De-Conflicted and efficient 4D Trajectories through a Centralised Service

Recently I have had a very interesting discussion about the future of air traffic management with a person who has for a long time studied future concepts to improve efficiency in Air Traffic Control.

4D trajectory: Image embedded from

I was in Bucharest on a 2 day trip to give presentations about ATM and ATC from an operational perspective to Aeronautical Engineering students at the Polytechnic University of Bucharest. The vice dean, who has read a PHD in 4D Trajectories Optimisation started a discussion on what the vision may be regarding 4D trajectories and on the computing of the most efficient trajectories for flights in terms of energy consumption and time.

He said that whereas he agreed with the common concept that the key towards further efficiencies in the above mentioned areas is to be able to push the traffic situation into proactive management of already de-conflicted situations (eventual gate-to-gate precision trajectory clearances) rather than legacy reactive / tactical ATC based achievement of separation, there were two elements that needed to be enforced. These were:

  • The idea that the trajectory of flights which are at once both de-conflicted from other flights and also the most efficient, should be entrusted to a centralised ground system, and
  • Secondly, that the main criteria for the most efficient trajectory was one based on time and fuel consumption and not on distance covered (that the most efficient route might not be the shortest one).

1. Ground based centralised de-conflicted and efficient precise trajectory clearance:

The idea makes sense, the system should be based on a centralised processor that analyses all flight requests and parameters stemming from all ANSPs and proposes the best trajectory to flights. The term best would be defined as a trajectory which would be de-conflicted from all other flights and most efficient. ATC would be then responsible for monitoring the flight and to act on last minute changes due to unforeseen circumstances. The efficient trajectory would be based on meteorological reports and forecasts, thus making the best use of prevailing winds, etc.

In this model, the Reference Business Trajectory for a flight would be prepared by a centralised regional service based on information from various stakeholders including ANSPs and weather services. The RBT would then be accepted by the airspace user who would fly it knowing it has a high degree of stability (it would not be likely to suffer major revisions whilst in flight), and being the most efficient.

This idea to me makes sense as it marks an evolution of the direction the management of the ATM network has been taking, from Flow Control, to Network Management to perhaps Enhanced Network Management. History has shown that a centralised responsible body, pushed and governed by its stakeholders (such as the case of CFMU and NM) are the most efficient responses to unleashing and enforcing the necessary situations to improve flight efficiency.

2. It is about time and fuel consumption and not distance

 Aircraft with sails. Image courtesy of

The second aspect that I found quite innovatory in this discussion I had is that efficiency should be seen in terms of flying time and in terms of energy consumption and that this does not always correlate with distance covered.

The way things are presented today is that the closest we get to the great circle in terms of distance covered between two airports, the more efficient our flight is. But this may not be true if to fly the great circle one ignores wind, for example. (If a head wind component is more important for a given flight, should it fly a great circle trajectory than it would be if the flight steers away from this great circle trajectory into more favourable wind and therefore saves more fuel and/or arrives earlier to destination by doing so – a bit like the case of a boat that has to choose between navigating against the current for a direct route or to let the current steer it in terms of maritime navigation. We need to become more sophisticated when talking about flight trajectory efficiency and how we calculate it. Is it the distance covered? (as we see it shouldn’t be), is it the flying time? Or the energy used to fly? (possibly it should be a calculation based on the component of the last 2).

The above for me is very simple to understand, yet it seems to me as a break through as we always tend to talk about trajectory inefficiencies based on the deviation from the shortest (distance) route and then by computing the fuel and/or time lost as a direct conversion of the distance, whereas what we need to be talking about is on how to make trajectories to be agreed based on the knowledge of winds and of engine performances.

The above seems to me more feasible to be achieved using a centralised service rather than a scattered one. In terms of applicability in the near future I see also that in terms of phases of flight, it may still be difficult to manage these type of trajectories in terminal airspaces and in and around airports, because the incognita in these areas seem to me still too complex to compute and manage (from passengers arriving late to the gate to GA traffic making unpredicted manoeuvres in the air and airside vehicles on the ground.)  However, I could imagine a centralised service presenting on behalf of ANSPSa trajectory contract of this type to the flight in question, based on the current situation and de-conflicted from other traffic from the moment the flight leaves the terminal area of departure to the point it arrives to the terminal are of arrival as something that if worked upon could be made feasible in a foreseeable future (This close to Sesar’s Operational Improvement Step:” Use of Free Routing from Terminal Area Operations-exit to Terminal Area Operations-entry” but rather than ‘free routing’ we should be aiming for a de-conflicted and optimum-efficient precision trajectory clearance from Terminal Area Operations-exit to Terminal Area Operations-entry)

I think this, based on a centralised service and on a real time system wide information management exchange and on the concepts of de-conflicted precision trajectory clearances  which are optimum efficient in terms of a computation of time and fuel consumption, should at least make the base for a serious study into feasibility and into a shared operational concept...

Such was my discussion in Bucharest. I went to teach and as often happens, I came back feeling I have learnt something very significant about the future of air traffic management!

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