This photo shows a typical approach lighting system as the pilot would see it when properly aligned from the runway end. The top image shows the approach during the day; the lower image shows the same approach at night.
A transmissometer provides visibility information to the pilot when landing. It is used in conjunction with the instrument landing system.
Elements of the Instrument Landing System.
This model of a distance measuring indicator allows the pilot to obtain distance, groundspeed, or time-to-facility.
History of Aircraft Landing Aids
One of the most difficult tasks a pilot has to perform is to achieve a smooth and safe landing. Early pilots landed on an open field, facing any direction that gave them the best angle relative to the wind. But as traffic grew, and more aircraft began to use airports rather than farms or fields, landings became limited to certain directions. Landing aids were developed to help pilots find the correct landing course and to make landing safer.
Airports had begun using lights in the late 1920s, when landing 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. These indicated the correct angle of descent and whether the pilot was right on target. Their approach path was 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. The Air Mail Service's intermediate, or emergency, landing fields that it established along the air route used rotating electric beacons and lights that were set around the perimeter of the field.
The introduction of the slope-line approach system was a first in landing aids. Developed in the 1940s, the aid consisted of lights in rows that showed the pilot a simple funnel of two rows that led him to the end of the runway. Other patterns showed him when he was off to the right or left, or too high or low. The system was inexpensive to build and operate although it had some limitations and was not suitable for certain airports.
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, the beams were widely separated and it was easy for the pilot to fly between them. 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 instrument landing system (ILS) incorporated the best features of both approach lighting and radio beacons with higher frequency transmissions. The ILS painted an electronic picture of the glideslope onto a pilot's cockpit instruments. Tests of the system began in 1929, and the Civil Aeronautics Administration (CAA) authorized installation of the system in 1941 at six locations. The first landing of a scheduled U.S. passenger airliner using ILS 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 ILS system.
More than one type of ILS system was tried. The system eventually adopted consisted of a course indicator (called a localizer) that showed whether the plane was to the left or right of the runway centerline, a glide path or landing beam to show if the plane was above or below the glide slope, and two marker beacons for showing the progress of approach to the landing field. 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 to visual contact with the runway. Approach lighting and other visibility equipment are part of the ILS and also aid the pilot in landing. In 2001, the ILS remains basically unchanged.
By 1945, nine CAA systems were operating and 10 additional locations were under construction. Another 50 were being installed for the army. 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 International Civil Aviation Organization (ICAO) adopted this army standard for all member countries. In the 1960s, the first ILS equipment for fully blind landings became possible.
The development of radar during World War II led to the development of a new precision-beam landing aid called ground control approach (GCA). GCA worked along with the ILS to help planes land at busy airports. By 1948, distance measuring equipment (DME) was being used to provide data relating to the plane's distance from the ground. The installation of other radar continued with the air-route surveillance type of radar and the airport-surveillance radars that were installed at a number of airports in the mid-1950s. These helped air traffic controllers with their job.
Microwave landing systems (MLS) were developed in the 1980s. These systems allow pilots to pick a path best suited to their type of aircraft and to descend and land from more directions than the ILS. Having different landing patterns can help reduce noise around airports and keep small aircraft away from the dangerous vortices behind large aircraft. MLS have been adopted in Europe as replacements for ILS. In the United States, however, the FAA halted further development of MLS in 1994. Instead, the FAA is considering the use of technology based on the global positioning system (GPS) instead of, or in addition to, existing microwave systems. The GPS uses satellites for navigation between airports and is exceedingly precise.
Lights still play an important part in landing. Modern 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.
Helicopters have used visual landing procedures for most of their history, and on June 12, 1987, the 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.
Selected Bibliography and Further Reading
Clausing, Donald J. Aviator's Guide to Navigation. Blue Ridge Summit, Pa.: Tab Books, 1992.
Illman, Paul E. The Pilot's Air Traffic Control Handbook. Blue Ridge Summit, Pa.: Tab Books, 1993.
Kershner, William K. The Student Pilot's Flight Manual. Ames, Iowa: Iowa State University Press, 1993.
Komons, Nick. Bonfires to Beacons. Washington, D.C.: Smithsonian Institution Press, 1989.
Spence, Charles F. Aeronautical Information Manual/Federal Aviation Regulations. New York: McGraw-Hill, 2000.
Whitnah, Donald R. Safer Skyways: Federal Control of Aviation, 1926-1966. Ames, Iowa: The Iowa State University Press, 1966.
FAA Historical Chronology. http://www.faa.gov/docs/A-INTRO.htm
“The Instrument Landing System.” Allstar Network. http://www.allstar.fiu.edu/aero/ILS.htm.