Do you really understand – Trajectory based operations (TBO)?

There is a misconception in some air traffic management circles that trajectory based operations is simply business as usual except that the current, notoriously imprecise ground generated trajectories are replaced by more accurate, 4 D trajectories and that is all there is to it. Some will add that parts of this 4D trajectory might be sourced from the FMS or an airspace user ground system… While there is truth in all this, TBO is much more. Much much more and significantly, if the other aspects of TBO are not considered, the potential for benefits inherent in TBO is reduced significantly.
So, what is trajectory based operations?
First and foremost we must look at the basis of the existing operation. Air traffic management has grown historically along an airspace based paradigm. Airspace as such was a given so it stood to reason that early ATM experts set out to define airspace volumes which they thought would best fit the traffic they expected and established air traffic control units to fit the task foreseen in those volumes. When aircraft arrived, they were obliged to fly within the confines of the defined airspace and if their needs differed from that envisaged, the aircraft trajectory was bent to fit the picture. Of course this is a bit of an oversimplification but to this day, ATM is being done on this basis.
The end-to-end trajectory played almost no role in this game. To illustrate the point, juts consider that until recently the Central Flow Management Unit calculated expected sector loads on the basis of a trajectory the vertical dimension of which was famously inaccurate while ground ATC systems generated their own trajectories for their own airspace and these often did not tie up with the trajectory dreamed up by the neighboring unit. All this time however scores of experts everywhere worked furiously on airspace design and organization… Only a blind person could fail to see that this legacy, airspace based paradigm had to go if the volume and efficiency demands of increasing traffic were to be met.
Things were not helped at all by the fact that controllers were handing flights as if they were born just outside their sector boundary and went into the big blue yonder when they exited their sector. In other words, they only ever looked at a small part of the trajectory with little regard to what was or was not happening further downstream. Conflict free handover was the almost the only aim.
Because of the way airspace was used in the past, popular ATM wisdom came up with the notion that airspace was a scarce resource and it had to be organized better to save the day. This notion was a dangerous one because for a long time it did divert attention and effort from looking at the real problem. Trajectories…

Airspace is not a scarce resource. In fact it is practically impossible to really saturate any given piece of airspace in normal operations. The problems arise when ground ATC has to deal with aircraft several miles long and wide and the position of which even 10-15 minutes into the future we are rather uncertain of, to put it mildly. Sounds crazy? Well, it is not. This is exactly how the air traffic management paradigm works to-day when we look at the required separation minima and the uncertainties of the predicted trajectories.
There is also a huge mismatch here between what existing ground systems can do and what the flight management system on the aircraft “knows”. The FMS has a pretty good idea of where it is going and where it will be say 15 minutes down the road but until recently nobody on the ground much cared about this. The ground generated trajectory is accurate only to a rather limited extent when it comes to the fourth dimension, time.
The solution? Trajectory based operations!

