The Sky Keeper

SESAR Between Bureaucracy and Innovation

The Single European Sky ATM Research (SESAR) project is an enormous plan that promises to completely overhaul the European air traffic management (ATM) environment during the next several years, with a total investment of more than 2 billion euros (US$2.5 billion). With the involvement of several leading aviation and ATM industries worldwide, it will have a significant impact on flight operations far beyond Europe.

Without doubt, the SESAR project is a massive, yet necessary, undertaking. Therefore, it must be broken down into smaller pieces. As such, the SESAR program is split into a high-level overall concept layer and a work-package layer. The work-package layer is divided into an operational and a system thread that, again, are split into several work packages. Most work packages are led by EUROCONTROL, Airbus and air navigation service providers. The work package for which airlines can articulate requirements related to flight operations is called work package 11.1 (WP 11.1). It is represented by a consortium called Fly4D, which consists of Sabre Airline Solutions®, Lufthansa Systems, Airbus Defense And Space (formerly known as Cassidian) and Honeywell, and it is led by Airbus S.A.S.

One of the fundamental concepts of SESAR is the trajectory. The trajectory is a four-dimensional (4D) entity that allows exchange of flight path data in much higher detail than the route defined in the current International Civil Aviation Organization (ICAO) flight plan format. WP 11.1 is taking a leading role in introducing all ATM players to the advanced trajectory prediction capabilities of modern flight planning systems.

A surprisingly common and deep-rooted misconception in the ATM world is the view that modern flight planning systems are “pseudo flight management system” applications that mimic onboard flight management computers to calculate flight paths. In the past, the ATM community widely ignored the essential fact that only modern flight planning systems have the ability to actually create trajectories from scratch and, more importantly, create optimum trajectories. Highlighting this fact can be seen as a small first achievement of SESAR in that ATM stakeholders get a better understanding of the gaps in trajectory creation and handling capabilities between modern computerized flight planning systems and systems used by air navigation service providers or flight management systems onboard aircraft.

The Fly4D consortium manages WP 11.1, from operational concept to system specification, prototyping and validation. Participation in working groups that create or evolve ICAO and ARINC standards already leads to global recognition of the SESAR research.

Two examples of the industry-driving power of SESAR are the changes proposed to the ARINC 702a standard and the creation of a new flight plan format called Extended Flight Plan.

The changes in the ARINC 702a standard will allow a richer temperature data set for climb and descent wind uplinks, which enables the flight management system to better predict the top-of-climb and top-of-descent position during flight execution. While this may not sound like a mind-blowing evolution, it is a big step toward flight management systems computing the accurate top-of-descent position for continuous-descend operations.

High-Resolution Airspace Structure

In conventional direct-route airspace, every possible segment is defined. To achieve a high-resolution airspace structure, this results in a large number of published connections, the maintenance of the published segments and the associated restrictions.

The Extended Flight Plan, on the other hand, is an important enabler for sharing the high-quality trajectory information of modern flight planning systems with all ATM stakeholders. Air-navigation service providers are already showing interest in integrating this new flight-plan format into their ground-trajectory-prediction tools.

However, there is also a focus on much tighter integration of the flight operation center into the ATM decision-making process. The core concepts here are collaborative decision making and a user-driven prioritization process.

Collaborative decision making is an already slightly worn term that describes many different processes during the execution of a flight in different phases (ground and airborne). A user-driven prioritization process describes processes that allow aircraft operators to submit flight prioritization, which will help reduce costs in irregular situations by implementing the most efficient departure sequence.

During the execution phase, there is also great potential for increasing flight efficiency, if modern flight operations systems (operations control and flight planning) are brought into the loop of the “reroute” process. In this context, “reroute” is actually a legacy term that does not describe the 4D character of the process, since the “route” is only two dimensional and the focus is on trajectory revisions and the trajectory revision process within SESAR.

As advocates for the airspace users, the Fly4D consortium fights to prevent proliferation of multiple time constraints as a simple, but for airlines costly, means to overcome conflicts in the execution phase, as well as the revival of outdated concepts such as the repetitive flight plan in its new European incarnation, “nominal preferred route.” Instead, the Fly4D consortium uses every opportunity to push for a wider and earlier implementation of real free-route airspace.

Even then, the challenge remains to avoid a step back for airspace users. The initial step to replace the airway route structure within SESAR is an attempt to introduce even more published directs (published via the RAD Appendix 4), a concept called direct -route airspace within SESAR. The difference between free-route airspace and direct-route airspace is not obvious at first.

Free-route airspace describes airspace where any connection can be used within the trajectory optimization process considering published constraints. The trajectory optimization algorithm just needs to follow well-described rules (e.g. maximum segment distance) within the airspace. Direct-route airspace is an attempt to bring the advantages of “more (unnamed) airways” into legacy systems.

This might sound like a logical step to take in evolving ATM. The risk, however, is that the legacy will be kept alive as opposed to using the research and development character of SESAR to drive innovation. With direct-route airspace, the doors are wide open to expand the route availability document, which contains the vast majority of airspace restrictions and constraints in European airspace, since published directs can be restricted in the same way as airways today. The data maintenance effort has to be carefully taken into consideration when validating the direct-route airspace concept.

