Free flight…or…Flight of Fancy?

On 08-04-2012, in Buzzwords explained, by steve

If you utter the words ”free flight” these days, a number of interesting reactions are likely. These can range from blank stares via violent reactions to sad headshaking. On one occasion somebody actually asked me.…”But you still have to pay airport taxes…?”

But jokes apart, there are plenty of experts who will remember how much discussion there was when the concept of free flight was first proposed. In its purest form this meant an air traffic management paradigm where, in most circumstances, separation is provided from the cockpit with controllers on the ground retaining a mainly strategic role. The original discussions, and to a certain extent the debate, focused on where the line should be drawn: to what extent should the responsibility for separation be transferred to the pilot, where is the limit of safe and cost effective transfer that brings real benefits?

Related work also quickly highlighted the fact that while free flight in its purest form was probably realistic only at very high altitudes with low density traffic, or in remote areas, variations on the theme were possible and some of these held real promise. Indeed, the term free flight itself was found to be too offensive for some and was replaced by terms like Airborne  Traffic Situational Awareness (ATSAW) and Airborne Separation Assistance System (ASAS).

Whatever you call it, the question of transferring some or all separation responsibility to the cockpit is a complicated one, as is the work currently under way to assess the possibilities and chart out the most promising directions.

In this article Capt. Phil Hogge, one of the most consistent supporters of the free flight concept, outlines the subject and shares his views on it with Roger Wilco’s readers.

I hesitate to use the term ‘free flight’ for the reasons stated above: it is neither free in terms of cost, nor is it an operational free-for-all. I prefer to use the terms ‘delegated spacing’, ‘delegated separation’ or ‘self separation’, and then only under tightly controlled conditions. For 5 years I chaired the European Commission’s ASAS Thematic Network workshops which brought together researchers and ATM specialists from many countries (Europe, Australia, Canada, Japan, Russia and the USA to name but a few) so that ideas could be exchanged and research coordinated. Through these workshops we encouraged international standardization, simulations and trials of equipment and operational procedures.

The results of those discussions inform the views I present below and show, I hope, that orderly progress can be made towards a different paradigm for ATM. The recent trial carried out in February by SWISS International Airlines as part of the CASCADE programme on the North Atlantic using ATSAW-ITP (albeit under radar control) gives me the opportunity to review some of the options available.

For those of you who are unaware of this work, I will describe the building blocks of the system and the basic groups of ASAS applications. The diagram below shows the interconnections between the FMS/navigation functions of the aircraft, ADS-B-Out, ADS-B-In and the airborne data display on a CDTI (Cockpit Display of Traffic Information). Clearly for such systems to be used in normal operations, the aircraft equipment has to be certified, the crews correctly trained, the rules to be used properly defined, the ATCOs and their systems equally well trained and certified, and clearly defined pilot/ATCO procedures established.

ASAS functional description

ADS-B-Out information is already being used by ANSPs in Australia, Canada, parts of Europe and in the USA to fill in the gaps of radar coverage, e.g. over Hudson Bay in Northern Canada and in the centre of Australia, where it is either impossible or too expensive to install radar. The data is displayed alongside conventional radar data at the controller work positions and permits reduced separation in non-radar airspace.

For ASAS this same data also needs to be received by other aircraft using ADS-B-In and displayed on a CDTI which looks like this:-

CDTI by Airbus

There are four sets of ASAS applications.

1. The first set covers situational awareness:-

• ATSA-AIRB (Airborne surveillance); this is the most basic application in which traffic information is displayed on the navigation display. It is for information only.
• ATSA-VSA (Enhanced Visual Separation on Approach); this application is similar to the above and used to achieve visual acquisition of preceding traffic and maintain visual separation on the approach.
• ATSA-SURF (Enhanced traffic situational awareness on the airport Surface); the traffic is displayed on a moving map of the airport surface to supplement out-the-window observations and see-and-be-seen procedures on the airport surface.
• ATSA-ITP (In-Trail Procedure in non-radar oceanic airspace) permits ‘climb-through’ manœuvres to climb past ‘blocking’ aircraft to a more economical level when in oceanic airspace.

These are all fairly simple applications, and many aircraft are being supplied with ATSA-AIRB already fitted. The trials in which SWISS and other airlines are invloved are assessing whether ATSA-AIRB (Airbus call it ATSAW-AIRB) can help pilots to decide whether it is worth asking ATC for a climb to a higher level when on the N Atlantic Track system.

