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Review

Impacts of COVID-19 on Air Traffic Control and Air Traffic Management: A Review

by
Armaan Kamat
1 and
Max Z. Li
2,*
1
Department of Aerospace Engineering, University of Michigan, Ann Arbor, MI 48109, USA
2
Department of Aerospace Engineering, Department of Civil and Environmental Engineering, Department of Industrial and Operations Engineering, University of Michigan, Ann Arbor, MI 48109, USA
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(15), 6667; https://doi.org/10.3390/su16156667
Submission received: 25 June 2024 / Revised: 20 July 2024 / Accepted: 22 July 2024 / Published: 4 August 2024
(This article belongs to the Special Issue Advances in Active Travel and Transportation Planning in Smart Cities: Past, Present and Future)
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Abstract

:
The global air transportation system continues to be greatly impacted by operational changes induced by the COVID-19 pandemic. As air traffic management (ATM) focuses on balancing system capacity with demand, many facets of ATM and system operations more broadly were subjected to dramatic changes that deviate from pre-pandemic procedures. Since the start of the COVID-19 pandemic when air travel became one of the first transport modes to be impacted by lockdown procedures and travel restrictions, a geographically diverse cohort of researchers began investigating the impacts of the COVID-19 pandemic on air navigation service providers, airline and airport operations, on-time performance, as well as airline network structure, connectivity, crew scheduling, and service impacts due to pilot and crew shortages. In this study, we provide a comprehensive review of this aforementioned body of research literature published during one of the most tumultuous times in the history of aviation, specifically as it relates to air traffic management and air traffic control. We first organize the reviewed literature into three broad categories: strategic air traffic management and response, air traffic control and airport operational changes, and air traffic system resilience. Then, we highlight the main takeaways from each category. We emphasize specific findings that describe how various aspects of the air transportation systems could be improved in the domestic and global airline industry post-COVID. Lastly, we identify specific changes in operational procedures due to the COVID-19 pandemic and suggest future industry trends as informed by the literature. We anticipate this review article to be of interest to a broad swath of aviation industry and intercity transportation audiences.

1. Introduction

The global air transportation system (ATS) was, and continues to be, greatly impacted by the COVID-19 pandemic [1,2]. Pandemic-related disruptions resulted in significant negative financial consequences across all key players in the aviation value chain, including airlines, original equipment manufacturers (OEMs) of aircraft and aircraft components, air navigation service providers (ANSPs), and airports. Airlines were impacted more than any other aviation subsector and faced $168 billion in losses for 2020, with revenues falling by 55% [3]. Cargo airlines were the only exception, and they benefited from a rise in demand for air cargo during the pandemic. Even though the International Air Transport Association (IATA) estimates that global revenues for airlines increased by 27% in 2021 relative to 2020, they were still 44% less than 2019 revenues [3].
The high fixed costs and primarily variable revenue flows inherent in the business models of most airports worked to their detriment during the pandemic. The drastic reduction in traffic amounted to $32 billion in losses to airports in 2020 [3]. While the Airports Council International estimates that airport traffic throughput performance recovered to some extent in 2021, their revenues were still more than 50% lower than the 2019 levels. Aircraft OEMs unsurprisingly also incurred a 55% drop in orders for new aircrafts and sustained $12 billion in losses in 2020, largely on account of canceled orders and a fivefold increase in deferred deliveries stemming from reduced passenger demand and outlook of airlines [3].
Approximately 73 percent of Americans report taking air trips for personal reasons in 2023, up from around 70 percent in the period from 2015 to 2019 [4]. Additionally, by the end of 2023, the number of passengers traveling domestically in the U.S. surpassed pre-pandemic levels for the first time (around 819 million in 2023 vs around 812 million in 2019), whereas the number of international travelers had not yet surpassed pre-pandemic levels (around 234 million in 2023 vs around 241 million in 2019) [5]. While some segments of the ATS (e.g., leisure travel and domestic air travel) continued to recover at the start of 2023, ongoing and dynamic policy changes related to quarantine procedures and traveler restrictions hindered overall passenger demand recovery. With this context in mind, as the aviation industry continues to recover post-COVID, increased air traffic has led to a resurgence in the need for balancing system demand and capacity. The variety of operational procedures that fall under the broad umbrella of air traffic management (ATM) are critical to ensuring a safe, efficient, and sustainable ATS.

1.1. Relevance of Air Traffic Management during COVID-19

The drastic reduction in demand, followed by a progressive recovery that impacted the aviation industry as a result of the COVID-19 pandemic is wholly unprecedented in scale and impact. Correspondingly, as ATM is focused on balancing capacity and demand in the ATS, many facets of ATM were also subjected to dramatic changes that deviate from pre-pandemic procedures. This dynamic range of operational contexts—for example, from extremely low flight demand to recovering and sometimes even exceeding pre-pandemic demand [5]—would have been impossible to replicate. In other words, it is neither feasible nor plausible to suggest artificially depressing air travel demand in order to study potential variations in ATM procedures.
At the beginning of the COVID-19 pandemic, air travel became one of the first transport modes to be impacted by widespread lockdown procedures and travel restrictions [1]. As a result, a geographically diverse cohort of researchers began investigating the impact of the COVID-19 pandemic on many aviation stakeholders and topics. These include ANSPs, airline and airport operations, and air travel on-time performance, as well as airline network structure, connectivity, crew scheduling, and service impacts due to pilot and crew shortages. This emerging and growing body of literature related to air transport operations, traffic management policies, and stakeholders (e.g., commercial airlines, general aviation, air cargo fleet operators) sheds light on phenomena and system behavior (e.g., necessary versus redundant ATM procedures, airline schedule robustness) that previously would have been difficult or impossible to observe had it not been for the range of impacts on air travel demand induced by different phases of the COVID-19 pandemic. Additionally, this review article provides insights on the systems-level behavior of a critical intercity transport mode when impacted by a large-scale and sustained disruption.

