Instrument Landing System
See Instrument Landing System Tutorial for a tutorial on how to use ILS in FlightGear.
The Instrument Landing System (ILS) is a ground-based instrument approach system which provides precision guidance to an aircraft approaching 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.
Instrument Approach Procedure charts (or "approach plates") are published for each ILS approach, providing pilots with the needed information to fly an ILS approach during Instrument flight rules (IFR) operations, including the radio frequencies used by the ILS components or navaids and the minimum visibility requirements prescribed for the specific approach.
ILS categories
There are three categories of ILS which support similarly named categories of operation.
- Category I - A precision instrument approach and landing with a decision height not lower than 200 feet (61 m) above touchdown zone elevation and with either a visibility not less than 2,625 feet (800 m) or a runway visual range not less than 1,800 feet (550 m). An aircraft equipped with an Enhanced Flight Vision System may, under certain circumstances, continue an approach to CAT II minimums. [14 CFR Part 91.175 amendment 281]
- Category II - Category II operation: A precision instrument approach and landing with a decision height lower than 200 feet (61 m) above touchdown zone elevation but not lower than 100 feet (30 m), and a runway visual range not less than 1,150 feet (350 m).
- Category III is further subdivided
- Category III A - A precision instrument approach and landing with:
- a) a decision height lower than 100 feet (30 m) above touchdown zone elevation, or no decision height; and
- b) a runway visual range not less than 655 feet (200 m).
- Category III B - A precision instrument approach and landing with:
- a) a decision height lower than 50 feet (15 m) above touchdown zone elevation, or no decision height; and
- b) a runway visual range less than 2,625 feet (800 m) but not less than 165 feet (50 m).
- Category III C - A precision instrument approach and landing with no decision height and no runway visual range limitations. A Category III C system is capable of using an aircraft's autopilot to land the aircraft and can also provide guidance along the runway surface.
- Category III A - A precision instrument approach and landing with:
In each case a suitably equipped aircraft and appropriately qualified crew are required. For example, Cat IIIc requires a fail-operational system, along with a Landing Pilot (LP) who holds a Cat IIIc endorsement in their logbook, Cat I does not. A Head-up display which allows the pilot to perform aircraft manoeuvres rather than an automatic system is considered as fail-operational. Cat I relies only on altimeter indications for decision height, whereas Cat II and Cat III approaches use radar altimeter to determine decision height.
An ILS is required to shut down upon internal detection of a fault condition as mentioned in the monitoring section. With the increasing categories, ILS equipment is required to shut down faster since higher categories require shorter response times. For example, a Cat I localizer must shutdown within 10 seconds of detecting a fault, but a Cat III localizer must shut down in less than 2 seconds.
Definitions
glideslope
Controlled descent of an aircraft with engines (close to) idle. Often this descent equals the sinkrate, saving fuel flying as a glider. Often this descent is controlled with the autopilot.
glidepath
The descending path an aircraft has to take towards a specified point. The glide path can be taken towards the glidescope or towards a lower altitude.
glidescope
The pre-defined limited path an aircraft has to take towards a specified point. The scope is most often defined by the ILS to provide a controlled landing. The pilot will have to fly the aircraft to within the scope before the ILS can help with performing landing.