2.1 Airport simulation model

The Shanghai Hongqiao Airport, selected as the subject of this study, is equipped with two closely spaced parallel runways and features a highly typical End Around Taxiway (EAT) configuration. This layout bears a striking resemblance to the runway structure observed at DFW, particularly runways 17R and 17C, which were among the earliest to implement EATs. The inclusion of Hongqiao Airport is crucial for an in-depth exploration of EAT utilisation. Hongqiao Airport operates under curfew regulations, limiting its flight activities to the period between 6:00 and 24:00. With a daily average of over 750 flights and an hourly published capacity of 51, it is a bustling and sizable airport.

The primary focus of this study is on the strategic planning and application scenarios of End Around Taxiway (EAT). Given the complexity of testing in real-world scenarios, the research employs AirTOP as a simulation tool. AirTOP was developed by TransSoft, a Canadian company. AirTOP, the leading airport and airspace simulation software, is a comprehensive suite of tools to model, simulate, and visualise airport and air traffic operations in fast time. It can be used for enroute, approach and departure simulations as well as airport ground movement research. It is capable of setting up operational scenarios to analyse complex concepts in a single simulation scenario [Reference Tee YY25]. Therefore, we can set up different simulation scenarios of EAT applications for analysis through AirTOP.

Runway and gate assignments emerge as pivotal factors influencing the frequency of runway crossings. The spacing between the two runways at Hongqiao Airport is 365 m. In principle, the 18R/36L (west runway) is mainly used for take-off and the 18L/36R (east runway) for landing. The airport is divided into northward operation and southward operation. Runway 18L is used as the landing runway, and Runway 18R is used as the take-off runway when operating southward. Runway 36L is used as the take-off runway, and Runway 36R is used as the landing runway when operating northward. Gate allocation rules at Hongqiao Airport predominantly hinge on the affiliations of different airlines, directing them to distinct apron areas. In this paper, the southward operation of the airport is used as the study scenario.

Figure 1(b) shows the AirTOP simulation model of Hongqiao Airport, in which runway 18R is used as the takeoff runway and runway 18L is the landing runway.

Figure 1.

Hongqiao Airport diagram with AirTOP simulation configuration.

Operational parameter settings

The airport field-related setup parameters are shown in Tables 1, 2 and 3. Due to the high attack angle during take-off and landing, wake turbulence becomes significant and extremely hazardous, so wake vortex separations on aircraft are necessary [26]. As such, ICAO has set minimum separation standards for departure and arrival flights [27]. The wake turbulence separation between departures is judged by time intervals, while arrivals are judged by distance [28]. In addition, there are separation requirements between approaching and departing aircraft, which are judged by distance.

2.2 Flight data analysis

To conduct a precise analysis of Hongqiao Airport’s operations, we utilised flight data from May 2023, a month during which the average daily flights at Hongqiao Airport amounted to 744. The decision to construct and operationalise the End Around Taxiway in the eastern section of the airport hinges on its potential impact on operational efficiency. Consequently, to simulate the airport’s future incremental flight demand, we integrated both actual flight data provided by the airport and anticipated future flight demands. In addition to addressing the existing flight volume scenario, we conducted incremental increases based on the current flight data to meet future flight demands. Our study delves into the ramifications of different scenarios, both pre- and post-flight volume increases, on the operational efficiency of the airfield.

The average daily hourly traffic distribution and the percentage of arrivals and departures at Hongqiao Airport in May 2023 are shown in Fig. 2. Hongqiao Airport is an airport with a curfew and its flight activities are mainly concentrated from 6:00 to 24:00 h.

Table 1.

Taxi speed limit

Table 2.

Operation parameters setting

Table 3.

RECAT-CN wake turbulence separation (km)

Figure 2.

Traffic flow distribution.

In various scenarios of EAT utilisation, EAT positioned at the approach and departure ends of the runway exerts influence on departing aircraft from Terminal 1 (T1) and approaching aircraft with gates at Terminal 2 (T2), respectively. Currently, EAT imposes restrictions on the use of aircraft classified as Class C and lower. Examining the structural layout of Hongqiao Airport, the terminals are dispersed on both sides of the runway. However, the west terminal (T2) boasts a significantly higher number of gates and passengers compared to the east terminal (T1). Hongqiao Airport designates the west runway for takeoffs and the east runway for landings. Consequently, there is a pronounced demand for arrivals to cross the departure runway to access the gates at T2, representing the primary terminal (as depicted in statistical data illustrating the terminals where arrivals and departures parking gates are located and the percentage of aircraft types, as shown in Fig. 3). As shown in Fig. 3, about 75% of the aircraft stands are close to the west terminal T2, with about 30% of the aircraft in category D and above.

