A tri-color glidepath system projects a three-color visual approach path.
Archie W. League is usually regarded as the first air traffic controller. The 1929 photo shows him dressed for cold weather at St. Louis, where the airport operator employed him to prevent collisions between aircraft. His communication tools were simple: a red flag for "hold" and a checkered one for "go."
League is shown on duty in his summer office. Note the rolled-up flags in the wheelbarrow, and the dangling lunch box. His other equipment included a folding chair, drinking water, and a pad for taking notes.
Aircraft Landing Technology
A variety of technological navigation aids all help pilots achieve a smooth and safe landing. Early pilots didn't worry about keeping away from other aircraft and landed in any direction on an open field that gave them the best angle relative to the wind. As traffic grew and more aircraft began to use airports rather than farms or fields, landings became limited to certain directions, and descending aircraft, competing for the same course, were in danger of colliding.
The earliest landing aids were people-powered. Flagmen provided aircraft separation and direction control. They stood on the field waving red, green, or white cloths that told pilots if they were approaching a clear field at the correct angle. Green flags would indicate a clear field or the proper direction; red flags meant danger or told the pilot to circle until further instruction.
Archie League was one of the airport system's first flagmen, beginning before 1920 and staying at the field in St. Louis, Missouri, until a radio tower was installed in the early 1930s. League then became the airport's first radio controller. The radio controllers could provide some information to the pilots to help them land.
As of 1935, there were only about 35 such radio controllers. More often, flagmen were replaced by a system of red and green airport lights to show the runway threshold and sides, to allow pilots to judge their remaining distance and angle they should take for landing based on how narrow the parallel beams appeared to them from a distance.
Airports had begun using lights in the late 1920s, when fields were marked with rotating lights so they could be found after dark. In the early 1930s, airports installed the earliest forms of approach lighting, which indicated the correct angle of descent and whether the pilot was right on target. These were called the glidepath or glideslope. Gradually, the colors of the lights and their rates of flash became standard worldwide based on International Civil Aviation Organization (ICAO) standards.
Approach lighting can be oriented to accommodate any obstructions located near the airport that the pilot may need to avoid before beginning his descent to the runway. Lights can even be set at a second angle for larger aircraft because those cockpits are farther off the ground and the angle of descent will look different to pilots in these planes. Pilots flying into fields without any staff can often turn landing lights on or off themselves or change their brightness by tuning their radio to a certain frequency and clicking their transmitter.
Radio navigation aids also assisted in landing. One type, introduced in 1929, was the four-course radio range, where the pilot was guided by the strength of Morse code signals. Another type that was tried experimentally was the low-frequency radio beam. These radio beams flared outward from the landing point like a "v," so at the point farthest from the runway, they were wide, and it was easy for the pilot to fly between the arms of the "v." But near the landing point, the space between the beams was extremely narrow, and it was often easy for the pilot to miss the exact centerpoint that he had to hit for landing. Another new method had a pilot tune into a certain frequency at a checkpoint far from the airport, then use a stopwatch to descend at a precise rate to the touchdown area of the runway. This method also proved difficult.
The development of RADAR at the beginning of World War II allowed the military to use a new landing aid called ground control approach (GCA). GCA consisted of two six-inch (15-centimeter) radar displays using cathode ray tubes (CRTs). One CRT displayed the approaching aircraft while the radar operator directed pilots into a waiting position using voice radio. The other CRT display helped determine how the pilot should steer to find the proper heading and approach angle to land. Then a controller literally talked the pilot down. GCA used mobile trailers that could roll to a new runway or even a new airport when needed. Today's military still uses an advanced version of this system, called precision approach radar. GCA proved its worth during the Berlin Airlift in 1948-1949 when as many as a thousand flights landed each day.
The instrument landing system (ILS) incorporated the best features of both approach lighting and radio beacons with higher frequency transmissions that painted an electronic picture of the glideslope onto a pilot's cockpit instruments. U.S. Army Air Service Captain Hegenberger and Lieutenant Jimmy Doolittle each made a blind flight and landing that tested a primitive form of the ILS in 1929.
