Airway beacon

An airway beacon is a rotating light on a tower once used extensively in the United States for visual navigation by airplane pilots along a specified airway corridor. Approximately 1,500 airway beacons were constructed to guide pilots from city to city, covering 18000 mi. Most of the beacons are gone, but Montana continues to maintain several in mountainous terrain. One is preserved in Saint Paul, Minnesota at the Indian Mounds Park on a bluff overlooking the Mississippi River. Recently the beacon at Grants, New Mexico was restored using original items found at other nearby sites.

Many of the concrete arrows are visible from satellite pictures, even in urban settings.

Light characteristics
An airway beacon has two distinct light characteristics: A revolving narrow white light beam about 5 degrees wide in azimuth and a set of fixed colored course lights of about 15 degrees width.

White rotating beacon
The rotating beacon features a 24 inch (610 mm) parabolic mirror and a 110-volt, 1 kilowatt lamp. spinning at 6 rpm, creating a quick 1/10 second flash every 10 seconds. In clear weather they could be seen for 40 miles (64 km). Montana took steps to modernize their beacons encasing newer light systems in clear domes.

Red or green course lights
Just below the white beacon, a set of red or green course lights point along each airway route. Red lights denote an airway beacon between landing fields while green denotes a beacon adjacent or upon a landing field. These course lights flash a Morse code letter identifying the beacon to the pilot. Each beacon is identified with a sequential number along the airway, and flash the red or green course lights with the Morse code of one of 10 letters: W, U, V, H, R, K, D, B, G or M. The letters represent the digits of 1 through 10 (W = 1, ..., M = 10). The course lights turn on for 0.5 second for a dot, 1.5 second for a dash with a 0.5 second between each dot or dash. A pause of 1.5 seconds separates each letter.

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 an example, beacon number 15 would have a code digit of 5 (the units digit), hence the letter R, and Morse code: "dit dah dit" (.-.).

History
Airway beacons were constructed by the Post Office and the Department of Commerce between 1923 and 1933. The Low Frequency Radio Range system began to replace this visual system in 1929. The last visual airway beacon was supposedly shut down in 1973, however some airway beacons are still operating in Western Montana, and are charted on the Great Falls sectional chart. They are maintained by the Montana Department of Transportation Aeronautics Division.