Lighting the Way

In Uncategorized on March 6, 2018 by hillermuseum

SFO Beacon on display

Visual Aids to Aerial Navigation
by Jon Welte

On October 7, 1903, Charles Manly sat at the controls of the Langley Aerodrome. The brainchild of Smithsonian Institution director Samuel Langley, the Aerodrome was the culmination of his research into heavier than air flight. A signal was given and the Aerodrome launched—and immediately plunged into the Potomac. A second attempt in December 1903 met the same watery fate. Manly, unharmed, was rescued each time.

Although the Aerodrome’s two ill-fated “flights” went nowhere, Charles Manly was clearly prepared for success. A pre-flight picture of Manly together with Langley shows him in his flying suit, which included a nautical compass sewn onto its left leg. Manly’s readiness showed that even in the earliest days of flight, navigation was a serious concern.

The potential for airplanes to travel rapidly over long distances was apparent from the start. In 1909, Louis Bleriot successfully flew across the English Channel—but without a compass, he initially navigated by following a destroyer of the French Navy. In the United States, initial efforts to cross wide expanses of countryside were undertaken by aircraft flying US Mail. The first flight, in May 1918, was scheduled from Washington, DC to New York City. The pilot became lost shortly after departure and landed to ask for directions, vividly demonstrating the ongoing shortcomings in aerial navigation.

Air mail served as an impetus to development of a nationwide aerial navigation system. By 1920 air mail service linked San Francisco and New York, yet the service was a hybrid. Mail flew by day and was transferred to trains overnight, taking nearly 3 days to complete the transcontinental journey. Night flight, especially over the vast and sparsely populated spaces of the western United States, was too dangerous to attempt. To solve this problem, a system of airway beacons was constructed.

Built between 1923 and 1933, airway beacons were intended to guide airmail pilots on night flights. Each beacon consisted of a 90’ tall tower topped by a rotating white beacon light, and a small building containing a gasoline-powered generator to power the beacon. The concrete footprint of each beacon formed the shape of a giant arrow, painted bright yellow to indicate the direction of the next beacon. Each individual beacon could be seen from 40 miles away in ideal conditions; a series of beacons about ten miles apart defined an airway, or route, across the country. The completion of the first transcontinental airway in 1923 permitted airmail to be flown coast to coast without the need to transfer to trains overnight, cutting the trip duration in half.

Airway beacons were developed in conjunction with airfields to service aircraft flying the mail. On the transcontinental airway between San Francisco and New York, stops at thirteen intermediate airports were made. These stops allowed for mail transfers, aircraft refueling and pilot changes. Airports received beacons of their own, with different color combinations to indicate airports of different types. Sequential green and white flashes denoted civilian land airports in the United States, a practice that continues today.

Airway beacons proliferated across the United States in the 1920s and 1930s, with over 1,500 installed. These beacons facilitated a form of navigation called pilotage, in which pilots compare landmarks on the ground with their plotted locations on an aeronautical chart. A network of airways soon spanned the United States, totaling more than 18,000 miles.

Even as the last airway beacons were being erected, however, their demise was at hand. Pilotage in general, and airway beacons in particular, required clear weather to provide a safe and sure means of navigation. In poor weather, early pilots relied on a different type of navigation known as dead reckoning—flying a known compass course at a known speed for a carefully measured period of time. Charles Lindbergh used dead reckoning to cross the Atlantic Ocean in 1927, but while the technique sufficed to find a continent at the end of a 33-hour flight, it was insufficiently precise to thread a mountain pass, or find a small airfield. For that, greater precision was required—the precision that radio navigation could provide.

Although invisible to the naked eye, radio waves are able to shine through haze, clouds and darkness. Following a series of experiments undertaken by Jimmy Doolittle in 1929, low-frequency radio range (LFR) equipment was gradually installed along airways across the country. LFR signals required elaborate ground stations, but aircraft needed only a simple AM radio receiver to follow the signal. The station transmitted two signals on the same frequency—the letters A and N in Morse code. The pilot listened to the radio frequency while following the defined airway. If the pilot heard a steady tone in the radio headset, the aircraft was on the course line. If either letter became audible, the pilot would know to turn left or right to regain the course. Following an LFR course for long periods of time was mentally demanding of the pilot, but the equipment requirements were modest and the precision sufficient to allow for instrument approaches to airports in extremely poor visibility. LFR systems were widespread across the United States by the mid-1930s, and were the mainstay of commercial aviation operations until VHF Omnidirectional Range (VOR) equipment became available following World War II.

Airway beacons remained in use following the adoption of LFR radio ranges, and in fact early LFR stations were often co-located with airway beacons to define the same airways. Following the adoption of VOR navigation, however, the beacons gradually fell into disuse. By the 1970s most had been decommissioned, and today only a handful of beacons located in Montana remain operational. By 2020, just one operational airway beacon—located at MacDonald Pass, Montana—is expected to remain. In contrast, the white-and-green rotating beacons identifying airports are still a welcome and familiar sight to 21st-century aviators.

In 2017 the Hiller Aviation Museum acquired a decommissioned aerodrome beacon from San Francisco International Airport, The beacon was reconditioned by Restoration Shop volunteers and placed on display in February 2018. Retired after many years of service guiding aircraft from around the world to SFO, this new exhibit serves as a reminder of aerial navigation in a bygone era.

Resources, downloaded 1 November 2017 , downloaded 1 November 2017 , downloaded 1 November 2017


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