1.2. Literature Survey Objectives and Contributions

In recognition of this newly formed and fast-paced sub-field of research within the air transportation research literature (e.g., the more than 60 papers reviewed in Table A1 in the Appendix A), as shown in Figure 1 and the list below, we defined the following objectives when designing and carrying out this literature review:
1.
Categorize research literature on COVID-19 within the operating context of ATM and ATC, as well as highlight main takeaways from each category.
2.
Emphasize specific findings that describe how ATM and related operations could be improved in a post-COVID domestic and global airline industry.
3.
Identify specific permanently changed operational procedures and suggest future research to improve the global ATS in the post-pandemic context.
The specific contributions of this study include a large set of pertinent, peer-reviewed literature, a synthesis and distillation of their main findings, and a comprehensive review of the state of ATM and ATC during one of the most tumultuous times in the aviation industry as a result of the COVID-19 pandemic [6].
Sustainability 16 06667 g001
Figure 1. Literature review objectives.
Figure 1. Literature review objectives.
Sustainability 16 06667 g001

1.3. Review Structure

As shown in Figure 2, the remainder of this review is structured as follows: In Section 2, we first describe the process we utilized to search for and retrieve relevant literature, as well as the criteria we adopted for paper selection. We then introduce the categories of COVID-19 and ATM/ATC research literature that we developed to organize our review. For each category, we summarize the relevant body of work and highlight the main takeaways from each category in Section 3. An in-depth discussion on particular findings follows in Section 4, with an emphasis on research results that demonstrate how ATM and ATC could be improved in a post-COVID airline industry. We conclude in Section 4 with future industry outlooks in terms of how the pandemic may have permanently changed specific operational procedures (e.g., a shift towards leisure markets that exacerbate airspace congestion due to increased demand), and we suggest future research that seeks to fully leverage data (e.g., on-time performance, schedule conformance and adherence) generated by the aviation industry during the COVID-19 pandemic in order to suggest potential improvements to the global ATS.

2. Literature Collection and Categorization

2.1. Paper Search and Retrieval

Similarly to [7], we utilized a literature search process involving searching a database using specific keywords and selection criteria. Using Google Scholar as the primary search engine, we considered a predefined set of inclusion criteria when determining whether or not a set of articles retrieved via search was relevant. The decision to select Google Scholar was made because it indexes millions of journal and conference papers from major publishers in the field of air transportation, including Taylor & Francis, Elsevier, Springer, IEEE, ACM, ASCE, and MDPI [8].
To begin the search process in Google Scholar, we utilized a group of keyword combinations that are frequently used in the field of ATM. Other than “air traffic management”, the keywords “air transportation”, “air transportation system”, “demand”, “capacity”, “air traffic control”, “air transportation operations”, “airside airport operations”, “landside airport operations”, “air transportation network”, “aircraft turnaround”, “flight safety”, and “flight efficiency” were combined and used. We based the selection of the keywords on the results of a preliminary search on papers in the ATM field. The common operational aspects of the ATS were focused on one or more ATM subfields represented by the keywords. To include studies that considered the relationships between the operational aspects of ATM and the impacts of the COVID-19 pandemic, we narrowed down the search criteria by including the keywords “COVID*” and “pandemic.” The search was based on the combined keywords that appeared in the titles, abstracts, keywords, and other metadata of the papers indexed by Google Scholar, and only papers published in 2020 and beyond were considered. Box 1 lists the combinations of keywords used in our search process. The ‘*’ character after a keyword indicates its possible variants. For instance, the search term COVID* would include COVID, COVID19, COVID-19, etc. in the retrieved papers. Similarly, air-transportation* would include air transportation, air transportation systems, air transportation operations, and other variations of the keyword.
Box 1. Keyword combinations used in our literature searches.
[“COVID*” OR “pandemic”] AND “air-transportation*” AND “demand”
[“COVID*” OR “pandemic”] AND “air-transportation*” AND “capacity”
[“COVID*” OR “pandemic”] AND “air-traffic-management”
[“COVID*” OR “pandemic”] AND “air-transportation-system”
[“COVID*” OR “pandemic”] AND “air-traffic-control”
[“COVID*” OR “pandemic”] AND “air-transportation-operations”
[“COVID*” OR “pandemic”] AND “airside-airport-operations”
[“COVID*” OR “pandemic”] AND “landside-airport-operations”
[“COVID*” OR “pandemic”] AND “air-transportation-network”
[“COVID*” OR “pandemic”] AND “aircraft-turnaround”
[“COVID*” OR “pandemic”] AND “flight-safety”
[“COVID*” OR “pandemic”] AND “flight-efficiency”

2.2. Inclusion Criteria and Paper Selection

The retrieved publications ranged from national and international ATM and air transportation operations (ATO) conference proceedings, papers in air transportation and transportation-specific journals, and technical reports published by universities and government agencies. Only papers written in English were considered. The results from multiple keyword combination searches were merged, and duplicates were removed manually. The retrieved papers were filtered based on an initial reading of the abstract, and only those papers that appeared to discuss one or more operational aspects of ATM or ATC in the context of the COVID-19 pandemic were retained. This process yielded 138 papers, which were selected for further consideration.
In the second phase of filtering, each retained paper beyond the first phase was rapidly scanned with the intention of identifying its objectives, research methods, and conclusions. Papers that did not directly address at least one operational aspect of ATM or ATC or papers that predominantly focused on various aspects of airline passenger experience during the pandemic were eliminated from further consideration. The specific operational categories (e.g., airline network planning, flight scheduling adjustments, airport operating procedures, ATC procedures, stakeholder resilience, demand, and capacity) of the literature that we reviewed are detailed in Section 2.3. In contrast, the papers that focused on the operational aspects of ATM and ATC allowed us to expand the search outwards using standard methods such as “snowballing” [9]. The conclusion of this phase and a second round of manually eliminating duplicates resulted in a comprehensive and well-balanced set of research literature, comprising of 67 papers that appeared to be of high relevance to the subject of our study. We based our review and findings on this finalized set of research literature that was identified. The papers retained for review were read in their entirety. For each study, we identified and recorded its motivation and objectives, research methods, data utilized in the analysis, and key conclusions.

