An instrument landing system (ILS) is used to provide precision guidance to an aircraft during the approach and landing phase of a flight. The main components of an instrument landing system are a glideslope, a localizer and one or more marker beacons. Vertical guidance is provided by the glideslope, which keeps the aircraft high enough to avoid any obstacles, yet low enough for it to make a stable approach for landing. The localizer system is responsible for keeping the pilot on course laterally while marker beacons serve to notify the pilot of his distance to the runway. Marker beacons are ground-based objects along the flight path that signal the pilot when he flies over one, while some instrument landing systems also include distance measuring equipment (DME), which provides the pilot with an exact distance to the runway at all times.
Instrument landing system approaches are used primarily during times of poor visibility or low cloud cover. Even during periods of good weather, when visual flight rules (VFR) are in effect, pilots often elect to use this system to assist in maintaining a stabilized approach for landing. Every approach is assigned a frequency and is named after the runway for which it is intended. A corresponding chart, which must be in the physical possession of the pilot operating the aircraft, provides critical information about the approach.
In order to execute an instrument landing system approach during poor weather, when instrument flight rules (IFR) are in effect, pilots must be instrument rated and current on their certification. Instrument ratings are issued to licensed pilots by certified flight instructors for the purpose of navigating low visibility conditions. The instrument-rated pilot is then required to stay qualified by completing routine flights and evaluations. Each aircraft is also required to be certified for flight in IFR conditions prior to flying instrument landing system approaches. This is accomplished by furnishing the aircraft with special equipment and obtaining authorization from a certified mechanic.
Most instrument landing systems allow aircraft to descend to 200 feet (about 15 meters) above the ground before requiring visual contact with the runway. More sophisticated systems, along with advanced pilot training, allow for descent to lower altitudes. Instrument landing system approaches also allow for expedited flow of traffic into busy airports during periods of poor weather. Large airports with multiple runways often feature multiple approaches to different runways to allow for an even high capacity of air traffic at any given time.