A Transitional System for Operating Both Sectorless and Sectored Airspace in Southeast Asia

A transitional system for operating both sectorless and sectored airspace in Southeast Asia

Hee Wei Gary Foo, Zhao-Wei Zhong
School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore

Abstract

As air traffic demand grows, airspace planners work around the increase in workload on air traffic controllers by dividing the airspace into sectors for better manageability. However, this method has its limits and also brings about inefficiency in the air traffic system. One possible solution to this problem is the implementation of a sectorless airspace – an airspace with a single unified sector. Its benefits include dynamic manpower allocation, shorter flight path and more.

It has been many years since the introduction of the sectorless airspace concept, and yet this idea has not become operational to date. This paper therefore discusses some of the key considerations with regards to the implementation of this idea – the stakeholders, the changes necessary, and the work already done by others.

To date, researches done on the sectorless idea are mainly confined to laboratory and simulations. For the sectorless idea to take off, there must first be a trial performed in the real-world. The trial is a necessary step to gain approval and acceptance from many stakeholders and must therefore be carefully designed. Following this, a transitional system for operating both types of airspace is conceptualized and discussed in this paper.

The proposed transitional system is a mixed-mode system for operating both traditional sectored and sectorless airspaces in tandem. Several aspects such as flight rules, scope of coverage, arrangement of air traffic controllers, as well as coordination strategies between agencies are presented. In addition, the benefits, trade-offs, and dangers of this transitional system are also briefly examined.

In this paper, the airspace of the Southeast Asian region is used as a case study. Preliminary analyses also showed that the effect of route lengthening as a trade-off on the regional traffic is minimal.

Finally, the future of the sectorless implementation in the Southeast Asia context is discussed. Several areas of necessary development and future study for this concept are also briefly presented.

Introduction

The current approach to handling increase in air traffic demand is to further divide the airspace into smaller portions (each known as a sector), where workload becomes more manageable for a single air traffic controller (ATCO). This method is preferred by many airspace planners as it is simple to implement, and can be reused for multiple times.

However, such method of divide-and-conquer has its limits. Increased number of sectors will lead to a decrease in efficiency due to difficulty in coordination [1, 2]. Pilots will also face the need to change radio frequency more often, resulting in an increase of their workload.

Furthermore, it is reasonable to assume that there is a physical limit to the extent of airspace division which can occur.

Two solutions are being researched on recently to reduce the workload on ATCOs. The first one is through dynamic sectorization. This approach works by actively adjusting the size of each sector to optimize the aircraft-to-ATCO ratio and keep the workload of ATCO manageable. Wong et al. presented some results on solving a multi-objective formulation of the concept using a genetic algorithm in the Singapore airspace [3]. In Europe, Amasuno Arrebola had also demonstrated a case study of Barcelona West Airspace that using this approach does generate some limited benefits [4].

The second alternative is not an evolution of the current sectored airspace approach, but a revolution towards an airspace with a single unified sector.

Sectorless ATM

The sectorless airspace concept was first introduced by Duong et al. in 2001 [5]. Rivière subsequently revisited the sectorless topic in 2004 and proposed a simple route network with improvisations via optimization techniques [6].

In the sectorless concept, ATCOs are no longer assigned responsibilities by geographical area (i.e. sectors), but assigned to several specific aircraft instead. The result is one unified airspace with no sector boundary. Since there is no sector boundary in this concept, radio handoffs are also no longer necessary, thereby reducing unnecessary radio chatter. In the new concept, an ATCO would ideally guide an aircraft from the point it enters till it exits the airspace. The same ATCO stays in communication with a single aircraft for the entire duration and provides navigation guidance. Essentially, the ATCO becomes an additional pilot for the navigation of the aircraft. The same ATCO is also in charge of ensuring the safety of the flight path as well as to resolve any possible aircraft conflict with other ATCOs ahead of time.

Since there is no longer a fixed number of sectors to operate in an airspace, the number of ATCOs needed on duty is now dynamic. Furthermore, the direct pairing of ATCO with specific flights and not via a predefined geographical sector gives better flexibility to the watch manager in assigning workload to all ATCOs on duty.

Since ATCOs are not allocated to a fixed geographical sector, every ATCO is therefore required to be equally well-versed with all regions. This therefore leads to greater flexibility in the allocation of work between the ATCOs. Pütz et al. also suggested that these changes could double the efficiency of ATCOs and the air traffic management system [7].

Emergencies also benefit from this concept since any ATCO can be allocated an emergency to handle regardless of the geographical location. In addition, the pairing of a flight to one ATCO throughout means that the pilots handling the emergency do not need to change radio frequency as they fly across sectors – a procedure which is necessary with the current system. This frees up time for the pilot to deal with the emergencies at hand. The controllers can benefit from the system as well, as it allows the ATCO handling the emergency to be not allocated any other responsibilities beside the emergency aircraft.