2.3. Taxonomy of Topics Surveyed

We developed a taxonomy to organize the surveyed literature under three ATM/ATC categories: (A) strategic air traffic management response; (B) air traffic control and airport operational changes; and (C) air traffic system resiliency. Within these main categories, there were additional notable sub-catgories. Category A included airline network planning (A1) and flight scheduling adjustments (A2), category B included airport operating procedures (B1) and ATC procedures (B2), and category C included stakeholder resilience (C1) and demand and capacity (C2). We also introduced a subcategory flag of geographical scope (domestic, international, both), to classify the reviewed papers. The primary ATM/ATC categories in our taxonomy are highlighted in Figure 3.
During our review of the papers, we associated each of the 67 papers with the set of categories in our taxonomy and their geographic scope. The entire listing of the surveyed literature, along with their assigned categories, are presented in Appendix A. In general, we note that the majority of the papers were associated with multiple attributes and were thus broad in contextual scope.

3. Key Observations and Insights

We kept track of each paper’s primary contributions, objectives, methods, data sources, key findings, and conclusions, along with the paper’s assigned category attributes using a spreadsheet-based database. Each row corresponded to a reviewed paper. We sorted the papers in order to, i.e., identify clusters of studies with similar objectives and/or findings, as well as to observe patterns where findings in similar studies either converged or were different depending on, for instance, the geographic location. This process was followed for each of the six primary categories in our taxonomy, and key ideas and findings were observed. We summarize these findings and insights in the following subsections.

3.1. Strategic Air Traffic Management Response

3.1.1. Airline Network Planning

As passenger demand continues to rise beyond the pandemic, airlines have grappled with various operational issues in the planning of network recovery strategies, as explained further below, that were not previously encountered during other economic or event-related disruptions. For example, airlines typically leverage historical data and use those as a baseline to anticipate future passenger demand. However, recent studies suggest that this approach is no longer feasible for airlines in planning strategies for post-pandemic network recovery [10]. In particular, due to the magnitude of disruption that COVID-19 caused, airlines continue to realize that historical pre-COVID data has largely been irrelevant in the context of post-COVID recovery for forecasting demand recovery and strategic network recovery planning. For this reason, the authors in [10] proposed a new approach to benchmark air travel demand at U.S. airports to support planning processes at airlines and adjacent industry stakeholders. For example, one goal of the study was to identify airports where travel demand recovers in similar patterns to help affected stakeholders predict future travel demand and thus support network planning. The authors found that by November 2021, business travel was still at significantly reduced levels, while leisure airports showed stronger evidence of demand recovery.
Some airlines have begun to develop new business models focused on ultra-long-haul (ULH) point-to-point flights, with continued growth during the progression of the COVID-19 pandemic [11,12]. The business models that emerged for ULH point-to-point flights during the pandemic often evolved to include flights that combined passengers and cargo. This approach not only de-centered the existing emphasis on hub-and-spoke networks but also met the burgeoning need of supporting the global supply chain through air cargo as the pandemic evolved [12]. Researchers also found that point-to-point ULH services, in combination with an effective domestic feeder system, not only reduce the operational changes needed to adapt to COVID-19 but also succeed in simultaneously creating higher load factors and yields, heightened network flexibility, and improved social distancing by avoiding the processing of transit passengers through crowded airport hubs [11]. The need to conduct a range of special repatriation and critical cargo flights during the early stages of the pandemic provided airlines with an ideal opportunity to test their operational and logistical readiness to successfully provide point-to-point ULH services for leisure travel both during and beyond the pandemic.
The fact that large aircraft such as the Airbus A380 and passenger variants of the Boeing 747 ceased production [13] during the pandemic has further compelled airlines to change network strategies, and this is expected to impact passenger demand as recovery from the pandemic continues [14]. The resulting evolution of airline business models towards point-to-point networks is also expected to impact other operational aspects of ATM [12]. For example, hub-and-spoke airports often run on banked schedules (e.g., morning/afternoon pushes), which heavily influences congestion and chokepoints within the system and the times at which they manifest. The growth of point-to-point network models post-COVID thus offers the possibility of easing such inefficiencies by better utilizing and scheduling airport and airspace resources.

3.1.2. Flight Scheduling Adjustments

The onset and progression of COVID-19 made it necessary for airlines and airports to allocate additional time to clean aircrafts and socially distance passengers and staff during check-in and boarding. Despite considering strategies such as boarding from an additional rear door, airlines and airports generally experienced a significant increase in aircraft turnaround times that in turn forced changes in scheduled push back and arrival times [15]. Under normal operating conditions, additional turnaround times are likely to impact ATM operations, as they cascade into the arrival processes (e.g., arrivals may now have to be delayed during the taxi-in because the gates are still occupied). However, the reduced frequency of flights during COVID-19 may have offset this potential impact on recorded delays, as airlines had some latitude to increase the block times of scheduled flights. As observed in [16], the overall reduction in traffic intensity during COVID-19 may also have contributed to reducing delays and their impact on flight schedules on account of the improved lateral efficiency, infrequent hold-on patterns, and reduction in flight trajectory extensions that occur from vectoring in high traffic conditions.
An important aspect of flight scheduling changes (or lack thereof), particularly in the early phases of COVID-19, concerns the operation of so-called “ghost flights” [17], where airlines were compelled to operate empty or nearly empty aircrafts on routes with insufficient demand with the primary intention of not losing allocated airport slots. The authors in [17] studied the extent of such abnormal services during the COVID-19 pandemic through an explorative, data-driven analysis using actual load factor data from 2017 to 2021 for European airlines. The study found that the peak of such abnormal flights occurred during the early phase of the COVID-19 pandemic, primarily during March–April, 2020. The first slot waiver program was approved and implemented in Europe starting on 30 March 2020. The authors concluded that it was specifically low-cost carriers that primarily operated abnormal flights, particularly on sectors that represented frequently served markets [17]. Additionally, ref. [18] noted that specific government intervention in terms of redistributing slots may be necessary. For instance, similar to Europe, the U.S. Federal Aviation Administration temporarily waived minimum slot-use requirements at specific slot-controlled U.S. airports in March 2020, allowing airlines to cancel flights at slot-restricted airports in response to drastically reduced demand. However, as noted earlier, cancellations due to low demand in the U.S. were also restricted in some cases based on the implementation of the Essential Air Service (EAS) program [19]. Finally, to account for multiple sources of increased volatility and uncertainty, improvements in terms of airline operations research technologies were noted in [20], with expectations that the adoption of these advanced models would continue to improve airline efficiency.

