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In page Instrument flight rules:
"Modern flight management systems have evolved to allow a crew to plan a flight as to route and altitude and to specific time of arrival at specific locations. This capability is used in several trial projects experimenting with four-dimensional approach clearances for commercial aircraft, with time as the fourth dimension. These clearances allow ATC to optimize the arrival of aircraft at major airports, which increases airport capacity and uses less fuel providing monetary and environmental benefits to airlines and the public.
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