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Special routes known as ocean tracks are used across some oceans, mainly in
the Northern Hemisphere, to increase traffic capacity on busy routes.
Unlike ordinary airways, which change infrequently, ocean tracks change twice
a day, so as to take advantage of favourable winds.
Flights going with the jet stream may be an hour shorter than those going
against it. Ocean tracks may start and finish about 100 miles offshore at
named waypoints, to which a number of airways connect.
Tracks across northern oceans are suitable for east–west or west–east flights, which constitute
the bulk of the traffic in these areas.
Complete routes
here are a number of ways of constructing a route. All scenarios using airways use SIDs and
STARs for departure and arrival. Any mention of airways might include a very small number of
"direct" segments to allow for situations when there are no convenient airway junctions. In
some cases, political considerations may influence the choice of route (e.g., aircraft from one
country cannot overfly some other country).
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Airway(s) from origin to destination. Most flights over land fall into this category.
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Airway(s) from origin to an ocean edge, then an ocean track, then airway(s) from ocean
edge to destination. Most flights over northern oceans fall into this category.
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Airway(s) from origin to an ocean edge, then a free-flight area across an ocean, then
airway(s) from ocean edge to destination. Most flights over southern oceans fall into
this category.
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Free-flight area from origin to destination. This is a relatively uncommon situation for
commercial flights.
Even in a free-flight area, air traffic control still requires a position report about once an hour.
Flight planning systems organise this by inserting geographic waypoints at suitable intervals.
For a jet aircraft, these intervals are 10 degrees of longitude for eastbound or westbound
flights and 5 degrees of latitude for northbound or southbound flights. In free-flight areas,
commercial aircraft normally follow a least-time-track so as to use as little time and fuel as
possible. A great circle route would have the shortest ground distance, but is unlikely to have
the shortest air distance, due to the effect of head or tail winds. A flight planning system may
have to perform significant analysis to determine a good free-flight route.
A - Z of Flight
July 2017
www.alliance-airways.net
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