2. The next set is rather more complicated. These applications are all spacing applications:-

• ASPA-S&M (Sequencing and Merging); where the ATCO instructs the pilot to identify and then maintain a given time or distance spacing behind a designated preceding aircraft in the arrival traffic stream. The pilot uses the CDTI and an indicator showing the spacing required to maintain his position in the stream, thus relieving a certain amount of the ATCO’s workload and improving the landing rate.
• ASPA-C&P (Crossing and Passing); again, the ATCO instructs the pilot to use his CDTI to identify a specific aircraft and then alter course to pass it at a specified spacing value.

ASPA-C&P has proved to be a lot more complicated than originally thought. However, ASPA-S&M (called Merging and & Spacing in the USA) has been trialled successfully by UPS at Louisville, Kentucky. They used equipment supplied by ACSS, an L-3 Communication and Thales company. This company has also certified equipment for ATSA-SURF, which they call SAMM (Surface Area Movement Manager). Both applications are fitted in Class II EFBs.


These trials in the USA and extensive simulations by EUROCONTROL show promising benefits in terms of runway throughput, fuel savings and noise reduction. BUT it does require a sufficiently large population of aircraft to be fitted before these benefits can be realised – and an ANSP who is willing to use it.

3. The third set of applications is intended for delegated separation:-

• ASEP-LC&P (Lateral Crossing and Passing)
• ASEP-VC&P (Vertical Crossing and Passing)
• ASEP-ITP (In-trail procedure)
• ASEP-ITF (In-trail Follow)
• ASEP-S&M (Sequencing and Merging)
• ASEP-ITM (In-trail Merge)

In all of these the ATCO delegates responsibility for separation between designated aircraft to the pilots and is then free to concentrate on other tasks. These applications require higher standards of integrity than the previous two sets, and also a lot more work on the procedures before they are ready to be trialled.

4. The last set of applications covers full self separation. They are:-

• SSEP-FFAS (Self separation in segregated airspace free flight airspace)
• SSEP-MAS (Self separation in managed airspace)
• SSEP-FFT (Self separation in an organised track system)

The SESAR Concept of Operations envisages the use of all these applications as and when they become operationally and economically viable.

Clearly, much more work still needs to be done both in Europe and the USA. The EC, EUROCONTROL, individual ANSPs, the FAA, aircraft and equipment manufacturers and standardization bodies have all undertaken a considerable amount of the initial work – simulations, trials and certification.

The Airbus A330-300 of SWISS International Airlines used fully certified ATSAW avionics as part of the CASCADE Programme which involves 25 Airbus and Boeing aircraft belonging to British Airways, Delta Air Lines, SWISS, US Airways and Virgin Atlantic. This programme is also working towards the use of ADS-B and Wide Area Multilateration in European continental airspace.

You can find more information about CASCADE here.

Both SESAR and NextGen envisage the use of some or all of the ASAS applications outlined above. What is needed now is a step by step approach towards the implementation of these applications where and when they can provide benefits. There is no need for a ‘big bang’ transition. The end goal is a major paradigm change in ATM using 4D trajectory management, airspace user ownership of trajectories, and the tactical freedom derived from delegated separation and self separation.

I am convinced that the technical and operational issues are all solvable. To those who say it can’t be done, I would say I have even flown a simulation in NLR’s laboratories in Amsterdam using self separation in the Maastricht airspace with three times the current traffic, as did other airline pilots, and found it perfectly feasible.

As always, with ATM developments, the economics will be the deciding factor. There needs to be a sound business case and a practical transition plan.

Having read Phil’s interesting account, one will no doubt wonder. What conclusions can we draw from all this?

The concept of free flight is a major paradigm changing idea in air traffic management which fits very well with the other concepts. Exactly because it is such a radical new approach to separation provision, it needs to be examined very closely and its potential assessed in terms of feasibility, safety and cost effectiveness.

As with all new developments, the problem is essentially twofold. The safety and technical/engineering aspects need to be resolved but this can be done in spite of the complexity of the issues. It is the financial (cost-benefit) aspects that will cause the most problems. It is not easy to prove to the airspace users that the costs involved in such a paradigm change will ultimately bring benefits that go way beyond the primary advantages of more capacity on certain routes, important as those may be.

Changing the traditional roles of pilots and controllers and redistributing certain tasks and responsibilities opens up an ATM future that can finally move in a direction where human limitations play a much reduced role.

There is a long way to go. It is possible to take one step at a time. The concept is ambitious but so is aviation.


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