Use
At a controlled airport, air traffic control will direct aircraft to the localizer via assigned headings, making sure aircraft do not get too close to each other (maintain separation), but also avoiding delay as much as possible. Several aircraft can be on the ILS at the same time, several miles apart. An aircraft that has turned onto the inbound heading and is within two and a half degrees of the localizer course (half scale deflection shown by the course deviation indicator) is said to be established on the approach. Typically, an aircraft will be established by at least two miles (3 km) prior to the final approach fix (glideslope intercept at the specified altitude).
Aircraft deviation from the optimal path is indicated to the flight crew by means of a display dial (a carry over from when an analog meter movement would indicate deviation from the course line via voltages sent from the ILS receiver).
The output from the ILS receiver goes both to the display system (head-down display and head-up display, if installed) and can also go to a Flight Control Computer. An aircraft landing procedure can be either coupled, where the autopilot or Flight Control Computer directly flies the aircraft and the flight crew monitor the operation; or uncoupled where the flight crew fly the aircraft manually to keep the localizer and glideslope indicators centered.
Read more about this topic: Instrument Landing System
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