3.2. Air Traffic Control and Airport Operational Changes

3.2.1. Airport Operating Procedures

Okulicz and Rutkowska [21] examined the impact of the COVID-19 related sanitary regime implemented at the Warsaw Chopin Airport. The study focused primarily on operational (airport operations) and organizational (human and material resources) factors and found that the average aircraft turnaround times in 2020 were almost double those in the same period in 2019 due to the additional aircraft cleaning and passenger handling requirements. The study also utilized the opportunity afforded by the pandemic to evaluate the efficacy of an airport collaborative decision-making (A-CDM) system for the timely and effective exchange of information in airport operating environments during highly dynamic and disruptive periods. Similarly, Teixeira et al. [22] analyzed the impact on aircraft ground times of new safety procedures at airports to avoid the spread of COVID-19. Using the Sao Paulo Congonhas airport as a case study, they analyzed the ground time durations before and after implementing the new cleaning and social distancing measures to confirm the expected increases in ground times. Their study found that compared to the period from April to July in 2019, where the Congonhas airport had a scheduled ground time between 30 and 40 min, in 2020, the scheduled ground time increased to between 50 and 60 min due to the new hygiene processes that were implemented. They also studied the impact of these increased ground times on airside airport operations using simulations.
COVID-19-induced turnaround procedures were necessary new procedures to reduce virus transmission, but they created additional incurred costs for airlines by decreasing seat load, requiring more cleaning personnel, and increasing overall ground times [15]. The authors found that a 67% load factor created a 20% increase in ground time with one boarding door and pre-pandemic levels of cleaning personnel. The same study [15] also showed that apron boarding with an additional rear door operational (compared to gate boarding using a single door) was the only scenario that could keep boarding times to pre-pandemic levels while complying with social distancing requirements and a 67% load factor (i.e., middle seat empty). The insights highlighted the potential of using such a strategy post-pandemic in the long-term to achieve faster turnaround times. It is yet to be determined if this would only be feasible if existing gates were retrofitted to enable simultaneous front and rear door boarding, or whether the existing busing and multi-door apron boarding strategies commonly in use today would achieve similar results.
In addition to longer aircraft turnaround times, the resulting inefficiencies cascaded into disruptions in airside and airspace operations as well [15,23]. The situation was further exacerbated because several airports cordoned off sections of terminal buildings to modify passenger flow and adapt to the reduced overall demand, and this in turn reduced the number of available and serviceable gates. The fact that several apron areas and taxiways had to be used as medium- to long-term parking areas for grounded aircrafts also contributed to a reduction in stands. Scala et al. [24] studied the effects of the reduction in the physical facilities at airports on airspace and airside airport capacity, especially in the terminal maneuvering area (TMA) airspace. Using Paris Charles de Gaulle as a case study airport, they adopted a state-based model that was perturbed by the evolution of the “rare” COVID-19 event. For several initial system states (or “situations”), the influence of COVID-19 created several “new situations” that affected airport facilities utilization and progressed until the system eventually reached its normal “pre-rare event” state. The study found that the main airport and airspace system bottleneck was the number of gates (i.e., stands) in the terminal system. This finding was particularly relevant to the recovery to pre-pandemic passenger levels and future growth in passenger demand. With regard to their specific case study, they determined that at Paris Charles de Gaulle, the system needed to reopen another runway and make more than 70 gates operational by the time the traffic recovery reached 55 percent of the pre-COVID-19 schedule.
Additional complexities arose in airside operating procedures and airport capacity because several areas including aprons, taxiways, and in some cases, runways were repurposed as parking locations for grounded aircrafts. For instance, multiple major airports in the U.S. (e.g., Chicago O’Hare, Houston Bush) had several runways that were closed for months at a time because they were being used as temporary storage of airframes by airlines and fleet operators. This required the recovering passenger aircraft traffic and the burgeoning cargo aircraft traffic to be handled within the constraints of the available airport resources during times of scheduled operations. Some studies proposed that careful coordination was thus required to maintain the safety and integrity of the airfields and any parked aircrafts to ensure the airports’ safe transition to normal operations and traffic levels [25]. For some airports, returning physical assets such as taxiways and runways back to service also involved the consideration of additional pavement maintenance due to the atypical, long-term static loading resulting from parked aircrafts on surfaces such as runways and taxiways [26]. Finally, longer-term effects stemming from the COVID-19 pandemic on the design and layouts of airport passenger terminals were discussed in ref. [27].

