The major traffic flow between Europe and North America takes place in two distinct traffic flows during each 24-hour period due to passenger preference, time zone differences and the imposition of night-time noise curfews at the major airports. The majority of the Westbound flow leaves European airports in the late morning to early afternoon and arrives at Eastern North American coastal airports typically some 2 hours later - local time - given the time difference. The majority of the Eastbound flow leaves North American airports in mid/late evening and arriving in Europe early to mid morning - local time. Consequently, the diurnal distribution of this traffic, has a distinctive tidal pattern characterised by two peaks passing 30° W - the Eastbound centred on 0400 Universal Co-ordinated Time (UTC) and the Westbound centred on 1500 UTC.
Baseline forecasts for 2015 show a 71 % increase of traffic compared to 1996, while optimistic figures show 96% increase (almost double).
The current North Atlantic tracks
There are three types of tracks above the North Atlantic Ocean:
Tracks in the Organised Track System (OTS),
The OTS is set up on a diurnal basis to facilitate a high throughput of traffic by ensuring separation for the entire oceanic crossing. Each core OTS is comprised of a set, typically 4 to 7, of parallel or nearly parallel tracks, positioned in the light of the prevailing winds to suit the traffic flying between Europe and North America. Examples are shown in Figure 1 and 2. Longitudinal separation between in-trail aircraft using the Mach Number Technique is 10 minutes and aircraft which satisfy MNPS are separated laterally by a minimum of 60 NM. Re-clearances for change tracks are normally limited to the provision of some step climbs and/or re-routes.
Figure 1: Example of a Day-Time, Westbound Oceanic Track System, Reference 
Figure 2: Westbound tracks, Reference 
Operators may also choose to flight plan random routes. However, random traffic competes for routes and flight levels on a first-come, first-served basis and aircraft flying random routes conflicting with the OTS are likely to be subject to flight level or routing restrictions. - unless the track is above or below the system. Aircraft electing to fly random tracks are required to flight plan on great circle tracks joining successive significant points defined by whole degrees of latitude intersecting meridians spaced by 10 degrees of longitude (20 degrees North of 70 degrees N).
Shortcomings of the current North Atlantic system
The North Atlantic System has a series of existing shortcomings, as identified by the North Atlantic Systems Planning Group, working on behalf of the International Civil Aviation Organisation (ICAO) Error: Reference source not found. These shortcomings are identified to be structural, that is, inherent to the system itself and make that the users currently do not enjoy maximum economy in their operations with minimum restrictions.
The existence of a track structure brings in rigidity, making the traffic flow over the Atlantic not efficient. As the variability of the wind patterns is higher than the variability of the OTS ammulgation, the system is penalising in terms of flight time and consequent fuel usage. Inaccuracies in the meteorological forecast may cause the operator to choose a track which does not make optimal use of prevailing conditions, which plays an important role in flight economics. In addition, the limitations of turbulence forecasting may operationally impact upon the application of RVSM levels for OTS and random flights resulting in reduced capacity and efficiency.
Besides the OTS system inevitably limits users to choose preferred cruise parameters as flight level and Mach number. Cruise climbs and tracks changes are possible during a crossing, but not always and certainly not real-time.
Furthermore, the surveillance and communication systems limit the ATC capabilities in the Atlantic region. Manual waypoint insertion errors for example can cause Gross Navigational Errors and the current surveillance system provides only a limited ability to detect those errors and to contain them by ATC intervention. The ATC limitations dictate relatively large separation minima and this in turn limits exploitation of the airspace capacity. The inability in certain areas to apply RVSM reduces system capacity further. While the North Atlantic Region as a whole is not saturated, a capacity shortfall can occur in the busiest part of the Europe/North America axis. This further constraints on aircraft profiles: aircraft may not obtain the desired route or flight level and therefore must be re-cleared on a less optimal flight profile.
In areas outside the OTS, some tactical flexibility is achieved, particularly where traffic operates on random routes. An efficient use of such tactical control is however again limited by present communication and surveillance systems.
A significant problem for the introduction of new ATM concepts, such as “Free Flight”, is that the main stakeholders in the ATM system, the airlines, require a quick return on their investments. Another problem for the introduction of “Free Flight” is the difficult transition path from today’s Core-European or US domestic ATM system to a new ATM system incorporating “Free Flight”. For this reason, a way to introduce “Free Flight” was figured out, while still meeting the short term, return on investment requirements of the airlines and the requirement for a smooth transition path. The solution is called: “North Atlantic Free Flight”.
NLR, in the ASSTAR (Avanced Safe Separation Technologies and AlgoRithms) project is heavily involved in this “North Atlantic Free Flight” concept, which forms part of ASAS Package II. This ASAS application will be assed on the Eastbound traffic flow, see figure 3
ASSTAR aims to improve management of domestic/oceanic interface and facilitate flow management where required, without compromising operational safety levels. ASSTAR is a Eropean Union co-funded project which started in January 2005.
Figure 3: Eastbound track
ASSTAR will perform research into the operational and safety aspects of the ASAS Package II type applications indicated below in order to realise the significant potential benefit to the user community in the 2010 plus time frame:
Reduced controller workload and improved flight efficiency is expected
Use of ADS-B to support new operations in oceanic and other non-radar airspace, enabling more optimal routing, including enhanced use of wind corridors and passing and level changing
Improvements to flight path efficiency currently restricted due to the procedural separation standards
Research objectives include:
Reference  : “Guidance and Information Material Concerning Air Navigation in the North Atlantic Region”, Seventh Edition, January 2002, Prepared by the ICAO European and North Atlantic Office, NAT DOC 001.
Reference : http://www.bcavirtual.com/crossing_the_north_atlantic.htm)