An Airway beacon was a rotating light on a tower used for visual navigation by airplane pilots along a specified airway corridor. Approximately 1,500 Airway beacons were constructed, covering 18,000 miles (29,000 km) in the U.S. to guide pilots from city to city. Construction by the Post Office and the U.S. Department of Commerce occurred between 1923 and 1933.[1] They were spaced 10 miles (16 km) apart and featured a 24 inch (610 mm) parabolic mirror and a 110-volt, 1000 watt lamp.[2] In clear weather they could be seen for 40 miles (64 km).[1] The beacons rotated at 6 rpm, creating a flash every 10 seconds for 1/10 second duration.[2] The Saint Paul, Minnesota beacon sits on top of a 110 foot (34 m) steel tower in Indian Mounds Park.[3] The Low Frequency Radio Range system began to replace this visual system in 1929.[1] The last visual airway beacon was supposedly shut down in 1973,[1] however fourteen airway beacons are still operating in Western Montana[4], and are charted on the Great Falls sectional chart. They are maintained by the Montana Department of Aeronautics and were transferred to the state in 1973.
The beacons were spaced 10 miles apart, each one was identified with a number, and they flashed the Morse code of one of 10 letters: W, U, V, H, R, K, D, B, G or M. The letters represented the digits of 1 through 10 (W = 1, ..., M = 10), and the beacons along an airway were numbered sequentially. To help remember the letters and their sequence number, pilots memorized the following phrase: "When Undertaking Very Hard Routes, Keep Direction By Good Methods." The beacons were depicted on navigation charts along with their number and Morse code. As example, beacon number 15 would have a code digit of 5 (the units digit), hence the letter R, and Morse code: "dit dah dit" (.-.).[5]