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Copyright: 1996 - 2024

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    Kirt Blattenberger,

    BSEE - KB3UON

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

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Landing Blind - the Instrument Landing System
July 1938 Radio News

July 1938 Radio News
July 1938 Radio News Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged.

Only three and a half decades had passed since the Wright Brothers made the first flight of an aeroplane taking off under its own power when this "Blind Landing" article appeared in a 1938 issue of Radio News magazine. By then, an entire World War had been fought with air power having been determined to be a primary strategic force, and a commercial airline industry was thriving as travelers everywhere entrusted pilots and air traffic controllers with their very lives. The main impediment holding back further progress from an navigational and scheduling perspective was inclement weather. Pilots had long ago learned to fly by instruments, and taking off into nearly zero visibility was not a problem, but landing confidently and safely under the same conditions was still impossible. Aviation researchers were hard at work trying to devise an instrument landing system (ILS) which would guide pilots to the runway threshold by providing electronically via narrowly focused radio beacons whose signals were measured and presented on cockpit instruments. Glide slope (elevation) and course line (azimuth) together creates a three-dimensional representation of the aircraft's position in space, and all the pilot needs to do is keep the needles centered until the runway is visible. At that point the airplane is flown in a normal manner to touchdown. That, of course, is easier said than done. Even in perfectly calm weather conditions a lot of skill is required to stay on the directed path, but throw in wind, wind gusts, rain, lightning, snow, and other variables and the job gets a lot tougher. Add to that the fact that just as a primary tenet of General Relativity asserts, there is no way to differentiate between acceleration due to gravity and acceleration due to a change in speed and you have a situation where without a visual reference outside the cockpit, he cannot tell for instance whether a particular G-force is due to being upright and level or to being upside down and flying a parabolic course toward the ground. That explains the aviator's admonition to "die by your instruments," meaning it is usually better to ignore your "gut" feeling and trust what the instruments say since when properly interpreted  they are more reliable.

Landing Blind

Radio eliminates one of the great hazards of aviation - RF Cafe

Radio eliminates one of the great hazards of aviation by making possible a safe landing in "zero-zero" weather.

by C. S. Van Dresser

For years it has been the dream of aviation experts throughout the world to perfect a blind landing device that works. Not that several of the inventions in the past do not offer some degree of exactness and reliability and are a distinct aid to aviation; but what was wanted was a device that would function perfectly - that would guide the pilot to an absolutely safe landing even if the field were blanketed by dense fog and not a floodlight would work.

A difficult task to develop such a device, but not an impossible one, for it appears at last that this dream of aviation has become a reality.

The story started ten years ago in the National Bureau of Standards, Uncle Sam's great scientific laboratory in Washington. In 1928 this vital work was undertaken by a group of brilliant young scientists of the Bureau and carried to partial completion in 1933. Then the necessary appropriations ran out, and the Washington Institute of Technology took over, engaged the same scientists, and carried on.

Photo-schematic diagram above shows how the Air Track blind landing system functions - RF Cafe

The photo-schematic diagram above shows how the Air Track blind landing system functions for Pennsylvania Central Airlines at the Municipal Airport at Pittsburgh. Marker beacons are installed at the ends of all runways. The transmitter trailer may be towed to the opposite end of any runway it is necessary for the incoming plane to use (due to the direction from which the wind is blowing), plug in on the power line, and send forth its localizer and glide-path beams. By the use of the "cross-pointer" instrument the pilot can then guide his plane down to a safe landing, even though visibility be limited to a short distance in front of his ship.

Cross-pointer instrument keeps the pilot informed at all times of his plane's position in relation to the glide-path beam - RF Cafe

The "cross-pointer" instrument keeps the pilot informed at all times of his plane's position in relation to the glide-path beam. (1) Indicator shows that plane is to the left of the course beam which marks the center of the runway, and above the glide-path beam which brings the ship gently to earth along a curved track. (2) Indicator shows plane is to the right of the course and below glide-path. (3) The incoming plane is now on course but is below the glide-path. (4) The pilot now has the plane on course and on the glide-path. By keeping the needles in this position the pilot can bring his plane down to a safe landing with its cargo of passengers, air mail and express.

Radio equipment for the localizer and glide path beams - RF Cafe

This trailer houses the radio equipment for the localizer and glide path beams.

Elevation and Plan of "Air Track" in Operation - RF Cafe

Elevation and Plan of "Air Track" in Operation.

1.   Off course, left and below flight path.

2.   Off course, right and below flight path.

3.   On course and on flight path.

4.   On course and above flight path.

5.   On course and on flight path.

6.   Marker beacon at end of runway, or edge of field, gives pilot audible signal.

7.   Air Track equipment installed in trailer which can be placed at either end of any runway on the airport.

8.   Airport control tower containing monitor box which shows operation of the Air Track. The Operator is in two-way voice communication with the pilot.

Monitor in the airport's control tower- RF Cafe

The monitor in the airport's control tower.

For four more weary years the painstaking work continued, until today it can be announced that the job at last has been successfully completed. With new developments perfected at the Institute, for the first time in the history of the world a blind landing device is in actual commercial operation. It is being used daily by Pennsylvania Central Airlines at the municipal airport in Pittsburgh.

