all aircraft according to a minimum-cost plan poses tremendous difficulties even for medium-sized airlines. Accordingly, the airline scheduling problem is historically decomposed into a collection of four subproblems: Schedule design, f...

SESAR Flightpath 2050 sets several ambitious goals for European transportation in the year 2050. One of these goals concerns door-to-door travel time, which aims at having "90% of travellers complete their journey, door-to-door within four hours". This goal, however, is missing a clear baseline, i.e., "How good is the current door-to-door accessibi...

Air transport delays are a major source of direct and opportunity costs in modern societies, being this problem is especially important in the case of China. In spite of this, our knowledge on delay generation is mostly based on intuition, and the scientific community has hitherto devoted little attention to this topic. We here present the first da...

The current outbreak of COVID-19 is an unprecedented event in air transportation. In this study, we investigate the impact of COVID-19 on global air transportation through the lens of complex networks different at different scales, ranging from worldwide airport networks where airports are nodes and links between airports exist when direct flights...

The current outbreak of COVID-19 is an unprecedented event in air transportation. While it was known that air transportation plays a key role in the spread of a pandemic, it is probably the first time that global aviation contributed to the planet-wide spread of a pandemic, with over 500,000 reported casualties related to the infection. In this stu...

Throughout the last decade, the rise of open data in research has led to the release of many scientific datasets. Air transportation data are of particular interest given their high impact on migration, epidemic models and so on. Existing datasets are often specifically hand-curated, with the support of volunteers. One major problem with such datas...

Network dismantling techniques have gained increasing interest during the last years caused by the need for protecting and strengthening critical infrastructure systems in our society. We show that communities play a critical role in dismantling, given their inherent property of separating a network into strongly and weakly connected parts. The pro...

Complex network metrics are used for ranking the importance of nodes in many different applications, e.g., spreader identification and percolation analysis. Examples for such node metrics include the degree centrality and betweenness centrality. The computation of some of these metrics is computationally expensive, e.g. the computation of betweenne...

A new comparative framework for estimating the road access of airports around the world at high resolution is proposed. While existing studies are spatially constrained and often require hand-collection of data, this framework relies on freely available datasets at planet scale: Population density database Gridded Population of the World and transp...

The analysis of real-world systems through the lens of complex networks often requires a node importance function. While many such views on importance exist, a frequently used global node importance measure is betweenness centrality, quantifying the number of times a node occurs on all shortest paths in a network. This centrality of nodes often sig...

Improving the efficiency and resilience of air transportation is one of the major challenges in the 21st century. Large cities are often in the catchment area of multiple airports. Accordingly, the services at these airports can be comple-mentary or competitive; an airport-specific view of resilience neglects the effects of such interactions. In th...

Robustness measures the toleration of complex networks against random failures and malicious attacks. A malicious attack removes the most important node iteratively and destroys the network quickly. It is crucial to strengthen the network robustness against malicious attacks. In this paper, we propose an algorithm to strengthen the robustness with...

Betweenness centrality, which measures the contribution of an individual node to network’s connectivity by counting the number of shortest paths a node appears in, is widely used for the analysis of complex networks. The computation of exact betweenness centrality is prohibitively expensive for large networks, given a worst-case complexity of O(N*E...

The tail assignment problem (TAP) is an pivotal part of the airline planning process with the goal of enabling efficient and safe operations. As the airline industry faces in increasing number of schedule disruptions, taking into account uncertainty during scheduling receives increasing interest by researchers. This paper presents a novel stochasti...

Addressing topological properties of real-world networks requires the use of null models, of which the most common are random Erdős-Rényi graphs with the same number of nodes and links than the network under study. Yet, these latter graphs are completely structure agnostic, and can therefore be disconnected. In this study we analyse the bias introd...

Research studies on accessibility, urbanization, and land usage, increasingly make use of the community-driven project OpenStreetMap(OSM). The amount of data collected throughout the world via OSM has grown tremendously throughout the last decade, with the current planet-scale dataset reaching almost one terabyte of uncompressed storage (in XML for...

When dealing with evolving or multidimensional complex systems, network theory provides us with elegant ways of describing their constituting components, through, respectively, time-varying and multilayer complex networks. Nevertheless, the analysis of how these components are related is still an open problem. We here propose a general framework fo...

We design and implement a door-to-door travel time estimation framework, which aims to analyze the potential competitiveness of on-demand air taxis in Europe when competing with existing transportation modes: car, railway and traditional air transportation. Our grid cell-based framework, opposed to previous studies, allows for fine-grained, high-re...

In this paper, we design and implement HUBBI, a heuristic-based method to minimize the transportation/setup costs for p-hub location problems with incomplete hub networks. First, HUBBI pre-computes an estimation of node pair quality for establishing hub links. The quality ranking over all node pairs is then used to guide an iterative network design...

Estimating, understanding, and improving the robustness of networks has many application areas such as bioinformatics, transportation, or computational linguistics. Accordingly, with the rise of network science for modeling complex systems, many methods for robustness estimation and network dismantling have been developed and applied to real-world...

This study aims to clarify a few concepts when assessing the robustness of complex systems with the usage of complex network methods. Many researchers simply use the degree centrality as a proxy for node importance in a network. We show that such assumption can lead to rather overestimated robustness values, neglecting much stronger attacks. Furthe...