3.2.2. ATC Procedures

The circumstances resulting from the COVID-19 pandemic provided researchers with a unique opportunity to observe the impact of an unprecedented disruption on both the efficiency of ATC procedures at the system level as well as the impacts of a prolonged disruption on the air traffic controller workforce. With respect to the controller workforce, ref. [28] documented the impact on mental well-being (e.g., anxiety) of air traffic controllers and other personnel involved in air traffic management. The pandemic also provided researchers with an opportunity to study the effects of the air traffic demand decreases on the maintenance or degradation of air traffic controller skills, as well as suggest solutions for their upkeep. Responses from air traffic controllers in one study [29] suggested that controllers operating at large international airports perceive higher degrees of skill decay and may be more susceptible to the effects of skill fade after prolonged exposure to low traffic levels, such as those encountered during the COVID-19 pandemic. The research highlighted that ATC skills associated with the implementation of declarative knowledge (e.g., utilizing factual information on the preceding aircraft category to implement arrival spacings in low-visibility conditions) were the most susceptible to decay, particularly if those skills were performed in isolation and without “integration complexity”. Another study [30] complemented these findings and considered fatigue management in ATC officers during the COVID-19 period in light of the reduced workforce stemming from financial cutbacks, which in turn resulted from an overall decrease in passenger demand. Citing an increased number of fatigue reports from ATC personnel during the pandemic, the study cautioned against excessive financial cutbacks in fatigue management investments and budgets, given its critical role in safe ATC operations.
Intuitively, it would be logical to expect that the reduction in commercial air traffic during the pandemic naturally results in fewer delays. This was consistent with the findings by EUROCONTROL [31] following their analyses of COVID-19 impacts on European ANSPs, where the general interpretation was that fewer flights during the pandemic allowed for more direct flight profiles, resulting in reduced congestion throughout Europe. However, improvements in ATC efficiency and corresponding reductions in congestion and delays were not globally consistent. For instance, a study that investigated the impact of COVID-19 on air navigation efficiency in Brazil [32] found that system performance in terms of delays did not improve from the decline in traffic during the pandemic, i.e., there was little correlation between the number of flights and the average flight times. The authors conjectured that, unlike Europe or the US [33], where the volume of air traffic appeared to be the primary determinant of congestion and inefficiency (e.g., ref. [33] found that a single standard deviation increase in COVID-19 case numbers resulted in a reduction of arrival and departure delays by 1–2 min), in Brazil, factors such as the airlines’ route structures, the share of domestic flights, and passenger demand patterns had a stronger influence on air navigation system efficiency.
Notwithstanding these contrasting observations in different parts of the world, the reduction in the number of flights during the COVID-19 period saw a significant increase in deviations from nominal operations in terms of ATC procedures, especially during the critical approach and landing phases of flights. For example, at Paris Charles de Gaulle Airport, there was an overall increase in aircraft approaches with energy atypicality that were primarily associated with the shortening of trajectories, glideslope interceptions from above, and approaches with late speed reductions [34]. Even when the traffic at the Charles de Gaulle airport decreased by around 90% by April 2020, the atypical flight ratio increased by approximately 50%. Another study by IATA [35] independently confirmed such sharp increases in the rate of unstabilized aircraft approaches during the period of COVID-19. Similarly, a study conducted at major Australian airports [36] aimed to evaluate an improved performance measure by identifying whether a continuous descent operation (CDO) was executed by an aircraft’s automation, i.e., the flight management system (FMS). ANSPs aim to report successful CDOs in their airspace as a measure of flight descent efficiency. The study found that the proportion of CDOs, measured using the conventional metric, showed no increase from March to May 2020 after the pandemic-related traffic decline that started on 10 March 2020. However, in contrast, the managed descent measure showed a steep increase, as traffic declined from March to May 2020, which is indicative of less tactical intervention for sequencing traffic. Since speed control is a primary form of tactical intervention applied by ATC, less tactical intervention yields fewer speed deviations on descent during the execution of a CDO. Another study analyzed the buffer encroachment trends in the terminal airspace of 24 airports in the U.S. from April 2019 to December 2020 [37]. The reviewed research indicated that the analyzed airports showed significant variability in encroachment levels, with some airports such as Dallas/Fort Worth and Charlotte Douglas showing encroachment levels that were higher than expected during the reduced traffic period. The preliminary estimates of buffer encroachment changes from this study are thus indicative of potential safety degradations during COVID-19. However, it must be noted that the observed trends also revealed noticeable spikes in encroachment levels during the pre-pandemic period at other airports such as Atlanta Hartsfield and Denver International. Therefore, overall, the observed trends suggest that there are benefits to real-time measurements of buffer encroachments across airports as a way of monitoring safety margin levels and supporting decisions related to enforcing appropriate vertical and lateral separation of aircrafts with nearby traffic.
The period of COVID-19 also presented an opportunity to analyze ATM performance by partially allowing for the isolation of influencing factors that collectively result in flight delays. A study conducted at the Stockholm Arlanda and Gothenburg Landvetter airports in Sweden [16] found that weather has a stronger influence than traffic intensity on the vertical efficiency (measured by deviation of flights from desired CDOs), while traffic intensity has a stronger effect on the lateral efficiency (measured by horizontal deviation from reference trajectories) of arriving flights. The research reviewed the historical data of arrivals and analyzed the impact of factors such as weather and traffic intensity on arrival efficiency in isolated scenarios when only one factor dominated (e.g., isolated scenario with low traffic; isolated scenario with good weather conditions). The authors attributed the impact of traffic intensity on the lateral efficiency to the frequent hold-on patterns and flight trajectory extensions resulting from vectoring in high-traffic conditions.
Another study [38] took a holistic view of ATC over Europe and proposed that the period of COVID-19 presented an opportunity to reinvent EUROCONTROL along with stakeholder relationships and workflows intended to overcome the pre-pandemic gridlock. The authors observed that despite the vision for and intent of EUROCONTROL, ANSPs spread over multiple European countries cause a lack of centralization and thus higher costs. They proposed having a single ANSP across Europe similar to the US FAA that would be authorized to optimally redesign the European airspace to eliminate artificial congestion points that exist at the interfaces between the ANSPs. Furthermore, this would help facilitate the dynamic balancing of demand and capacity across the network.