The equipment consists of three radio transmitters and antennae located on the ground and the necessary equipment in the plane to indicate signals from them. The ground antennae actually produce a so-called curved beam, or radio track down which the plane glides to land gently and securely on the runway.

Two of the ground transmitters and antennae are housed in an automobile trailer. These give the incoming pilot vertical and horizontal guidance. The third transmitter and antenna are located at the approach end of the runway and throw up a vertical radio "curtain" through which the plane passes.

To receive the signals the plane is equipped with a cross pointer instrument which is a dial with two needles, one to indicate whether it is to the right or left of the air track, and the other to tell if it is too high or too low. One of the radio beams is projected in a straight line down the center of the runway on which the plane will land.

The horizontal guidance transmitter, or localizer as it is called, is in effect a miniature range beacon with four distinct courses, each separate course with a range of twenty miles or better. When the pilot approaches the airport, the vertical needle informs him whether he is aiming his ship correctly on this particular beam. When this needle points directly up, the plane is headed precisely for the runway, and the pilot is in a position to receive signals as to his altitude.

This is where the horizontal needle on the cross pointer instrument comes in. All the pilot has to do is maneuver his ship until the two needles are at exact right angles and he will come out of the sky in a gentle glide to make a safe and easy landing.

The transmitter, located at the approach end of the runway, which sends up the vertical curtain, referred to previously, is known as the marker beacon. As the plane passes through this curtain, the pilot receives a signal in his earphones and a light flashes on his instrument panel. Thus he knows he is crossing the boundary of the field at a known altitude, at which point he cuts the throttle and simply follows the glide-path which leads him gently to the ground.

Two-way radio communication between the pilot and the operator of the airport control tower is not interrupted at any time during the landing procedure.

As a further check in safety, the complete operation of the entire air track system is constantly under the observation of the operator of the control tower. An ingenious monitoring device, with lights to correspond to the various air track radio beams, instantly informs the tower control man of the operation of all ground equipment.

One of the most amazing properties of the curved beam set-up is its complete maneuverability. As we know, planes must land into the wind. Landing in a violent cross wind might prove fatal to these leviathans of the air.

Suppose, for instance, a 30 mile gale is blowing from North to South which is directly in line with the long runway of an airport. That would mean that the plane must land facing North, or into the wind. Half-an-hour before the ship arrives, the wind changes direction, blowing from East to West. With stationary equipment, Air Track would be of no avail, and the pilot would be forced to land in the regular manner, either by sight in the daytime, or floodlights at night. (Naturally if visibility was too poor, the plane would have been grounded.)

However, with this new system the trailer with necessary ground equipment can be shifted at a moment's notice. All that has to be done it to hitch a tractor to it and tow it to any position desired and plug in to the airport's electrical system. It is ready then to guide the plane to the field on a different runway.

Another advantage of the new system is that it presents practically no obstruction to departing or arriving transports. Some landing fields are menaced by dangerous obstructions such as radio towers, high tension wires and the like. The trailer, situated at one end of the field, is but eight feet in height. The beam insures any incoming plane plenty of clearance, while a ship taking off, which might be headed directly at it, has the entire length of the field to dear eight feet. If a pilot can't make that altitude. in a half-mile runway, he's in trouble. already.

Officials and scientists of the Washington Institute of Technology point out positively that the flights and landings now taking place at the Pittsburgh airport are not experiments, not demonstrations, and not tests of any nature. As Dr. Frank G. Kear of the Institute states: "This is the emergence of instrument landing from the demonstration stage to that of pilot training during scheduled airline operations. The air track system has been developed over a period of nine years. It is now in actual service."

During the nine years of development, many hazardous experiments were undertaken at the laboratory and airport which are located at College park, Maryland, just outside of Washington, D. C., near the University of Maryland. Courageous and skillful pilots risked their lives in testing various phases of the instruments. Literally thousands of landings were made, many "under the hood" when the pilot could see nothing but his instrument panel. Some of the birdmen received severe jolts and bruises due to inaccuracies which in time were corrected, but no one was seriously hurt and no one was killed.

Colonel Charles Lindbergh later flew the bent beam, as it was called then, at Newark and expressed great interest in the development to date. The highlight of the early demonstrations was the flight by James L. Kinney, Department of Air Commerce pilot, from College Park to the Newark airport. This epochal aerial journey was undertaken when every plane in the East was grounded due to one of the heaviest fogs in years. Pilot Kinney took off in this fog, flew to Newark, and landed without ever having seen the ground. It was the first actual zero-zero, cross-country flight ever made.

According to W. E. Jackson, Chief of the Radio Development Section of the Bureau of Air Commerce, an amendment to the Air Commerce Act of 1926 will be presented at this session of Congress to make possible the expenditure of addition Federal funds on airports as well as airways. This will make possible the installation of blind landing equipment at every important airport in the United States.

Credit for the evolution of the curved beam system goes to Harry Diamond, radio expert of the National Bureau of Standards; Gomer L. Davis and Dr. Frank G. Kear, formerly of the Bureau and now with the Washington Institute of Technology; W. E. Jackson, of the Bureau of Air Commerce and Pennsylvania Central Air Lines.

As their motto, the men of science who have developed their newest aid to safety in the air, have adopted these words, "A safe flight must end in a safe landing."

 

 

Posted January 13, 2022

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