We simulated the operation of Hongqiao Airport in May 2023, and to verify the validity of the simulation model, we compared the simulated runway throughput and taxiing time with the actual runway throughput and taxiing time obtained from ALDT, AIBT, AOBT and ATOT.

Figure 4 shows the empirical and simulated total throughput (departures and arrivals) for the average daily hourly flights. It can be seen that the empirical curves match the simulated curves with root mean square error (RMSE) of 2.63 and 3.67 (number of aircraft), respectively.

Figure 3.

Percentage of aircraft type and slot distribution.

Figure 4.

Comparison of arrival and departure cumulative flights based on empirical and simulated data.

Figure 5.

Comparison of taxi-in and taxi-out times based on empirical and simulation data for terminal distinction.

Figure 5 shows the comparison of taxi-in and taxi-out times based on empirical and simulation data. The average taxi-in times at Hongqiao Airport are 8.9 min (simulation) and 10.0 min (empirical), with a difference of 1.1 min, and the average taxi-out times are 13.8 min (simulation) and 16.7 min (empirical), with a difference of 2.9 min. The difference can be explained by the statistical range of the data and the difference in the operation rules. Firstly, there is a difference in the statistical range of taxiing time between simulation and empirical data; ALDT refers to the touchdown moment of the aircraft, and the simulation taxiing-in time starts at the moment when the aircraft vacates from the runway. The simulation does not take into account the effect of controlled airspace (flow control) for departing flights, so there is a discrepancy between the situation for take-off queues and the actual operation. In addition, the simulation taxi-out time starts with the moment of leaving the stand, and the simulation leaves the stand later than the AOBT. Hence, the difference between the simulation and empirical taxi time is within an acceptable range.

The simulation model can produce reasonable estimates that match empirical data in terms of taxi time and runway throughput. In addition, the simulation can set up scenarios outside of actual operations to analyse complex concepts in individual simulation scenarios, such as the expansion and activation of EATs, as described in Section 3.

2.3 Operational efficiency evaluation indicators

The main objective of constructing EAT is to provide a taxiing route for aircraft without crossing the runway, so that arrivals can complete “runway crossing” without affecting departing flights, hence significantly reducing the possibility of runway incursion. In addition to the above, the use of EAT reduces holding for inbound flights and eliminates the need for frequent start/stop of aircraft during taxiing. While the adoption of EAT diminishes the holding duration, it substantially increases the taxi distance compared to runway crossing. Consequently, the use of EAT may lead to prolonged taxi times for aircraft, resulting in elevated time and fuel costs.

Based on the advantages and disadvantages of EAT mentioned above, we choose holding duration, taxi time and taxi distance as the evaluation indexes of field operation efficiency under different EAT operation scenarios.

Holding duration signifies the waiting time—an aircraft’s taxi time minus its unimpeded taxi time based on the planned route—attributable to taxiing separation, conflicts or runway occupancy [29]. It serves as a metric reflecting the field’s congestion level. Unimpeded taxi time in simulations represents an aircraft’s necessary operating time within the simulation system, covering the distance from the parking position to the runway without any interference. This excludes time spent on taxiing stops, waiting, etc., prompted by potential operational conflicts. While the calculation methods for holding duration and additional taxi time are similar, this study introduces changes to aircraft taxi routes through distinct simulation configurations. Consequently, the unimpeded taxi time for the same combination of runway and stand varies across scenarios, influencing the calculation of additional taxi time. Hence, the additional taxi time of aircraft in different scenarios is collectively referred to as holding duration.

\begin{align*} {\rm Holding\;duration} = {\rm Taxi} \hbox{-} {\rm in/out\;time} - {\rm Unimpeded\;taxi} \hbox{-} {\rm in/out\;time}\end{align*}

Simulation taxi time encompasses the operational time of a flight within the simulation, spanning from the parking position to the runway. This duration comprises both taxi-in time and taxi-out time. Taxi-in time denotes the duration from runway exit to reaching the on-block position, while taxi-out time indicates the duration from leaving the stand to departing the take-off waiting queue. Statistics about taxi distance align with those of taxi time.