3.3. Air Traffic System Resiliency

3.3.1. Stakeholder Resilience

Several studies have attempted to understand how operational strategies played a role in airlines’ and aviation stakeholders’ ability to overcome the impact of the COVID-19 pandemic and remain as viable organizations or businesses. For example, in [39], the authors used a PESTLE analysis to study the impact of six external factors (political, economic, sociological, technological, legal, and environmental) on the aviation industry after the onset of the COVID-19 pandemic. Using publicly available information from United Airlines and Southwest Airlines as industry representatives, the study concluded that government support and regulation of the industry and stakeholders post-pandemic can influence recovery. In particular, the authors observed that government support programs such as the U.S. CARES Act were pivotal in increasing stakeholder resilience by protecting vulnerable service routes and preventing worker furloughs [39], although stratification in terms of air service-related airport impacts was noted [40,41,42], with notable airport clusters in terms of responses to changes in passenger demand and available capacity [43]. PESTLE analyses and other frameworks were collectively examined in [44] for use in studying the management of air transportation systems. Ref. [45] also mentioned that airline markets with more competition pre-pandemic had a higher chance of making a full recovery, attributing this to the fiercer competition often associated with more successful and agile airline business strategies. Finally, multiple studies note complex interactions between government intervention, airline responses, and industry dynamics that arose from the reactions of various stakeholders to COVID-19 [18,46,47,48,49].
Passenger demand and the number of operated flights drastically declined globally around mid- to late-March 2020 in response to border closures and public health advisories. The number of flights in Europe declined more than in most other parts of the world, while many domestic flights continued to operate in the U.S. [50]. Since the onset of the COVID-19 pandemic, several researchers have studied the impact of the initial sharp reduction and still-ongoing recovery in passenger demand on the evolution of airline networks and seat availability between origin–destination (O-D) pairs, countries, and regions. For example, using a multi-granularity network analysis that considered global airport networks as well as country-specific networks in the U.S. and other countries from January 2020 to May 2020, Sun et al. [51] observed that the airline networks and flight connectivity in the southern hemisphere of the globe underwent a more drastic reduction compared to the northern hemisphere. The study also highlighted that, unlike the U.S., China and Europe experienced a significant decrease in domestic flight connectivity; an observation the study attributed to the extensive rail networks in those countries. Similar network analyses were conducted in [52,53], using network metrics such as average shortest path lengths, bridge/core nodes, and augmented network efficiency metrics. Notably, air freight networks were less impacted, as shown in studies such as [54]. These findings were consistent with refs. [55], which observed that COVID-19-related disruptions had a more significant impact on airline O-D flows that were connected with airports in China than those O-D flows without such connections. Another study [56] found that the disconnection of a single country had little impact on the connectivity of the Chinese International Air Transport Network (CIATN) when it was treated as an unweighted network, no matter what policy was implemented. However, the authors also found that when after flight frequency was considered, the impact of various countries’ entry policies on the connectivity of the CIATN varied significantly.
A study by Fuellhart et al. [19] similarly complemented the observations made for the U.S. described in [51] by analyzing the impact of the government-subsidized Essential Air Service (EAS) Program on the stability of the U.S. airline network during the pandemic. Using a comparative complex network analysis as a primary method, their analysis confirmed that, in aggregate, EAS airports in the U.S. performed better than non-EAS airports in preserving seat capacity during the peak of the COVID-19-related disruptions. This observation points again to more resilient domestic flight connectivity in the U.S. compared to other countries in Europe and Asia. Another study [57] undertaken to analyze the network dynamics of individual U.S. airlines from January 2019 to December 2021 observed that full-service carriers were less flexible in changing their network structure and therefore suffered higher revenue losses during the pandemic. The study applied a framework for analyzing temporal networks wherein the network structure changes over time, and they used monthly reports provided by the airlines to the U.S. Bureau of Transportation Statistics. The authors concluded that even though the number of nodes and edges returned to pre-pandemic levels around July 2021, the structure of the entire U.S. domestic airline network remained altered, attributed in large part to regional carriers who shifted to new route structures in an attempt to reduce their revenue losses. For European air carriers, ref. [58] describes typical deployed responses including changing fleet compositions, staff reductions, and network re-configurations.

3.3.2. Demand and Capacity

Several recent studies in the literature focus on describing long-term policy and investment opportunities to better meet the demand and capacity constraints that are expected to emerge post-COVID. For instance, in [59], the authors acknowledge that even though prior plans for developing and deploying the Single European Sky Air Traffic Management Research (SESAR) program involved a linear plan, the lessons learned during the pandemic and the need for ATM modernization post-COVID require that SESAR continue to evolve through continuous life cycle decisions.
The COVID-19-related disruptions in demand and capacity have also highlighted other adjacent operational priorities for aviation stakeholders. For instance, the aviation sector has broadly embraced a long-term goal of working to build a carbon-neutral sustainable aviation system by 2050 [60,61]. The drastic drop in passenger demand and the resulting reduction in scheduled flights offered researchers the opportunity to highlight the aviation sector’s carbon footprint prior to and during the pandemic. Taking two European airports (Zagreb and Split in Croatia) as case studies, the authors in [62] attempted to estimate emissions reduction by combining the drop in passenger numbers with the ICAO Carbon Emissions Calculator, which estimates passengers’ CO2 contributions based on flight durations. Given that the number of flights in Europe fell by as much as 89 percent in April 2020, the authors computed that the CO2 emissions for Zagreb were reduced by a factor of 1.81 and for Split by a factor of 3.49 based on 2019 and 2020 passenger numbers. However, fuel consumption and emissions have been increasing as expected during recovery [63], with increased couplings between fuel usage and air operation counts [64]. Additionally, the impacts of COVID-19 on the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) implementation was studied in [65]. On a positive note for potential new avenues towards aviation de-carbonization, ref. [66] developed a model to project post-COVID-19 demand recovery with the assumptions of synthetic fuel adoption and carbon pricing policies.

