Air corridor

An airway is a legally defined corridor that connects one specified location to another at a specified altitude, along which an aircraft that meets the requirements of the airway may be flown. Airways are defined with segments within a specific altitude block, corridor width, and between fixed geographic coordinates for satellite navigation systems, or between ground-based radio transmitter navigational aids (navaids) (such as VORs or NDBs) or the intersection of specific radials of two navaids.

To guide airmail pilots on their delivery routes, the United States Postal Service constructed the first airways in the United States. These airways were between major cities and identified at night by a series of flashing lights and beacons which pilots flew over in sequence to get from one city to the next. However, these visual airways required the pilots to be in visual contact with the ground which precluded flying in fog or clouds. Subsequently, the Department of Commerce funded the development of other means of airway navigation.

The first airways to be delineated by radiofrequency were based on the old A-N system. The pilot listened for the stronger of the Morse codes transmitters ("· –" for A and "– ·" for N) (indicating left or right of the course); the objective was to be centered on course hearing a steady tone (the A and N Morse codes merge to form a steady tone when the receiver is equidistant from both transmitters).

Later airways were based on low / medium frequency ground stations, like the beat frequency oscillator (BFO) and the non-directional beacon (NDB). These L/M frequency airways were the colored airways. Colored airways still exist, mostly in Alaska. There are only a few colored airways remaining in the contiguous United States. There is one colored airway off the coast of North Carolina called G13 (Green 13). Additionally, there are several colored airways connecting to, and transitioning through, the Florida Keys. B9 (Blue 9) connects the Southern Florida mainland to Marathon Key. B646 (Blue 646) connects Mérida, Mexico to Nassau, Bahamas and transitions through the Florida keys. A portion of B646 connects Key West to Marathon Key and serves as a bypass for aircraft unable to meet the 14,500 minimum altitude along V3. Finally, G448 (Green 448), which begins at the Marathon NDB, is the only low-altitude airway that connects the United States to Cuba; the airway continues through Cuba to the Grand Cayman and terminates at San Andres.

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