Free-route airspace, on the other hand, demands a completely new way of dealing with and defining constraints and restrictions. Such constraints and restrictions would not be linked to distinct segments within the airspace but would be defined as a four-dimensional volume of airspace with certain conditions tied to it that constrain how the volume can be utilized.

The trajectory optimization algorithm will still evaluate discreet connections within the airspace and verify them against such “flow-constraining volumes,” but the maintenance of such volumes of airspace is significantly easier compared to thousands of defined segments with distinct conditional restrictions linked to each of these published directs. Apart from that, when using flow-constraining volumes, the intention of the airspace designer is much more transparent, more direct and published in a clearer way.

Route Availibility Document

Airspace users today are already facing a high number of complex restrictions associated with individual segments of direct-route airspace. Such restrictions are published in a PDF document called the Route-Availability Document. Navigations officers of airlines or flight planning system providers have to work through that document every 28 days to keep the airspace structure updated.

Within the SESAR trajectory-management processes, the so-called shared-business trajectory (comparable to an early filed flight plan), the reference-business trajectory and the flight object are the key elements.

Concepts that involve the shared-business trajectory include requesting airlines to share flight plan data even six months before departure. Shared-business trajectory data is intended to support the demand- and capacity-balancing process completed by the network manager (EUROCONTROL). Validation will eventually show if a flight plan produced six months prior to the departure utilizing, at best, a significant sample size of historical winds or at least statistical winds is worth the effort compared to using historic flight data immediately.

The reference-business trajectory is per definition the trajectory that a user agrees to fly and the air navigation service providers and airport agree to facilitate. It is an attempt to increase predictability within the highly unpredictable ATM environment. The likelihood that the agreement will be kept, without any renegotiation, until the flight has landed can be considered rather low. Hence, predictability is questionable. The reference-business trajectory in SESAR was initially planned to only exist between air traffic controllers and aircraft. Changes to the reference-business trajectory can be done through a reference-business-trajectory revision process, which was initially defined excluding the airline’s flight operations systems, as well.

The reference-business trajectory revision process is currently designed to be mainly initiated by air traffic controllers or flight crews. For several reasons, it is critical that the airline’s flight operations staff is involved wherever feasible (e.g. where the time horizon allows a closed loop with the airline’s operations control). Fly4D is participating in the SESAR trajectory-management framework meetings to ensure that the trajectory-management process is modelled, including airlines’ flight operations centers.

Thus far, SESAR planned to rely heavily on enhanced ADS-C data to frequently update the precision-trajectory data residing in ATM ground systems. However, these new capabilities won’t be widely available in aircraft systems for many years. Flight operations systems are already capable of processing aircraft-position-report data and updating the reference-business trajectory for all stakeholders. In addition, these systems can generate an optimum solution when a new constraint is identified and have the complete data set available to create the optimum trajectory for the given situation. Complete data set in this context means an airline’s network; its cost structure; the most up-to-date wind, temperature and weather data; precise aircraft performance data for regular and aircraft defect situations (such as gear-down operations); and a holistic view of flights coming into Europe.

In addition, a few issues remain with aircraft flight management system optimization capabilities. The biggest challenge is that certain flight management systems calculate slightly higher cost-index speeds (up to 1 percent faster) than optimum. This type of cost-index-speed calculation originated when fuel was less expensive, flight planning systems were primitive and carbon emissions were not a factor. Obviously, things have changed in that respect.

Another group of flight management systems do not allow the execution of speeds, which are lower than the max-range cruise speed under no-wind conditions. This implies that the aircraft cannot fly the optimum speed under tailwind conditions but rather flies faster.

This is considered by many airlines as a significant cost penalty that can only be partially eliminated by entering an incorrect shifted cost-index value into the flight management system or by flying the speeds calculated on the operational flight plan. Unfortunately, the “on-the-spot” optimization opportunities of the flight management system, which is still advantageous in many cases (e.g. when the actual wind is significantly different compared to the predicted winds of the operational flight plan), cannot be utilized in the latter case.

Direct Definition Strategy

By understanding and defining airspace as a four-dimensional volume, which is what it actually is, the intention of the airspace designer and the technical description are brought much closer together, allowing a much more direct-definition strategy. This approach avoids misunderstandings and brings the maintenance effort down to a manageable level. In free-route airspace, a volume has a usage rule as opposed to every segment within an airspace having its own restriction.

In the SESAR precision-trajectory environment, this is not an option anymore due to the associated loss of predictive accuracy. Conversely, turning the flight planning system into a “pseudo flight management system” by adopting the same trajectory prediction model and algorithm should not be considered either.

Undoubtedly, SESAR is large and complex. But the bottom line is that SESAR is progressing and its first fruits can be harvested in the near future. For the first time, SESAR offers flight planning system providers the opportunity to directly influence development of new ATM systems on a global scale and ensure that proposed changes closely follow the interest of airspace users.

However, there is still plenty of work to do, and the initial plan seems to have been a bit too ambitious from both timing aspects and willingness to accept political change from the member states and industry.

In addition, due to the gigantic and slow-moving nature of SESAR, all participants and work packages want to ensure that nothing is conceptually validated that might potentially have the smallest negative impact on their domains. This approach is understandable since the SESAR implementation timeline does not realistically allow for a second try.

Unfortunately, the project has reached a critical stalemate situation where innovation is stifled. But to make the innovative leap, for which SESAR is considered to be a fundament, all parties involved must be more open to significant changes.