Transatlantic Routes
Unlike over land, transatlantic flights use standardized aircraft routes called North Atlantic Tracks (NATs). These change daily in position (although altitudes are standardized) to compensate for weather—particularly the jet stream tailwinds and headwinds, which may be substantial at cruising altitudes and have a strong influence on trip duration and fuel economy. Eastbound flights generally operate during night-time hours, while westbound flights generally operate during daytime hours, for passenger convenience. For European readers it should be mentioned that the "Eastbound Flow", as it is called, generally makes European landfall from about 0600UT to 0900UT. The westbound flow generally operates within a 1200–1500UT time-slot. Restrictions on how far a given aircraft may be from an airport also play a part in determining its route; in the past, airliners with three or more engines were not restricted, but a twin-engine airliner was required to stay within a certain distance of airports that could accommodate it (since a single engine failure in a four-engine aircraft is less crippling than a single engine failure in a twin). Modern aircraft with two engines flying transatlantic have to be ETOPS certified.
Gaps in air traffic control and radar coverage over large stretches of the Earth's oceans, as well as an absence of most types of radio navigation aids, impose a requirement for a high level of autonomy in navigation upon transatlantic flights. Aircraft must include reliable systems that can determine the aircraft's course and position with great accuracy over long distances. In addition to the traditional compass, inertials and satellite navigation systems such as GPS all have their place in transatlantic navigation. Land-based systems such as VOR and DME,because they operate "line of sight", are mostly useless for ocean crossings, except in initial and final legs within about 240 nautical miles (440 km) of those facilities. In the late 1950s and early 1960s an important facility for low-flying aircraft was the Radio Range. Inertial Nav. systems became prominent in the 1970s
Read more about this topic: Transatlantic Flight
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