This month, in 1935, all major airports between New York and Los Angeles were to be installed with the latest blind-landing radio equipment developed by Captain Albert Hegenberger of the U.S. Army Air Corps. But then things became a little complicated….
During the early 1930s, two government organizations, the U.S. National Bureau of Standards (NBS) and the Aeronautics Brand of the Department of Commerce, faced the challenge to overcome radio wave instability. Radio engineers and physicists were also motivated to find methods of locating radio transmissions because of wartime demands for improved navigation.
Fixing the instability problem meant finding a way around nature’s inconsistency. The pavement had been the answer to the inconsistency of the Earth’s physical surface, providing planes a smooth, flat place to land. Unfortunately, consistency in radio-based blind landing equipment proved much harder to achieve.
Hegenberger’s system, formally named the A-1 system by the army, was more commonly called the Hegenberger system. It relied on a radio compass to provide directional information to the pilot. Radio compasses, often called radio direction finders (now referred to as automatic direction finders), were a derivative of World War I radio research.
Hegenberger used two low-frequency, omnidirectional radio transmitters called compass locators to provide navigational references for the compass. Pilots were to fly between them using a specific pattern. The two locator stations, which were truck-mounted, were to be placed just outside the airfield boundary and 1.5 miles out. Pilots set their radio receiver for station A’s frequency and used the radio compass to fly toward it.
Once the plane had passed over the inner station, a pilot was to:
1) Make a 180-degree turn
2) Reset the receiver to station B’s frequency
3) Fly to B
4) After continuing past B for about three more miles, pilots were to execute a standard rate turn and line up on B again, and fly over at 500 ft.
5) The radio receiver was then returned to A’s frequency. Descending at 400 ft. per minute, pilots were supposed to fly over A at 150 feet and maintain the 400 ft. per minute descent until they touched down.
While testing of this development occurred during a desperate airmail crisis, the army’s Hegenberger, or A-1, system became the first officially sanctioned blind landing system in the United States. Even though it didn’t test well, it was given the go-ahead. Unfortunately, commercial pilots were having some issues with the equipment. The Hegenberger system did not define an adequate point of contact on the field. Several other pilots wrote the bureau with similar misgivings about the system’s ability to provide safe blind landings at commercial fields.
While all agreed the system’s radio compass was an excellent navigation aid for finding an airport, it became obvious the Air Corps and the commercial airlines had different ideas about what worked as a landing system.
The differences mostly were rooted in their operating environments. The Air Corps in 1934 still relied overwhelmingly on all-over turf fields for its flying, for which an accurate point of contact was unnecessary. The corps also used smaller, lighter aircraft, mostly biplanes. The airlines had already junked such planes for faster, heavier all-metal monoplanes. One consequence of the airlines’ adoption of surfaced runways is that landing on one required more precise guidance than the A-1 could give. What worked for the army’s all-over fields thus did not work for the airlines and their runways.
Soon after, United Airlines would contact Bendix Aviation to develop an instrument approach system rather than a landing system. Without the demand for blind landing, one could reduce the need for stability and precision.
While Hegenberger’s initial design may have fallen short, it’s a great example of how we learn from our mistakes and constantly improve technology. After the United-Bendix system, later came the Lorenze, and the GCA radar. All of which led to today’s more precision-based radar systems.
Conway, Erik M. Blind Landings: Low-Visibility Operations in American Aviation, 1918–1958. Johns Hopkins University Press, 2006. Project MUSE. doi:10.1353/book.3283.