Ground-based components and instrument indications of an Instrument Landing System.

The Instrument Landing System (ILS) is a landing navigation system that is used only within a short distance from the airport. Its purpose is to help the pilot land the airplane. It is generally used only when visibility is limited and the pilot cannot see the airport and runway.

The system, which is ground-based, broadcasts very precise directional signals. These signals provide a lateral and vertical path to the runway to a distance of 18 nautical miles from the runway.

The ILS broadcasts marker beacons that are directed upward within a relatively narrow space. These beacons serve as checkpoints to tell the pilot the airplane's position. Some systems use three marker beacons-an outer, middle, and inner-while others use only an outer and middle beacon. These marker beacons tell the pilot that he has reached an important place along the approach path. For instance, it might tell the pilot that the plane's landing gear should be lowered.

Equipment on board the plane that allows the pilot to use the ILS consists of a glide slope receiver and a marker-beacon receiver. They show the pilot whether the airplane is to the right or left of the centerline and whether it is above or below the glide slope. The marker beacon receiver has a light display that shows when the plane passes over each marker beacon. As the plane crosses each marker beacon, the radio speaker can also broadcast a tone if the pilot has turned on this feature.

The pilot has to fly within range of the ILS in order to use it. When the pilot is approaching an airport, the FAA controller directs the pilot to where the plane will be in range of the ILS. The pilot also tunes his navigational receivers to the ILS frequency.