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. Planes 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, airplanes 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 planes. 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.
Read more about this topic: North Atlantic Tracks
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