North Atlantic Tracks
||This article lacks historical information on the subject. (November 2012)|
North Atlantic Tracks (NAT) are transatlantic flight routes that stretch from the northeast of North America to western Europe across the Atlantic Ocean. They ensure aircraft are separated over the ocean, where there is little radar coverage. These heavily traveled routes are used by aircraft traveling between North America and Europe, flying between the altitudes of 29,000 and 41,000 feet inclusive. Entrance and movement along these tracks is controlled by special Oceanic Center air traffic controllers to maintain separation between airplanes. The primary purpose of these routes is to provide a Minimum Time Route (MTR). They are aligned in such a way as to minimize any head winds and maximize tail winds impact on the aircraft. This results in much more efficiency by reducing fuel burn and flight time. To make such efficiencies possible, the routes are created twice daily to take account of the shifting of the winds aloft and the principal traffic flow, eastward in North America evening and westward twelve hours later.
The specific routing of the tracks is dictated based on a number of factors, the most important being the jetstream—aircraft going from North America to Europe experience tailwinds caused by the jetstream. Tracks to Europe use the jetstream to their advantage by routing along the strongest tailwinds. Because of the difference in ground speed caused by the jetstream, westbound flights tend to be longer in duration than their eastbound counterparts. North Atlantic Tracks are published by Shanwick Center (EGGX) and Gander Center (CZQX), in consultation with other adjacent air traffic control agencies and airlines.
The day prior to the tracks being published, Airlines that fly the North Atlantic regularly send a Preferred Route Message (PRM) to Gander and Shanwick. This allows the ATC agency to know what the route preferences are of the bulk of the North Atlantic traffic. 
Provision of North Atlantic Track air traffic control
Air traffic controllers responsible for the Shanwick flight information region (FIR) are based at the Shanwick Oceanic Control Centre at Prestwick Centre in Ayrshire, Scotland. Air traffic controllers responsible for the Gander FIR are based at the Gander Oceanic Control Centre in Gander, Newfoundland, Canada.
Flying the routes
Prior to departure, airline flight dispatchers/flight operations officers will determine the best track based on destination, aircraft weight, aircraft type, prevailing winds and Air Traffic Control route charges. The aircraft will then contact the Oceanic Center controller before entering oceanic airspace and request the track giving the estimated time of arrival at the entry point. The Oceanic Controllers then calculate the required separation distances between aircraft and issue clearances to the pilots. It may be that the track is not available at that altitude or time so an alternate track or altitude will be assigned. Aircraft cannot change assigned course or altitude without permission.
Contingency plans exist within the North Atlantic Track system to account for any operational issues that occur. For example, if an aircraft can no longer maintain the speed or altitude it was assigned, the aircraft can move off the track route and fly parallel to its track, but well away from other aircraft. Also, pilots on North Atlantic Tracks are required to inform air traffic control of any deviations in altitude or speed necessitated by avoiding weather, such as thunderstorms or turbulence.
Despite advances in navigation technology, such as GPS and LNAV, errors can and do occur. While typically not dangerous, two aircraft can violate separation requirements. On a busy day, aircraft are spaced approximately 10 minutes apart. With the introduction of TCAS, aircraft traveling along these tracks can monitor the relative position of other aircraft thereby increasing the safety of all track users.
Since there is little radar coverage in the middle of the Atlantic, aircraft must report in as they cross various waypoints along each track, their anticipated crossing time of the next waypoint, and the waypoint after that. These reports enable the Oceanic Controllers to maintain separation between aircraft. These reports can be made to dispatchers via a satellite communications link (CPDLC) or via High Frequency (HF) radios. In the case of HF reports, each aircraft operates using SELCAL (Selective Calling). The use of SELCAL allows an aircraft crew to be notified of incoming communications even when the aircraft's radio has been muted. Thus, crew members need not devote their attention to continuous radio listening. If the aircraft is equipped with Automatic Dependent Surveillance, (ADS-C & ADS-B), voice position reports on HF are no longer necessary, as automatic reports are downlinked to the Oceanic Control Centre. In this case, a SELCAL check only has to be performed when entering the oceanic area and with any change in radio frequency to ensure a working backup system for the event of a datalink failure.
Maximizing traffic capacity
Increased aircraft density can be achieved by allowing closer vertical spacing of aircraft through participation in the RVSM program.
Additionally from 10 June 2004 the Strategic Lateral Offset Procedure (SLOP) was introduced to the North Atlantic airspace to reduce the risk of mid-air collision by spreading out aircraft laterally. It reduces the risk of collision for non-normal events such as operational altitude deviation errors and turbulence induced altitude deviations. In essence, the procedure demands that aircraft in North Atlantic airspace fly track centreline or one or two nautical mile offsets to the right of centreline only. However, the choice is left up to the pilot.
The tracks reverse direction twice daily. In the daylight, all traffic on the tracks operates in a westbound flow. At night, the tracks flow eastbound towards Europe. This is done to accommodate traditional airline schedules, with departures from North America to Europe scheduled for departure in the evening thereby allowing passengers to arrive at their destination in the morning. Westbound departures typically leave Europe between early morning to late afternoon and arrive in North America from early afternoon to late evening. In this manner, a single aircraft can be efficiently utilized by flying to Europe at night and to North America in the day. The tracks are updated daily and their position may alter on the basis of a variety of variable factors, but predominantly due to weather systems.
While the routes change daily, they maintain a series of entrance and exit waypoints which link into the airspace system of North America and Europe. Each route is uniquely identified by a letter of the alphabet. Westbound tracks (valid from 11:30 UTC to 19:00 UTC at 30W) are indicated by the letters A,B,C,D etc., where A is the northernmost track, and eastbound tracks (valid from 01:00 UTC to 08:00 UTC at 30W) are indicated by the letters Z,Y,X,W etc., where Z is the southernmost track. Waypoints on the route are identified by named waypoints (or "fixes") and by the crossing of degrees of latitude and longitude (such as "54/40", indicating 54° latitude, 40° longitude).
Concorde did not travel on the North Atlantic Tracks as it flew to the United States from the United Kingdom and France from a much higher altitude, between 45,000 ft and 60,000 ft, its relatively high service ceiling. The weather variations at these altitudes were so minor that Concorde followed the same route each day, traveling to and from Europe to North America on fixed tracks. These fixed tracks were known as 'Track Sierra Mike' (SM) and 'Track Sierra Oscar' (SO) for westbound flights and 'Track Sierra November' (SN) for eastbounds. An additional route, 'Track Sierra Papa' (SP), was used for seasonal British Airways flights from London Heathrow to/from Barbados.