An instrument landing system (ILS) is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway.
An instrument landing system (ILS) is a ground-based instrument approach system that provides precision guidance (thus making it a precision-approach) 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 aircraft performing an ILS approach (colloquially known as "shooting" the approach") is, at least initially, guided by two sets of radio beam transmitters:
- The localizer, mounted slightly beyond the far end of the runway being approached, provides lateral guidance, hopefully keeping the aircraft from flying off to the side of the runway; it can also be used on its own for guidance during a nonprecision approach.
- The glideslope transmitters are mounted next to the sides of the runway, usually ~1,000 feet past the threshold. They provide vertical guidance, guiding the aircraft down a safe descent profile to the runway and (ideally) keeping it from hitting the ground before reaching the runway.
It is also possible to have a localizer and glideslope that do not lead directly to the runway itself (for instance, if high terrain in the vicinity of the airport makes a safe straight-in approach path impossible) but, rather, guide the aircraft down to a point reasonably close to the runway, from where the pilot can fly visually to a landing. This is known as an instrument guidance system (IGS); it is also sometimes called a localizer-type directional aid (LDA), but the latter designation more properly refers to a type of approach with only localizer guidance, and no glideslope. IGS approaches are rare, as they are more complicated and difficult (sometimes much more) to fly than a bog-standard ILS approach, and most airports do not have the severe terrain issues that require the use of an IGS.
With most ILS approaches (and all IGS approaches), the pilot has to make visual contact with the runway, or the approach structures thereto, at or before a certain point in the approach, and then fly down visually to the runway; if the visibility is too low for the pilot to be able to do this, a missed-approach is required. To help with this, ILS-equipped runways generally have extensive airport-lighting along the approach path leading up to the runway (the lower the visibility for which the runway is certified, the more extensive its approach lighting).
This requirement comes about because even a good human pilot is - not to put too fine of a point on it - somewhat imperfect at manually guiding their airplane precisely along the ILS approach path in the absence of external visual cues. A manually-flown ILS approach will generally approximate this ideal path fairly well, but still exhibits small deviations that create a slightly "bumpy", uneven descent path; these small deviations could be fatal if they occurred very close to the ground. To remedy this, British researchers (Great Britain being famous for its frequent, dense, opaque, aviation-impairing fogs) worked from the 1940s through the early 1960s on a system to allow the aircraft's autopilot to fly the aircraft down the ILS beams more precisely and accurately than any human pilot could hope to do - an autoland system. The first airliner with autoland capability was the hawker-siddeley-trident, which entered service in 1964; autoland systems are ubiquitous on modern large airliners, and utterly essential for operations in extremely low visibility (where the pilot might well be unable to make visual contact until the aircraft touches down, or even later). Some autoland systems are even (theoretically) capable of guiding the aircraft through a safe touchdown and rollout in literal completely zero visibility, although no airports are, as of yet, equipped for this (as making effective use of this capability would require some way to safely taxi in zero-zero visibility - ironically, a much more difficult problem to solve than that of merely landing in these conditions!).
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.
ILS was to be replaced by the microwave-landing-system (MLS), developed in the 1970s and 1980s, which required far less in the way of equipment and was far more flexible than ILS, but, by the time MLS was ready for widespread deployment, GPS (the first gnss) had come on the scene for civilian use. Since then WAAS approaches saw widespread use in USA for airports where an ILS would have been expensive, as well as LAAS/GBAS/GLS approaches in select major airports worldwide. (Note that GLS is not RNAV.)