Key Considerations fora Sectorless Airspace

The Role of Controllers

While the sectorless concept promises many benefits, its implementation has been rather slow. Despite its introduction in 2001, the concept has yet to become operational even after 16 years (at the time of writing this paper).

One key reason was that ATCOs have been reluctant to accept the new framework. Despite studies showing that sectorless gave higher efficiency, ATCOs still expressed reservations with regards to its permanent implementation [8].

Several other works have been done by Birkmeier et al. in improving the impact ATCOs have on the performance of a sectorless airspace. They include studies on the mental model of controllers [9], the teamwork strategies ATCOs can apply [10], and the design of the controller work position to augment the performance of ATCOs [8, 11].

One major change to the work pattern of ATCOs is the shift of responsibility towards monitoring [12]. The change stemmed from the introduction of much conflict detection and advisory algorithms, which is necessary for the sectorless concept. This therefore reduced the amount of “control inputs” necessary from ATCOs, and some are uncomfortable with this reduction.

Several flight assignment strategies for ATCOs in the sectorless context were also studied. The flight assignment center is an agent (a person or a machine) who is in-charge of allocating a specific flight to a specific ATCO. The strategies may include grouping of flights from the same destination, same origin, or same direction etcetera. Schmitt et al. had devised and analyzed a mathematical model for the flight assignment problem in the form of a mixed-integer programming [13].

Adjustments to Flight Rules

Ideally, a sectorless airspace comprises elements of free flight airspace design. Therefore, it is necessary that the existing flight rules be extended to cater for free flight scenarios.

In 1996, Duong et al. published a document under Eurocontrol Experimental Centre with regards to this. The document detailed the possible conflict patterns of two aircraft and the resolutions for each scenario. In addition, Duong et al. also considered the category of flight (normal, emergency, search and rescue etc.), phase of flight (climb, cruise, descent), as well as maneuverability of the aircraft [14]. Known as the Extended Flight Rules (EFR), this set of rules laid the foundation for many subsequent studies on free flight.

Although the document only discussed the conflict situation for two aircraft, there is no reason why the EFR cannot generalize to any number of aircraft in conflict. However, this claim remains to be tested.

In 2009, the DFS (the German air navigation service provider) and DLR (German Aerospace Center) started the Airspace Management 2020 (LRM2020) program and looked at sectorless in particular. Using the EFR published by Duong et al. earlier, Pütz et al. [7] came up with an expanded set of flight rules for sectorless which included more intricate details such as overtaking of flights, and aircraft close to top of descent.

Subsequently in 2011, Birkmeier et al. published a paper discussing the results of a human-in-the-loop experiment performed with a small group of ATCOs using the rules defined by Pütz et al. The consensus was that the flight rules defined were sound but not fully sufficient. The ATCOs involved in the experiment also expressed preference for flexibility in following or disregarding flight rules in favor of efficiency in some cases.

Safety Consideration

Birkmeier et al. had also investigated several safety considerations with regards to the sectorless concept. The Swiss cheese model was adopted as the analogy to describe the air traffic system – where a multi-layered approach is used to catch a mistake should one fall through a single layer. [15] argued that the current safety nets for the sectored concept are largely compatible with the idea of a sectorless airspace. With a few additions and modifications, the new safety net would be suitable for sectorless operation.

Birkmeier et al. also presented a safety assessment of sectorless airspace operation using a success-based approach [1]. The paper presented several tables of possible hazards, the hazard classification, as well as some recommended mitigation plans.

Transitioning toa Sectorless Airspace

Over the years, much work was done to demonstrate the feasibility and benefits of a sectorless airspace, as well as to expose the problems it might bring about. However, it must be recognized that no matter how promising the new system might be in the laboratories and simulations, no air navigation service provider (ANSP) will switch to the new system without any experiment and proof of use in reality. Therefore, some sort of trial system must be devised to give the new sectorless concept a chance to be operated on a portion of the real-world traffic. The remaining majority of the traffic shall remain under the traditional sectorized concept of management. Such is the nature of an operational trial, and it must be carefully planned to ensure that the larger majority of traffic is unaffected by the test.

Five transition strategies were raised by Birkmeier et al. [16] in deploying a mixed-mode system. They are summarized as follows:

• Element-Wise: Only integrate certain sectorless concepts into the current system for testing, such as the conflict detection system or the priority rules.
• Aircraft-Wise: One or more specific aircraft will be controlled using the sectorless framework whereas others will follow the traditional method.
• Time-Restricted: Switch between sectorless or sectored airspace depending on the time of the day, possibly to try out the sectorless concepts during periods of lower traffic density.
• Area-Restricted: Combining several (but not all) sectors in the airspace to form one bigger supersector. The supersector will then behave like one sectorless airspace, allowing trials to be performed within.
• Top-Down: Start the trials with the upper flight levels before moving down towards the lower altitudes.

