An Instrument Landing System (ILS) provides precision guidance during approach and landing. It includes a glideslope, localizer, marker beacons, and DME. ILS is used in low visibility and requires instrument qualification for pilots and certification for aircraft. It allows for accelerated traffic flow and multiple approaches to different runways.
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, but low enough to make a stable landing approach. The localizer system is responsible for keeping the pilot on course laterally while the marker beacons serve to notify the pilot of his distance from the runway. Marker beacons are ground objects along the flight path that indicate to the pilot when he is flying over one, while some instrument landing systems also include distance measuring equipment (DME), which provides the pilot with an exact distance on the track at all times.
Instrument landing system approaches are used primarily at times of low visibility or low cloud cover. Even during periods of good weather, when Visual Flight Rules (VFR) are in effect, pilots often choose to use this system to help maintain a stabilized approach for landing. Each approach is assigned a frequency and is named after the runway it is intended for. A corresponding box, which must be in the physical possession of the pilot operating the aircraft, provides critical approach information.
To execute an instrument landing system approach in severe weather conditions, when instrument flight rules (IFR) are in effect, pilots must be instrument qualified and current in their certification. Instrument ratings are issued to licensed pilots by certified flight instructors for the purpose of navigation in low visibility conditions. The instrument qualified pilot is then required to maintain qualification by completing routine flights and evaluations. Each aircraft is also required to be certified to fly in IFR conditions before approaching the flight instrument landing system. This is accomplished by providing 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 runway visual contact. More sophisticated systems, along with advanced pilot training, allow descent to lower altitudes. Instrument landing system approaches also allow for accelerated traffic flow to busy airports during periods of bad weather. Large multi-runway airports often feature multiple approaches to different runways to allow for even high air traffic capacity at any one time.
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