Beam Me Down Scotty: How Airplane Auto-Landing Came to Be

You dream of becoming a pilot. It’s 1919, only 16 years after the Wright brothers took their first flight. You are airborne with an instructor. Suddenly, the weather turns bad, and you can’t see anything. With the realization that you are now “flying blind”, you begin to panic. How will you safely navigate and land?

We talked last week about how a gyroscope can help a pilot navigate without visual context. Today we’ll look at Instrument Landing Systems (ILS), a sophisticated technology for “landing blind” that began to emerge in the early 20th century. These days, ILS helps pilots land in all sorts of weather, making air travel safer and more reliable than ever before.

So, where did this amazing ILS technology come from?

Researchers designed early radio navigation techniques for seaborne, not airborne, vessels. To apply these ideas to aviation, they had to adapt them from 2D maritime space to 3D aerial space. To account for all three dimensions, they concluded that any ILS needed three essential pieces of information:

1. The distance to the landing touch-down spot;
2. How far away from the center of the runway the plane’s heading was;
3. Altitude information for a smooth “glide path” down to the runway.

Learning from one another, the worldwide radio research community devised many competing solutions to the ILS problem.

The German scientist Dr. Ernst Ludwig Kramar, working for the Lorenz company, is credited with developing the first ILS system using VHF in 1932. This system came to be known as the Lorenz beam, and following it came to be known as “flying the beam”.

How did it work? A radio transmitter was situated at the far end of the runway with two transmitting antennas. It could be tuned to transmit in the 30 MHz – 33.3 MHz range. One antenna was to the right of the runway centerline, and the other was to the left. The radio alternately transmitted Morse dots and dashes. The dots went to one antenna, and the dashes went to the other.

At an altitude of 200 meters, the pilot would want to align his plane with the runway’s center. This “sweet spot” was where the radiation pattern of dots and dashes overlapped and generated a continuous signal. Too far to the left or right, the pilot would receive an intermittent signal of dots or dashes. A needle indicator in the cockpit helped him to see if he was to the left or the right of this continuous signal “localizer beam” guiding him to touch-down. A second needle indicated the signal strength and the range to the transmitter. When the pilot flew over an outer marker beacon 3 km from the landing spot, he knew how far it was to the airport. This beacon transmitted a signal vertically at 38 MHz and an audio tone at 700 Hz. The plane continued to descend at a constant rate while the pilot monitored the localizer beam. An inner marker beacon was situated 300 m from the airport, operating at 3 MHz with an audio tone at 1700 Hz. Both beacons caused an indicator light to illuminate when the plane was over them. The pilot needed to be able to see the runway just before touching down.

In the United States, the National Bureau of Standards (NBS) began designing radio navigation tools for military aviation during World War I. After the war, NBS refined this technology for the civilian market. In 1921, it demonstrated a new ground-based system that used direction-finding (DF) stations at airports and radio communications. This DF system allowed the ground to radio locational information to the aircraft upon the pilot’s request.

The next NBS project addressed the “flying blind” concern of our 1919 pilot-in-training. This system used radio beacons to keep the pilot on course to his terminus. It also replaced the old radio communications link to the cockpit with a visual indicator. The indicator responded to the radio beacons in such a way as to guide the pilot to his destination. In 1929, a pilot demonstrated that he could use this system to fly blindly from Philadelphia to the College Park Airport, where he had never landed before.

Building on these achievements, researchers Harry Diamond and Francis Dunmore took on our pilot-in-training’s blind landing problem. They devised a new distance-from-the-runway indicator for the plane. It used the radio signals’ strength and calibrated itself using two marker beacons, one 2000 feet from the runway and a second at the end of the runway. They refined the stability and sharpness of the runway localizer beam. They solved the problem of vertical guidance – descending via a glide path to the ground – using:

“another transmitter antenna located near the runway localizer transmitter. Using ultra-high frequencies this time, the second antenna would send out a radio wave pattern, parabolic in shape and sharply enough defined to form an invisible ramp that the pilot would follow as the plane glided down to earth.”

In 1931, a pilot using the Diamond-Dunmore system made the first blind landing in aviation history “using only radio signals for lateral, longitudinal, and horizontal guidance.” People considered this American system superior to Lorenz’s because it provided a smooth glide path down to the landing strip.

Now we understand the origins of instrument landing systems. Predictably, World War II interrupted this ILS development cycle with urgent military requirements. Next week we will talk about how these technologies were adapted for wartime use.

References:

https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4502167

http://www.radarworld.org/flightnav.pdf

https://permanent.access.gpo.gov/lps1514/nvl.nist.gov/pub/nistpubs/sp958-lide/038-042.pdf