These strategies are great start to the discussion, and offer multiple angles of inspiration. The following section extends on the aforementioned ideas into a system for operation.

Proposed System

In the proposed transitional system, the air traffic picture of Southeast Asia (SEA) is being used as a case study. SEA was forecasted by Boeing [17] to have a traffic growth of 6.2 percent, a 1.5 percent higher than the global average. In addition, the region will also see an increase in more than 4000 new aircraft over the next two decades. This makes the SEA region most prone to capacity insufficiency problems in the near future, and also a good candidate on which to study the use of the sectorless concept.

The scope of the study therefore revolves around the 10 busiest airports in SEA in the year 2016. They are as ranked in Table 1.

Due to the proximity of the two airports in Bangkok, the midpoint between both is assumed and Bangkok will be treated as having only one airport. In essence, only nine airports are being included in the study.

Table 1. 10 Busiest Airports in SEA

a.This statistic is actually ranked by total passengers per year. However, due to the unavailability of aircraft movement statistics, this alternative is used instead.

Route Network

Figure 1 shows the route network generated by connecting all nine airports with each other using a great circle track. In this case, there are 92 crossing waypoints and they are indicated on the figure as blue crosses. The routes drawn only consider the origin and destination coordinates and disregard flight paths of SIDs and STARs for simplification purposes. Although this network is the most efficient in terms of distance travelled by aircraft, it is also perhaps the most undesirable due to the large number of crossing waypoints to manage and the high frequency of needing to deconflict crossing traffic. Therefore, the number of crossing waypoints must be reduced to a reasonable level.

In the new system, it is proposed that several waypoints be merged (in this case, into 6 waypoints) and results in a network with fewer crossings such as in Figure 2. It can be seen that the resultant network is simpler, has fewer crossing waypoints, less messy, yet does not cause a large detour for all origin-destination (O/D) pairs.

In reality however, the presence of restricted airspaces due to military areas, adverse weather, or geographical terrain necessitates a further deviation

Figure 1. Routes generated from nine busiest airports in SEA. (Crossing waypoints = 92)

of the flight path. As seen inFigure 3, the route network will be further modified in the presence of a restricted airspace. The algorithms behind the generation of the route networks will be discussed in a separate paper.

The 3D Picture

The red lines shown in Figures 1, 2 and 3 will become reserved ‘highways’ for aircraft participating in the sectorless trial. These highways ideally extend across all feasible flight levels. However, no aircraft can fly precisely on track – there will always be some error such as path definition error, flight technical error, or navigation system error in the path following system. In line with ICAO Asia Pacific Seamless ATM Plan [18], the recommended PBN for these tracks should be RNP4 at the minimum. Hence, factoring in the probable total system error in tracking of the flight path by the aircraft, as well as to build in a margin of safety, a “virtual wall” of 8 NM from the track centerline is constructed on both sides of the highways. These virtual walls serve to

Figure 2. Routes generated after crossing waypoints are merged. (Crossing waypoints = 6)

demarcate a corridor for sectorless aircraft to operate in. Non-participating aircraft (i.e. flying the traditional sectored airspace) will be assigned a lower priority should there be a need for them to intersect a reserved highway to get to the other side.

However, it must be clarified that the concept is currently 2D only. There is no variation in the vertical nor temporal dimension.

Flight Rules

Sectorless traffic flying within the virtual walls (shown earlier) will adhere to the flight rules presented by Korn et al. in [7]. There is no difference expected between a fully sectorless airspace and a trial system, and therefore the rules should be applicable. Hence, there should be no need to modify the flight rules for this purpose.

However, the operation of both sectorless and sectored airspace in tandem necessitates some additional rule for their interaction. The most likely situation for the interaction of both type of traffic is

Figure 3. Modified route network as a result of a restricted airspace.

when there is a need for the non-sectorless aircraft to cross a sectorless highway onto the other side. If there is no sectorless aircraft near the intersection point, then the intersection problem is easily resolved – the non-sectorless aircraft is simply cleared to cross the highway.

Yet, should there be a sectorless aircraft approaching the intersection point from a distance that warrants concern, then there is a need to implement some sort of prioritization between the aircraft. In most cases, the sectorless aircraft will not have to deviate whereas the non-sectorless aircraft will have to make additional maneuvers. Table 2, describes the possible scenarios and solutions where prioritization is necessary. (In the table, Aircraft (1) refers to the sectorless aircraft and aircraft (2) refers to the non-sectorless aircraft.)