The Bureau of Air Commerce adopted ILS in 1934 and began installing it at 36 airports. But airlines protested that the system was unflyable, partly because it was expensive to install the necessary instruments in their aircraft. An alternate form of ILS called air track, which had been tested in 1928 by engineers from the U.S. Bureau of Standards, eventually became standard. The first landing of a scheduled U.S. passenger line using air track was on January 26, 1938, as a Pennsylvania-Central Airlines Boeing 247-D flew from Washington, D.C. to Pittsburgh and landed in a snowstorm using only the air track system.
Air track consisted of an electronic glidepath signal that is beamed to the aircraft. This indicated the correct angle of descent to the runway. The system also used two marker beacons that let the pilot know when checkpoints were passed as the airport neared. Finally, a beam called a localizer let the pilot know whether to steer to the right or left to stay on the runway centerline. Equipment in the airplane allowed the pilot to receive the information that was sent so he could keep the craft on a perfect flight path.
On January 15, 1945, the U.S. Army introduced an ILS with a higher frequency transmitter to reduce static and create straighter courses, called the Army Air Forces Instrument Approach System Signal Set 51. In 1949, the ICAO adopted this Army standard for all member countries. In the 1960s, the first ILS equipment for fully blind landings became possible.
The ILS in 2001 remains basically unchanged. Pilots fly along a glideslope that is determined by information received by the pilot electronically. But landing still requires that he actually see the runway. At a particular height and distance from the runway, called the decision height, the pilot can see the runway well enough to land the plane.
Microwave landing systems were developed in the 1980s and were intended to replace the ILS in the United States. This system would have allowed pilots to enter a path to land from more directions than the ILS and descend at a choice of paths best matched to their type of aircraft. These different landing patterns can help reduce noise around airports and keep small aircraft away from the dangerous vortices behind large aircraft. But microwave systems have been delayed in the United States due to funding problems and uncertain developments from competing technology. In Europe, however, the microwave landing system is replacing the ILS. In the United States, the FAA is considering the use of the global positioning system (GPS) instead of or in addition to microwave systems. The GPS uses satellites for navigation between airports and is exceedingly precise.
Helicopters have used visual landing procedures for most of their history, but on June 12, 1987, the Federal Aviation Administration (FAA) opened its national concepts development and demonstration heliport. This research heliport was fully equipped with items such as a microwave landing system as well as precision approach path indication lights like those used by fixed-wing aircraft.
Pilots cannot simply navigate to an airport and then fly into the runway using one of these landing aids. Federal regulations and procedures require aircraft to approach airports at certain altitudes and in certain patterns well before they are ready to land. If excess traffic requires them to wait their turn, they must follow a holding pattern that keeps them up to 50 miles (80 kilometers) from the airport. Holding patterns look like a series of invisible racetrack ovals, with each circling aircraft in a stack at 1,000-foot (305-meter) intervals at altitudes as high as 23,000 feet (7,010 meters). As each aircraft is cleared to enter its final approach to land, the pilot exits the bottom of the stack while all others descend to the next lowest level, as though on a circular ladder. If a pilot misses his touchdown, he must climb and then follow strict procedures in leaving the airport to circle and hold again.
Selected Bibliography and Further Reading
Clausing, Donald J. Aviator's Guide to Navigation. Blue Ridge Summit, Pennsylvania, Tab Books, 1992.
Illman, Paul E. The Pilot's Air Traffic Control Handbook. Blue Ridge Summit, Pennsylvania, Tab Books, 1993.
Kershner, William K. The Student Pilot's Flight Manual. Ames, Iowa, Iowa State University Press, 1993.
Spence, Charles F. Aeronautical Information Manual/Federal Aviation Regulations. New York: McGraw-Hill, 2000.
Airport Infrastructure. http://www.airportnet.org/infra.htm