Even in clear weather, GCA is better than human eyes! That’s a bald statement which is hard to believe. But there are facts to back it up.
Stripped of all the gobbledygook, GCA, or Ground Control Approach, is just a radar talk-down system of helping the pilot make a landing when the weather is atrocious. You don’t need any special equipment in the plane itself. All the pilot has to know is how to fly on instruments and carry out simple instructions like “turn to heading of 350” or “descend to 2,000 feet.”
It is that simple. In fact, it is the simplicity of it that decided the Navy and the Air Force to adopt it as the standard low visibility approach for their pilots. No special training for the pilot, no special equipment in the plane. Just a mobile, reliable, self-sufficient unit on the ground with some calm, unruffled voice telling you what to do.
Oddly enough, it was this feature that was the one point against it as far as the commercial airline pilots were concerned. They felt it was taking control of the aircraft out of the cockpit. They couldn’t visualize that final controller as just another reliable flight instrument which was not subject to mechanical breakdown. To them, GCA usurped the pilot’s authority, and they were against it.
Pilots returning to the airlines from the armed services after the war brought back the “word,” however. And now even the commercial airlines are interested. CAA is presently installing remote sets in the towers of many key airports to be used in conjunction with their ILS, Instrument Landing System.
The idea of Ground Control Approach started back in 1941 at the Massachusetts Institute of Technology. But the Navy didn’t accept its first GCA Unit until 1944. Since that time over 530,000 Ground Control Approaches have been made, not counting the 2,500 per month used to train new operators at the GCA School which operates under the Naval Air Technical Training Command at the Naval Air Station, Olathe, Kansas.
Over twenty-five thousand GCA landings have been made during instrument weather. It is a conservative estimate that about ten percent of these, or over 2,500 landings, could not have been made at the fields concerned had it not been for GCA.
The statement that GCA is better than human eyes obviously requires some qualification. In the first place, the eye must be unaided. No high powered telescopes or binoculars. In the second place, the weather must be clear enough to see the plane.
At the GCA School in Olathe, a demonstration under such conditions was conducted to show the comparison. At the end of the runway a portable radio hookup was established with the plane. A sight, consisting of a round piece of tubing with cross wires at each end, was accurately lined up with the runway extension at the standard three degree glide path. All the controller had to do was to talk the plane down the line of sight. A recording was made of the number of course changes and the number of times the plane went above or below the glide path. Also, the range at which the aircraft first intersected the glide path (i.e., entered the visual field of the sight) was noted.
A number of runs were made visually and the results were compared with data taken using the radar control of the Ground Control Approach Unit.
GCA was able to direct the plane to the glide path sooner (i.e., at a greater range from the field) and to keep it on with less corrections. Ground Control Approach was actually more accurate than the unaided eye of the controller! In fact, it was at times difficult to keep the aircraft in sight, especially at the extreme ranges of the pattern.
Of course, this was just a demonstration and not a scientific evaluation, but the results were rather impressive.
There are 39 GCA Units in the Navy including five Marine Corps sets. Seven of these units are overseas. More than half of these units will land a plane even if the ceiling is as low as 100 feet. The others have 200 feet as the minimum weather condition. Minimum visibility is either one-half or one- quarter of a mile. On the other hand, the Air Force has established higher minimums, quite a few as high as 400 feet and none less than 250 feet. All, of course, can land a plane in zero zero weather in an emergency. For normal operation, however, the weather minimums established by the Navy provide a wide margin of safety.
These lower minimums of the Navy have permitted the operation of some flights which would otherwise have been cancelled or diverted to alternates. This was dramatically brought out in the winter of 1947 when the Naval Air Transport Service flew every scheduled trip into the San Francisco bay area during a three day period of bad weather which grounded all other commercial and military planes.
Each GCA unit has two complete radar systems: the search system and the precision system. The search system is similar to the search radar found aboard ships, except that there is no altitude discrimination and the shape of the lobe is designed to minimize the effect of ground return. This system covers 360 degrees and is used to direct the aircraft around the traffic pattern to a position for the final straight-in approach.
The precision system is used by the final controller as he talks the plane down the glide path. It consists of two separate elements which determine the position of the plane in relation to the on-course line in azimuth and in relation to the glide path in elevation.
A high degree of accuracy is attained on the two precision radars by the angular expansion of the two scope presentations. The azimuth scope covers twenty degrees of arc and is expanded three times. The elevation scope covers seven degrees of elevation and is expanded eight times. Rapid scanning gives practically a continuous revision of the picture of the plane as it approaches the field. The position is accurate within five feet of the glide path.
Duplicate sets of most of the essential elements in each unit are kept on a stand by status. This insures dependability and a high availability of the GCA unit at all times.
The Navy is still working for the ideal: True all-weather flying. So far, some horizontal and vertical visibility, however small, is required for landing if any margin of safety is to be maintained. Each improvement in GCA brings us nearer to the goal.
Sometime during fiscal 1952, the Navy will replace the present GCA units with more up- to-date equipment. One type, AN/CPN-4 made by Gilfillan will incorporate quite a few new wrinkles.
An MTI, or Moving Target Indicator, circuit will cut out all targets on the radar scope except those actually moving. Actually eliminating the ground echoes from water towers, buildings, etc. and showing only the aircraft flying in the area simplifies the control problem tremendously.
Another feature enables the GCA operator to identify the “blip” of the aircraft on the radar scope simply by having the pilot transmit over his voice radio.
With the new equipment the range is increased from 50 to 60 miles and three times as many planes can be handled in the traffic pattern. This is especially important in Carrier Control Approach where large groups of planes have to be brought aboard at a high landing rate.
The AN/CPN-4 equipment will be completely “air transportable.” It can be flown as a unit to any air base in the world and immediately set-up to give service. It also breaks down into small units that can easily be shifted for necessary maintenance and replacement.
But until this new equipment is received, the present MPN-1B units will keep on adding to its box score of “saves.” At the Naval Air Station, St. Louis, an Air Force plane was brought in during a blinding snow storm with one engine out and only ten minutes of fuel remaining. At Grosse He, Michigan, the Navy GCA unit talked down a civilian Mallard Amphibian with seven persons aboard that was lost in the vicinity of Detroit during severe icing conditions. Since the radio in the plane was on a different frequency from the Navy bands of the GCA Unit, all transmissions from the aircraft were relayed to GCA by the Detroit City tower by telephone. He was brought in with only 7 gallons of fuel—less than ten minutes. At the same station, an SNJ was brought in during blowing snow and had to be towed off the runway, out of gas. At Minneapolis, visibility was one-eighth of a mile with heavy fog and no ceiling, when GCA landed an Air Force C-47. Down on the runway, the visibility was so bad that the pilot had to stop and wait for a jeep to conduct him off the field. These are typical examples of how GCA saves lives.
The Navy loves its GCA. You don’t need a super-pilot to fly it. You don’t need any special equipment in the plane. A pilot who can fly basic instruments and follow simple instructions is a pilot who can be saved. Simplicity of operation, reliability, and accuracy spells GCA.
When the field is “socked in” by fog and the port engine is sputtering and you’re running low on gas, there is nothing so reassuring as the calm voice of the final controller telling you what to do.
“You are now ten feet above glide path, your course is 350 . . . You are on glide path, turn right to heading 353 ... On glide path ... On course . . . One quarter mile from touchdown point... On glide path . . . Center line of runway is dead ahead ... Over end of runway . . . Over touchdown, take over visually.”
Another safe landing chalked up by GCA! Sight without eyes!
