First airways, which were delimited by high frequencies, were built on the old A-N system. Subsequent airways were basing on low/medium frequencies earth station, such as the BFO and the Non-Directional beacon ( NDB ). Those L/M frequent airways were the coloured airways. Coloured airways still abound, especially in Alaska.
Only a few coloured airways remain in the neighboring United States. Off the North Carolina coastline there is a coloured respiratory tract known as G13 (Green 13). In addition, there are several coloured airways that connect to and pass through the Florida Keys. Blue 9 (B9) links the South Florida continent with the Marathon Key.
Blue 646 (B646) links Mérida, Mexico with Nassau, Bahamas and leads over the Florida Keys. Part of the N646 links Key West with Marathon Key and acts as a ring road for planes that cannot reach the 14,500 min height along the V3. After all, the G448 (Green 448), which begins at the NDB marathon, is the only deep respiratory tract connecting the United States with Cuba; the respiratory tract leads through Cuba to the Grand Cayman and ends at San Andres.
Lower airways (below 5,500 m), basing on VOR stops, appearing on cut maps, aviation maps of the globe and low level maps on the way, marked with the suffix "V" (winner outright, hence winner V.Airways, see V.v.). Air highways ( from 5,500 meters (18,000 feet) MSL to FL450) using VOR are known as jets; they appear on elevation maps (which usually do not show maps of low altitudes) and are preceded by the character "J".
The airways are 19 km broad and comprise a restricted space with a lower basis, usually FL070-FL100, to FL195. Every respiratory tract has an identifier that contains one character and one to three numbers. The entire FL195 area is a Category C managed area corresponding to the airways referred to as upper airways and marked with the letters "U".
When a top datum follows the same lane as a datum, its identifier is the U character and the identifier of the base datum. All airways in the United Kingdom are Category A under FL195 and VFR operations are therefore banned. Aerial passage is a particular part of the sky in which an aeroplane must be present during its passage through a particular area.
For example, during the Berlin Blockade pilot aircraft that flew over Soviet-controlled Germany had to observe very precise positions in the aerial passages specified by the commanding officer of the Berlin Wall. During the Cold War, following civil and army missions between West Germany and West Berlin had to stay within the planned range in order not to be fired.
Do not confuse ventilation ducts with airways. Aerial airways are navigation aides from which a pilots can generally depart when conditions require, while adherence to a particular aerial passage is compulsory.