4D Trajectory Management – What is this?

On 31-05-2012, in Buzzwords explained, by steve

The problem with traditional systems

In the traditional scheme of things, an aircraft will file a flight plan, containing a rather rudimentary description of its intentions. Air traffic management and air traffic control organizations then decipher the plan and create a trajectory for the flight as best they can… Very often this is but a poor image of what the airline concerned had in mind and then even this version is further distorted due to the need to limit sector loads or to provide separation.

We tend to think of the trajectory as being three dimensional but in fact the fourth dimension, time, is as important as the three spatial dimensions. This means that a delay on the ground is in fact a distortion of the trajectory which affects “only” the time dimension, but which can have serious consequences for the flight concerned.

Aircraft operators do develop the trajectories they want to fly taking many considerations into account and in the end, the trajectory represents their business intentions, the path on the ground and in the air they want to proceed along to ensure the most cost-effective conduct of their flight.

Traditional air traffic control is based on managing aircraft rather than trajectories. They do of course use the trajectory created in their systems from the flight plan to check ahead of the aircraft to see whether there is a conflict with another flight but this look-ahead is very short (in the order of 20 minutes or so) and tactical interventions rarely take into account their effect on the trajectory as a whole. Multi-sector planners are starting to appear but even these tend to have a limited scope and ability to keep the integrity of the trajectories intact.

Aircraft with sophisticated Flight Management Systems (FMS) can fly a trajectory with phenomenal accuracy but the prediction capability of the FMS is not always what it should be, especially because of shortcomings in the weather-modeling capability built into them.

As I said already, the ground-systems of ATC are not that great when it comes to trajectory prediction. The fact that they use their own logic which may not always line up well with the original flight intentions creates a peculiar situation exacerbated even more by the fact that each ATC system does its own calculations which often fail to match up at the system boundary…

Basically we have a situation where we can usually say fairly accurately which positions an aircraft will occupy in the future but we have a hard time figuring out just when…

This is one of the reasons why ATC has always kept their look-ahead time short… with longer look-ahead the uncertainties grew exponentially and hence it was useless.

Predictability is a word that is almost synonymous with efficiency in air traffic management. The adverse effects of delays and other trajectory distortions can be limited if the situation is predictable, i.e. we know about them in sufficient time.
If only we could manage the complete trajectories rather than just the aircraft…

Trajectory based operations to the rescue

Trajectory based operations (TBO), sometimes also referred to as 4D trajectory management, is one of the cornerstones of the concept of operations in both the European SESAR project and the US NextGen.

The most important enhancement involved is the integration of time (the fourth dimension) into the handling of the trajectory, together with enhancements to the weather model in the FMS as well as improvements to its ability to meet time constraints.

Ground systems are taught to work with the trajectory sourced from the aircraft and to integrate their own calculations with the aircraft data, resulting in a very accurate, predictable trajectory that can be shared among neighboring centers. The constrained look-ahead time horizon is replaced by common situational awareness that brings the complete trajectory into the picture. The effects of decisions can also be seen and this makes it possible to choose solutions to conflicts with the least overall distortion of the trajectories concerned.

The operational concept of 4D trajectory management is really rather simple. It aims to ensure flight on a practically unrestricted, optimum trajectory for as long as possible in exchange for the aircraft being obliged to meet very accurately an arrival time over a designated point (CTA, Controlled Time of Arrival sometimes also referred to as Constrained Time of Arrival).

Since these arrival times are highly accurate, it is possible to organize the arrival sequence over the given point in a reliable and predictable manner.

Managing traffic this way ensures optimal flight operations on the one hand and a predictable organization of traffic in the congested areas on the other, benefiting both the airspace users and air traffic service providers. Of course the benefit increases as the area of trajectory based operations, and hence the number of trajectories involved, increases.

Conversely, if only part of a given trajectory is subject to TBO, the achievable benefit is limited since the optimized section of the trajectory will be, by definition, shorter and disturbances in the non-TBO sections ripple over in unpredictable ways. This is something to think about for fans of the FAB concept… TBO limited to one’s own FAB is of little value if the trajectory extends beyond the FAB boundaries.

It will be clear that TBO requires improved FMS and ground systems, the appropriate air/ground and ground/ground digital link communications with the proper services and a new ATC paradigm that enables the proper management of the trajectories and CTAs while minimizing the number of tactical interventions. Once all this is in place, it becomes possible to introduce other, even more advanced, operations using the trajectory based approach.

SESAR, as part of its Release 1, has completed a validation exercise of what they call Initial 4D (i4D) which focused on the ability of aircraft to exchange trajectory information with the ground and the ability of the aircraft to meet its assigned CTA.

Obviously, the original continental Controller-Pilot Digital Link Communications (CPDLC) message set had to be enhanced to include new messages covering the trajectory exchange cases. These latter are sent as ADS-C messages, bringing a flavor to continental airspace so far used mainly over the oceans only.

Validation of various aspects of i4D will continue in SESAR Release 2. Of course these validation exercises are only the beginning and although a lot had been written about the single flight from Toulouse to Arlanda and a video or two published, the real proof of the cake will come when trials in a real operational environment with multiple aircraft take place. There is little doubt that Airbus and Boeing, together with the avionics suppliers will make their aircraft suitable for TBO. The uncertainty is more in how the ANSPs will measure up, especially if inter-FAB operation is involved.

Another element to watch is the inescapable fact that Trajectory Based Operations is as much a way of working as a technology challenge. The latter is no rocket science and the required technology is basically available. Training pilots and controllers to change to the new paradigm will require serious effort and attention to detail. This is not yet visible on a European scale. Of course the issues of transition and operations in a mixed environment will have to be addressed also.

No simple matter, that is for sure.

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  1. […] not go into the details of TBO in this article, if you are interested, read more about it here and here. Let it suffice to say that we left out any mention of FABs in the original SESAR concept of […]

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