We have all grown up with the idea that airspace was the most important single thing aircraft needed. While it is true that aircraft need both air (in which the wings can generate lift) and space (the room to move around in) but airspace? This word has grown over the years and held us hostage to an air traffic management (ATM) paradigm that is one of the main causes of inefficiencies and scarce ATM capacity to-day.
If we look around, we will see plenty of instances where the term “airspace” is used in ways that mask much more essential things, things that need to be considered first and foremost before we think about the space in which those “things” exist.
States have their sovereign airspace, airspace management is an element of the global ICAO ATM operational concept, EUROCONTROL has an Airspace and Navigation Team, and there is the concept of flexible use of airspace… Even the quarterly publication of CANSO, the Civil Air Navigation Services Organization is called “Airspace”. Air traffic control centers have their airspace… Airspace is the magic term we all grew up with and think we understand.
We also tried to solve ATM problems by “improving” airspace. When those efforts did not quite work out the way we had hoped, we pronounced airspace to be a scarce resource almost saying that it was airspace that actually put a limit on how many aircraft there may fly around at any given moment.
In fact, airspace is an almost limitless resort. It appears to be limited only because of the way we use it.
Precise airspace with imprecise trajectories
It is telling to consider how the legacy process goes… We design the airspace to serve a given purpose, be it the containment of flights or allocating responsibilities to control units. The airspace design process is a precision exercise and when the designers are done, they put forward a product characterized by a very high precision in terms of all dimensions of the airspace concerned. It is then published and we can wait for aircraft to come…
And they do and what happens? ATC has only rudimentary information on the intention of the flights, receiving only an extract (the filed flight plan) of the carefully crafted trajectory description the airspace user had put together. Very often even this extract is further removed from the original (it becomes the current flight plan) as clearances demanded by the traffic situation are implemented. As if to add insult to injury, a given control center and the sector controllers within it only see and really care about that stretch of the flight trajectory that is inside their airspace and possibly a bit further down the route… Finally, the precision with which this trajectory is flown and known is not exactly spectacular. Longitudinally, we talk about minutes.
This is not something we should blame air traffic control for. They are doing their best in the system that grew up around them. A system where airspace is designed to a very small tolerance while inside it, the all important aircraft trajectories are accepted with tolerances that have improved very little since the earliest times. The result is low predictability and a huge waste of the airspace resource, which combine to create inefficiencies that are avoidable to a very large extent.
Instead of starting by managing the airspace, we must manage the trajectories and then ensure that the airspace is there to accommodate them
The concept of Trajectory Based Operations (TBO)
Of course it is not realistic to envisage that it will ever be possible to adjust airspace to the trajectories without there being any airspace imposed constraints. But by shifting the balance towards trajectories, it is possible to achieve a situation where it is mainly the trajectories that influence airspace rather than vice versa.
TBO requires that the trajectories be expressed in four dimensions (3 spatial and 1 time) = 4D with a very high precision, which is
then carried over into actual execution of the trajectories. The SESAR Concept of Operations for example talks about navigation accuracy improving to +/- 10 seconds longitudinally.
This very precise trajectory is then shared between all the partners concerned via System Wide Information Management (SWIM) resulting in common situational awareness and knowledge of the consequences of all interventions, end-to-end. It becomes possible to make decisions on required ATC interventions that take into account the need for minimizing the overall distortion to the trajectory.
In other words, precise 4D trajectories enable both airspace users and the service providers to understand the impact of the trajectories on each other and the resources being made available so that constraints that need resolution can be addressed on a timely basis.
Flying precise 4D trajectories and TBO doe not mean a “planes on rails” scenario. Far from it. TBO is flexible exactly because it is so predictable. This may sound like a contradiction in terms, but it is not. On the one hand, precise trajectories enable precise planning and conflict resolution while predictability enables the early identification of the need for intervention. At the same time, the impact of interventions can be easily evaluated and minimum impact solutions sought.
The legacy ATC paradigm is often referred to as “clearance based” and this is set off against the new paradigm of TBO. This does not mean that there will be no clearances in the future system. The difference is really in the trajectory attaining such a central role whereas in the legacy system clearances were issued with a much more limited overall knowledge of their effect on the trajectory further downstream.
The benefits of TBO
Precise management of trajectories dramatically reduces the volume of airspace needed for a given flight and this translates into more flights per unit of airspace on the one hand (increased capacity) and a reduced need for intervention with a given trajectory on the other, resulting in less deviation from the original intent of the airspace user. Even when an intervention becomes inevitable, this can be much smaller and of shorter duration than is the case in the legacy system.
TBO also enables the introduction of much more powerful and effective queue management and separation assistance tools that reduce controller task load and hence the inherent capacity in TBO based airspace can be exploited even further.
The nature of the trajectory
With a bit of exaggeration we could say that an aircraft’s trajectory begins when it first rolls out from the manufacturing plant and ends when she finally goes to rest in a scrap yard. While we do not need to use this extreme view of the trajectory when designing TBO, it is important to remember that each section of the trajectory (whether the aircraft is moving on the ground or in the air) actually impacts all subsequent ones. We should not forget that if an aircraft is stationary on the ground, docked at the gate or waiting on a remote stand, the trajectory is not broken. It is in an idle state but it is still consuming resources. There may be a security guard there; others will be working on the plans for the next flight… Clearly, the contention that the trajectory, rather than airspace, is central to the life and operation of an aircraft is easy to prove. The next step, that the trajectory must be central to air traffic management, is a logical and unavoidable conclusion.
The extreme proof of the cake
Originally, at this point I was planning to bring some kind of snappy example from civilian air traffic management to make the point of this article. In the meantime however, our colleagues in uniform have produced an introductory video that talks about trajectories and trajectory management. They have provided the best possible example for me to use and believe me, when the military sign up for trajectory based operations, the thing needs no more proof of its value.
Imagine the old, airspace based system where you have two fighters in an exercise area that will need to be refueled. The military have blocked off the exercise area plus a biggish volume of airspace where the tanker is circling, waiting for its charges to find it… Wasteful in terms of airspace as well as fuel consumption. Change now to a trajectory based operation where the precise 4D trajectory of the tanker is designed to arrive where the fighters are at just the right time and while the babies are slurping, mama flies with the minimum of restrictions to other traffic. Instead of trying to segregate everything on an airspace basis, the trajectories are managed to ensure minimum interference and maximum compliance with the user intent, both civil and military.
Won’t we just love TBO?
