How Is an Instrument Landing System Used?
An instrument Landing System is used by a pilot in bad weather to bring down an aircraft in perfect line with an airport’s runway. It enables aircraft to land if the pilots are unable to establish visual contact with the runway. It does this by way of transmitted radio signals. Two radio beams are sent out by a ground transmitter.
The “localizer” is a narrow, upright beam which positions the aircraft in line with the runway. The “glide path” is a thin, flat beam which gives the aircraft a perfectly angled approach path to the end of the runway.
In the case of large, busy airports such as London’s Heathrow, a pilot is in the close contact with Air Traffic Control on his approach to the airport for, at peak periods, aircraft can be landing at 45 second intervals. The Ground Control operator tells him to “lock on”, a phrase used to tell the pilot to switch on his Instrument Landing System.
The pilot informs Control that he is “established”, which means that he has locked on to the two radio beams and is keeping them crossed. The point at which the beams intersect is the spot on the runway where the aircraft must land.
An instrument landing system operates as a ground-based instrument approach system that provides precision lateral and vertical guidance to an aircraft approaching and landing on a runway, using a combination of radio signals and, in many cases, high-intensity lighting arrays to enable a safe landing during instrument meteorological conditions (IMC), such as low ceilings or reduced visibility due to fog, rain, or blowing snow.
An instrument approach procedure chart (or ‘approach plate’) is published for each ILS approach to provide the information needed to fly an ILS approach during instrument flight rules (IFR) operations. A chart includes the radio frequencies used by the ILS components or navaids and the prescribed minimum visibility requirements.
Radio-navigation aids must provide a certain accuracy (set by international standards of CAST/ICAO); to ensure this is the case, flight inspection organizations periodically check critical parameters with properly equipped aircraft to calibrate and certify ILS precision.
An aircraft approaching a runway is guided by the ILS receivers in the aircraft by performing modulation depth comparisons. Many aircraft can route signals into the autopilot to fly the approach automatically. An ILS consists of two independent sub-systems. The localizer provides lateral guidance; the glide slope provides vertical guidance.
At a controlled airport, air traffic control will direct aircraft to the localizer course 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 is said to be established on the approach. Typically, an aircraft is established by at least 2 nautical miles (3.7 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 carryover from when an analog meter movement indicated deviation from the course line via voltages sent from the ILS receiver).
The output from the ILS receiver goes to the display system (head-down display and head-up display if installed) and may 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 monitors the operation, or uncoupled where the flight crew flies the aircraft manually to keep the localizer and glideslope indicators centered.