In order to understand TBO, we have to be familiar with two further novelties: the idea of the business trajectory and the concept of trajectory ownership. The following may sound like hair splitting but looked at closely, it is easy to see that they do represent two of the most important paradigm changes on which the future ATM system is to be built.
So, what is a business trajectory? The most important commodity of an airline is its network. That carefully balanced operation is ultimately expressed in the form of the trajectories its aircraft will want to fly. That line in space that the aircraft wants to follow is therefore the expression of the business intention of its operator. Taking all relevant (from their point of view) considerations into account, the airspace user has established that on a given day, flying that trajectory will meet best his business interests. Any distortion of that trajectory, however necessary, will inevitably result in a smaller or bigger change in the cost effectiveness of the operation. This is a vitally important consideration that air traffic management has to keep in mind at all times and strive to keep the overall business trajectory intact as much as possible. Even the choice of a solution to ensure separation must keep this in mind, right after the all important safety consideration. As we are talking about the overall trajectory, choices made must consider downstream effects also and not only those within a control sector or even control center.
What about ownership of the trajectory? Clearly, something as important as a trajectory representing the business intention must have an owner entitled to make changes to it but also to claim the quality of service it is paying for. Logically, there can only be one owner and that is the airspace user which has created the trajectory. This ownership entails certain rights as well as obligations. The owner will submit the trajectory to air traffic services for one simple reason only: they want to have air traffic services provided to the flight on a level and quality the owner is paying for. In exchange, the owner accepts the obligation of sharing information, reacting to requests, following clearances and so on. But at all times, the owner remains the master over the trajectory. Why is this important? There are several reasons but, after providing a means of clarifying who does what, perhaps the most important is that in our information age, where data has tremendous value, the trajectory itself also has value and it can be traded for good money. This is an important potential source of revenue and in fact at least one air navigation service provider has already latched on to the idea… This has resulted in the incredible situation where an airline was refused access to its trajectory data… data that was created using people and systems fully paid for by the users! Recognized ownership should take care of this kind of aberration.
So far, so good. We have the business trajectory owned by the airspace user and this is inserted into trajectory based operations… where do the benefits come from?
If we look at the first part of this article in which we detailed the shortcomings of the legacy system, it is easy to see why TBO will solve many of the problems of the airspace based paradigm. TBO will do all of the following:
• Aircraft will fly a very accurate 4 dimensional trajectories (3 spatial dimensions and time);
• The details of the trajectory will be shared by all concerned through system wide information management and this common awareness will extend end-to-end;
• Intervention in the trajectory will happen in full knowledge of the downstream effects and hence it will be possible to pick the option causing the least amount of trajectory distortion; and
• Because of the removal of most of the uncertainty around the trajectory, it will be possible to pack many more of them into a given airspace volume.
Of course TBO can only work well if we drop the idea that airspace is a monolith into which the trajectories must fit. Airspace must be seen as the unlimited resource it actually is and the trajectories must pass through it without being distorted by artificial bounds.
TBO does not mean that airspace will be permeable everywhere at all times. There will always be airspace reservations for things like military flying but the precise trajectories will mean that such airspace and the distance needed to go around it can be limited in both space and time.
Come to think of it, what is the scope of those famous 4D trajectories? To understand the nature of this animal, getting used to a bit of new thinking in the process, consider the following. When a new aircraft rolls out of the manufacturer’s assembly hall, it embarks on a long, long trajectory that ends only when the aircraft is withdrawn from service or, heaven forbid, is destroyed in an accident. During this life-time the trajectory of that aircraft will get various “names” attached to it: it will be KL123 then KL456 and so on. Sounds familiar? Well, taking the legacy approach, those are the flights we tended to focus on while we did not much care about the fact that all those flights were connected by the not negligible circumstance that they would all be carried out by the same aircraft. A fragmented view that led to many of the problems we know in air traffic management to-day. In TBO, we recognize that there is this long long trajectory and at any given time we will focus on a given section of it (which may be called KL123) but all the time remaining mindful that a kink in that portion of the trajectory will affect the rest of it also down the line. This is a huge and vital step towards really understanding the actual nature of our operating environment.

I can almost hear you say now: but what happens when the aircraft is in turnaround or parked for the night? There is no trajectory there… Wrong! Of course there is a trajectory except that it is idling in 3 dimensions while the 4th dimension, time, continues to tick away! That aircraft is being amortized, it is consuming resources (parking fee, guarding, etc.) and so the trajectory is as real as it gets except that it lack a few dimensions…
That the 4D trajectory is present when the aircraft is moving on the ground goes without saying.
A consequence of all this is that air traffic controllers will have to change their thinking in respect of both the planning and the executive tasks. To-day’s view focusing on individual flights and actions to separate them tactically will have to be replaced by a view focusing on the trajectories, managing them in such a way that tactical intervention is reduced to a minimum. This is where capacity is created enabling the humans to deal with a much larger number of trajectories… A number that would be impossible to handle on a tactical, aircraft by aircraft basis.
Of course this does not mean a future scenario that one might call “planes on rails”. Not at all! Managing trajectories in a shared environment creates flexibility on a scale not previously possible.
Trajectory based operations work best when they are combined with the concept of control by constraint. In this concept in most circumstances ATC will issue constraints to be met and the airspace user will decide what the most economical way to meet that constraint is. This method of working needs to apply in all but the really time critical situations. Because of the shared environment, agreeing the preferred method of meeting the constraint can take place in a dynamic manner in seconds, irrespective of whether it is the pilot who decides or the airline dispatcher assigned to the flight.
In the past, several promising concepts died before they could be adopted widely simply because they were pushed out into the field without the necessary changes in the operating environment having been completed. TBO does require a number of changes to be present if it is to be the success it has the potential to be.
Getting rid of the preoccupation with airspace management, educating all concerned about the significance of the business trajectory and trajectory ownership, developing the details of managing trajectories and the decision making tools fit for the TBO environment are but the most important in a range of things that must be present for trajectory based operations to succeed and bring the benefits.
It is therefore a great pity that in several recently produced papers on trajectory management we see little of any of the above. Most papers are a rather meek mixture of legacy descriptions with a few buzz-words mixed in but with little insight into how TBO will be achieved and run. Another misconception that seems to pervade current work is the misconception that trajectory based operations is a technical/engineering challenge. While there is a lot in TBO that engineers will have to solve, the paradigm change will have to happen in the heads of the humans first and foremost. The computers will follow easily…