3.1 Scenario 1: Baseline scenario

The baseline scenario is simulated based on the existing operation mode of Hongqiao Airport, as shown in Fig. 6. Currently, the EAT operation procedure at the departure end of Hongqiao Airport is: provided for arrivals whose parking positions are on the west apron (T2); aircraft of category C and below can use EAT without restriction; aircraft of category D and above need to cross the runway, and need to pause the use of runway 18R/36L for takeoff if they enter EAT by mistake.

Figure 6.

Scenario 1 operation configuration (southward operation).

Figure 7.

Scenario 2 operation configuration.

Figure 8.

Scenario 3 operation configuration.

Figure 9.

Scenario 4 operation configuration.

Figure 10.

Scenario 5 operation configuration.

Since aircraft in category D and above need to cross the take-off runway, their waiting at the holding point will obstruct the taxiing of subsequent arrivals. Therefore, the proportion of aircraft in category D and above will affect the holding duration of arrivals, and the higher the proportion, the longer the holding duration.

3.2 Scenario 2: Unrestricted taxi around for category D and above arrivals

The OLS of Annex 14-Airports to the Convention on International Civil Aviation restricts the use of EAT at Hongqiao Airport to aircraft in category D and above. The use of EAT at DFW Airport is subject to the terminal instrument procedures (TERPS) standard.

For Hongqiao Airport, if it follows the FAA standards, aircraft of category D and above types can also operate on the EAT without restriction, as shown in Fig. 7. Therefore, the study of different operation scenarios of EAT at the airport is carried out in advance for the possible future increase of the slope of the takeoff-restricted surface.

All arrivals with parking positions on the west apron choose EAT so that approaching aircraft will not have crossing waiting time due to takeoff runway occupation. This approach diminishes the likelihood of congestion, markedly mitigating the influence of the aircraft type ratio on holding duration.

3.3 Scenario 3: Category D and above arrivals using EAT at the departure end of the East zone

In line with the current operational practices at DFW Airport, arrivals requiring the use of EAT will diverge from both sides of the landing runway and access the EAT via distinct entrances based on their aircraft type [30]. As illustrated in Fig. 8, Hongqiao Airport has similarly implemented EATs on both its east and west sides. To enhance EAT utilisation, arrivals of category D and above can leverage the east EAT.

An approaching aircraft using the East zone EAT will result in an increased taxi distance, and thus a longer taxi time, compared to an aircraft vacating from the west side of the runway to enter the EAT.

3.4 Scenario 4: Departures using approach end EAT

Currently, according to the FAA’s study on the implementation of approach end EAT, aircraft operating on approach end EAT will have a psychological impact on the pilot who is landing, which is not conducive to the safety of flight [Reference Reisweber21, Reference Skiver22]. The EAT at the departure end adopts the measure of setting up a shelter to avoid the psychological impact on the pilot taking off on the runway. Since there is no effective measure to avoid the psychological impact on the landing pilots, the approach end EAT is still in the research and demonstration stage. However, approach EAT is important for reducing the risk of runway incursion, decreasing the holding duration of departing aircraft, and easing the congestion of the field, so it is possible to analyse the impact of the feasibility of approach EAT on the operational efficiency of the field in the future through the simulation study on the use of approach EAT by the departing aircraft, and to reflect the necessity of realising the approach end EAT in this way.

Given the operational mode at Hongqiao Airport, departing aircraft with gates at T1 are required to cross the landing runway, heightening the risk of runway incursion. Additionally, the priority given to arriving aircraft over ground-waiting aircraft increases the holding duration for departing flights awaiting clearance to cross the runway. Consequently, an exploratory study on the feasibility of enabling approach end EAT is initiated. This study, conducted within the framework of the baseline scenario that augments the use of approach end EAT by departing aircraft, aims to analyse its potential impacts on field operation efficiency and safety. Figure 9 shows the taxiing route of the departures using the approach end EAT.

3.5 Scenario 5: Category D and above arrivals using EAT on the departure end of the East zone and departures using EAT on the approach end

Building upon the four aforementioned scenarios, this particular case contemplates the concurrent activation of EAT at the East zone’s departure end and the approach end, as shown in Fig. 10. The activation of EAT at both the east departure end and approach end serves to eliminate the need for runway crossings, thereby significantly diminishing the risk of runway incursion. This strategic approach not only enhances the safety of airport operations but also alleviates the workload on controllers.