4. Discussion on Post-COVID Operational Changes in Air Transportation Systems

As summarized in the previous sections, the COVID-19 pandemic induced several operational changes within the air transportation system in response to the drastic reduction in demand and the resulting drop in capacity stemming from, e.g., more stringent sanitary requirements during aircraft turnaround. As airports, airlines, and other air transportation system stakeholders grappled with new operating conditions, several new procedures and business models emerged, primarily to meet the evolving demand (passengers and cargo) and to remain viable during and beyond the pandemic. As the air transport industry continues to emerge from the impacts of the pandemic, we expect several operational changes and stakeholder strategy adoptions observed during this period to persist in the future as demand and capacity continue to grow [67]. Several specific operational procedures and changes that were instituted by airlines, airports, and regulating bodies were detailed previously in Section 3. While implementation of some of these strategies by various stakeholders began prior to the COVID-19 pandemic, the unique circumstances during the pandemic provided an environment for evaluating their feasibility and in some cases accelerated their deployment.
The pivot to carrying increasing amounts of cargo emerged as a key strategy for some passenger airlines to remain viable in the face of drastic reductions in passenger demand. As airline operations rapidly evolved with the pandemic, empty “ghost” flights turned into primarily cargo flights, with some hybrid configurations as well. As the demand for critical cargo continued to grow during the pandemic, airlines began to leverage non-passenger revenue streams (e.g., cargo operations) more efficiently to improve their financial resilience. We expect that airline and airport business models will base their long-term profitability on nimble strategies that dynamically adapt to demand changes in both passenger and cargo revenue streams as a normal part of post-pandemic operations [23].
Another trend we expect to continue and grow beyond the pandemic is the operation of point-to-point ULH flights by airlines to an increasing number of destinations. The motivation to reduce passenger processing through connecting airports and experience gained operating special ULH repatriation flights during the pandemic have contributed to subsets of the industry transitioning away from hub-and-spoke models [11]. For example, in March 2020, Qantas operated several repatriation flights between Sydney, Australia, and London, U.K. using an Airbus A380 aircraft. A fuel stop in Darwin, Australia was necessary because the Singaporean government had banned transit passengers and stops in the Middle East were not possible due to airspace closures [68]. Subsequently, after putting the project on hold for two years due to the pandemic, in May 2022, Qantas placed an order for 12 Airbus A350-1000 aircrafts. This new addition to their fleet will enable the implementation of a long-desired element of Project Sunrise: direct flights of over 19 h long between Sydney, Australia and London, U.K., beginning in 2025 [69].
The onset of the COVID-19 pandemic provided the aviation industry with a low-demand environment to implement several ATM procedures. One particular set of enhancements that the ATM community is focused on implementing during the recovery are trajectory-based operations (TBOs), which leverage modern aircraft capabilities and improved ATC procedures. The systems, capabilities, and processes in TBOo can help manage flight trajectories by scheduling and metering aircrafts through constraint points (time-based management), enabling aircrafts to accurately navigate along their trajectories (performance-based navigation), and automating the sharing of aircraft trajectory information (enterprise enablers). This portfolio of ATM improvements hopes to achieve system-wide increases in flight efficiency [70].

5. Conclusions

As the aviation industry navigates through the recovery phases of the COVID-19 pandemic, increased air traffic has led to a resurgence in the need for strategic and tactical ATM, streamlined airport and airline operations, and more robust, resilient air transportation networks. The drastic reduction in demand followed by a progressive recovery experienced by the aviation industry as a result of the COVID-19 pandemic is wholly unprecedented in scale and impact. As evidenced by the breadth of material presented in this review, there are several areas of future research inspired by the impact of the COVID-19 pandemic. Potential research questions include the following, among others: What is the role of ultra-long-haul flights in a post-COVID market setting? Are “mixed operations” involving airlines carrying both air cargo and passengers a viable business strategy [1]? In addition to simulators, what future training methods can be developed to reduce air traffic controllers’ skill fade due to a sudden reduction in air traffic [29]? In this review paper, we first gave a broad overview of how the COVID-19 pandemic has impacted the aviation industry from a multitude of perspectives and operational characteristics. We then presented a summary of a collection of research literature pertaining to the impact of the COVID-19 pandemic on air transportation system operations, and we highlighted the main takeaways from each category. For each category, we also tried to incorporate a broad range of geographies and stakeholders whenever possible to highlight the differential impacts that the COVID-19 pandemic had on the aviation industry. We then provided an in-depth discussion on particular findings, with an emphasis on research results that are relevant to how ATM and ATC operations could be sustainably improved post-pandemic. We conclude our review with outlooks in terms of how the pandemic may have permanently changed specific operating procedures and processes within the air transportation system. The overarching goal of the many papers surveyed is to critically examine how the global air transportation industry handled the COVID-19 pandemic and how to best prepare the industry—upon which an increasing number of people rely on for transport and services—for the next pandemic.

Author Contributions

All authors contributed equally to the study presented in this manuscript, including the identification of relevant papers, their review, and summarizing of key observations and insights. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new shareable data were created in this study.

Acknowledgments

The authors are grateful to the comments and suggestions from anonymous reviewers, whose contributions have helped to strengthen the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Table A1. Summary of the literature reviewed in this study.
Table A1. Summary of the literature reviewed in this study.
Paper InformationTaxonomy
Organization		Domestic/
International
[1] COVID-19 pandemic and air transportation:
Successfully navigating the paper hurricane		A; CBoth
[2] Airline market exit after a shock event:
Insights from the COVID-19 pandemic		A; CDomestic
[3] Taking stock of the pandemic’s impact on
Global Aviation		CBoth
[4] Air Travelers in America: Annual SurveyN/ABoth
[5] Passengers (All Carriers-All Airports)N/ADomestic
[6] An early assessment of the impact of
COVID-19 on air transport: Just another crisis or
the end of aviation as we know it?		N/ABoth
[7] Climate change influences on aviation:
A literature review		A; CBoth
[8] Google Scholar, Microsoft Academic,
Scopus, Dimensions, Web of Science,
and OpenCitations’ COCI: a multidisciplinary
comparison of coverage via citations		N/AN/A
[9] Guidelines for snowballing in systematic literature
studies and a replication in software engineering		N/AN/A
[10] Benchmarking the recovery of air travel demands
for US airports during the COVID-19 Pandemic		A; CDomestic
[11] Ultra Long-Haul: An emerging business
model accelerated by COVID-19		A; CInternational
[12] Feeling the Pulse of Global Value Chains:
Air Cargo and COVID-19		A; CBoth
[13] Boeing’s last 747 has rolled out of the
factory after a more than 50-year production run		CBoth
[14] The role of very large passenger aircraft in
global air transport—a review and outlook to
the year 2050		A; CInternational
[15] Future aircraft turnaround operations considering
post-pandemic requirements		A; B; CBoth
[16] Arrival flight efficiency in pre- and
post-COVID-19 pandemics		BInternational
[17] Ghostbusters: Hunting abnormal flights in
Europe during COVID-19		A; CInternational
[18] Hub airport slot Re-allocation and subsidy
policy to speed up air traffic recovery amid COVID-19
pandemic—case on the Chinese airline market		A; CInternational
[19] The U.S. Essential Air Service Program and
SARS Cov-2, 2019–2020		CDomestic
[20] Airline OR Innovations Soar During
COVID-19 Recovery		A; CBoth
[21] The Impact of COVID-19 on Airport OperationsB; CInternational
[22] COVID 19 Impact on Aircraft Ground Time
at Congonhas Airport (CGH)		B; CInternational
[23] Exploring the Performance of International
Airports in the Pre- and Post-COVID-19
Era: Evidence from Incheon International Airport		B; CInternational
[24] Air Traffic Management during Rare Events Such
as a Pandemic: Paris Charles de Gaulle Case Study		B; CInternational
[25] Grounded aircraft: An airfield operations perspective
of the challenges of resuming flights post COVID		A; BInternational
[26] COVID-19 Grounded Aircraft - Parking and StoringBBoth
[27] The impact of COVID-19 pandemic on the
future airport passenger terminals design		BInternational
[28] The Effect to Human Performance and
Wellbeing of Air Traffic Management Operational
Staff Through the COVID-19 Pandemic		B; CInternational
[29] The Effect of the COVID-19 Pandemic on Air
Traffic Controllers’ Performance: Addressing Skill
Fade Concerns in Preparation for the Return to
Normal Operations		B; CInternational
[30] Post COVID-19 Fatigue Management for ATCOsB; CInternational
[31] What COVID-19 did to European Aviation in 2020,
and outlook 2021		CInternational
[32] Air Navigation and COVID-19: ATM Efficiency
in Pandemic Crisis		B; CInternational
[33] The airline on-time performance impacts of the
COVID-19 pandemic		CDomestic
[34] Flight safety during COVID-19: A study of
Charles de Gaulle airport atypical energy approaches		B; CInternational
[35] Unstable Approaches During Reduced OperationBBoth
[36] ANSP Measures of Flight Descent PerformanceA; BInternational
[37] Buffer Encroachment Trends in the Terminal
Airspace across Major US Airports and Impact
of COVID-19		B; CDomestic
[38] Reinventing European air traffic control based
on the COVID-19 pandemic experience		BInternational
[39] Based on COVID-19, the Strategy Research
of Airlines Companies–Case Study of Southwest
Airlines & United Airlines		A; CDomestic
[40] Quantifying the Resilience of the U.S. Domestic
Aviation Network During the COVID-19 Pandemic		A; CDomestic
[41] Evaluation of Airport Size and Delay Causal
Factor Effects on Delay Propagation Dissipation		B; CDomestic
[42] The impact of COVID-19 on domestic U.S. air
travel operations and commercial airport service		A; CDomestic
[43] Airport Performance Metrics Analysis: Application
to Terminal Airspace, Deicing, and Throughput		B; CDomestic
[44] Developing a strategic framework of analysis
for air transport management		A; CBoth
[45] A data-driven analysis of the aviation recovery
from the COVID-19 pandemic		B; CBoth
[46] European airlines’ strategic responses to the
COVID-19 pandemic (January–May 2020)		A; CInternational
[47] Mayday, Mayday, Mayday! Responding to
environmental shocks: Insights on global airlines’
responses to COVID-19		A; CInternational
[48] The Aviation Industry: Tackling the turbulence
caused by COVID-19		A; CBoth
[49] U.S. network and low-cost carriers’ performance
in response to COVID-19: Strictness of government
policies and passengers’ panic		A; CDomestic
[50] The Impact of COVID-19 on Flight NetworksABoth
[51] How did COVID-19 impact air transportation?
A first peek through the lens of complex networks		A; CBoth
[52] Vulnerability of the worldwide air transportation
network to global catastrophes such as COVID-19		A; CBoth
[53] Examining COVID-19-triggered changes in
spatial connectivity patterns in the European air
transport network up to June 2021		A; CInternational
[54] An analysis of the Chinese scheduled freighter
network during the first year of the COVID-19
pandemic		A; CInternational
[55] The impact of the COVID-19 pandemic on O-D
flow and airport networks in the origin country and
in Northeast Asia		AInternational
[56] Impact of entry restriction policies on international
air transport connectivity during COVID-19 pandemic		AInternational
[57] Dynamics of the US domestic airline network
during the COVID-19 pandemic		A; CDomestic
[58] European airline response to the COVID-19
pandemic—Contraction, consolidation and future
considerations for airline business and management		A; CInternational
[59] SESAR: The Past, Present, and Future of European
Air Traffic Management Research		B; CInternational
[60] Working to Build a Net-Zero Sustainable Aviation
System by 2050		A; B; CDomestic
[61] 2050 ICAO Vision for Sustainable Aviation
Fuels		BInternational
[62] Impact of coronavirus (COVID-19) pandemic
on air transport mobility, energy, and environment:
A case study		B; CInternational
[63] Impacts of COVID-19 on aircraft usage and
fuel consumption: A case study on four Chinese
international airports		A; CInternational
[64] The impact of COVID 19 on the aviation
fuel supply chain in the face of changes in air
traffic service: case study of one of the polish airports		CInternational
[65] How does COVID-19 affect the implementation
of CORSIA?		A; CBoth
[66] COVID-19 and pathways to low-carbon air
transport until 2050		A; CBoth
[67] Aviation Safety and Risk Management
During COVID-19		B; CInternational
[68] Qantas to operate first ever non-stop
Darwin-London Flight		AInternational
[69] Qantas Group announces major aircraft order
to shape its future		A; CInternational
[70] Session 5: Looking Beyond COVID-19BDomestic

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Figure 2. Literature review structure and process.
Figure 2. Literature review structure and process.
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Figure 3. Taxonomy of categories and sub-categories utilized in this review paper.
Figure 3. Taxonomy of categories and sub-categories utilized in this review paper.
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Kamat, A.; Li, M.Z. Impacts of COVID-19 on Air Traffic Control and Air Traffic Management: A Review. Sustainability 2024, 16, 6667. https://doi.org/10.3390/su16156667

AMA Style

Kamat A, Li MZ. Impacts of COVID-19 on Air Traffic Control and Air Traffic Management: A Review. Sustainability. 2024; 16(15):6667. https://doi.org/10.3390/su16156667

Chicago/Turabian Style

Kamat, Armaan, and Max Z. Li. 2024. "Impacts of COVID-19 on Air Traffic Control and Air Traffic Management: A Review" Sustainability 16, no. 15: 6667. https://doi.org/10.3390/su16156667

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