Definition

Instrument Landing System (ILS) is defined as a precision runway approach aid based on two radio beams which together provide pilots with both vertical and horizontal guidance during an approach to land.

Description

An Instrument Landing System is a precision runway approach aid employing two radio beams to provide pilots with vertical and horizontal guidance during the landing approach. The localiser (LOC) provides azimuth guidance, while the glideslope (GS) defines the correct vertical descent profile. Marker beacons and high intensity runways lights may also be provided as aids to the use of an ILS, although the former are more likely nowadays to have been replaced by a DME integral to the ILS or one otherwise located on the aerodrome, for example with a VOR.

The ILS LOC aerials are normally located at the end of the runway; they transmit two narrow intersecting beams, one slightly to the right of the runway centreline, the other slightly to the left which, where they intersect, define the "on LOC" indication (see Figure 1). Airborne equipment provides information to the pilot showing the aircraft’s displacement from the runway centreline.

The ILS GS aerials are normally located on the aerodrome; they transmit two narrow intersecting beams, one slightly below the required vertical profile and the other slightly above it which, where they intersect, define the "on GS" indication (see Figure 2). Aircraft equipment indicates the displacement of the aircraft above or below the GS. The GS aerials are usually located so that the glide-slope provides a runway threshold crossing height of about 50 ft. The usual GS angle is 3 degrees but exceptions may occur, usually to meet particular approach constraints such as terrain or noise abatement.

If marker beacons are provided, they will be located on the ILS approach track at notified distances from touch-down (see Figure 2). Typically, the first marker beacon (the Outer Marker) would be located about 5 NM from touch-down while the second marker beacon (the Middle Marker) would be located about 1 NM from touch-down.

An approach may not normally be continued unless the runway visual range (RVR) is above the specified minimum. When an approach is flown, the pilot follows the ILS guidance until the decision height (DH) is reached. At the DH, the approach may only be continued if the specified visual reference is available, otherwise, a go-around must be flown.

Special categories of ILS approach are defined which allow suitably qualified pilots flying suitably equipped aircraft to suitably equipped runways using appropriately qualified ILS systems to continue an ILS approach without acquiring visual reference to a lower DH than the Category I standard of 200 feet above runway threshold elevation (arte) and do so when a lower reported RVR than the 550 metres usually associated with Category I:

• Category II permits a DH of not lower than 100 ft and an RVR not less than 300 m;
• Category IIIA permits a DH below 100 ft and an RVR not below 200 m;
• Category IIIB permits a DH below 50 ft and an RVR not less than 50 m;
• Category IIIC is a full auto-land with roll out guidance along the runway centreline and no DH or RVR limitations apply. This Category is not currently available routinely primarily because of problems which arise with ground manoeuvring after landing.

The special conditions which apply for Category II and III ILS operation cover aircraft equipment; pilot training and the airfield installations. In the latter case, both function, reliability and operating procedures are involved. An example of the latter is the designation of runway holding points displaced further back from the runway so as to ensure that aircraft on the ground do not interfere with signal propagation. Reliability requirements for Category II and III ILS include a secondary electrical power supply which should be fully independent of the primary one.

The transmission of ILS signals is continuously monitored for signal integrity and an installation is automatically switched off leading to the immediate display of inoperative flags on aircraft ILS displays selected to the corresponding frequency if any anomaly is detected. The reliability of this monitoring function is increased where approaches to minima lower than Category I are permitted and all ILS systems are subject to regular calibration flights to check that signals are being correctly transmitted. These checks only validate that the ILS is performing as intended and do not routinely investigate the indications which aircraft would receive if flown beyond signal validity.

It is very important to note that only a full ILS with LOC and GS signals is a precision approach. If only the LOC is transmitting then it can only support a Non-Precision Approach with increased minima, albeit this should be a lower minima than an equivalent VOR would enable.

Validity of ILS Guidance

An ILS is only valid if used within strict boundaries either side of the transmitted LOC and GS beams as documented on the corresponding AIPs Instrument Approach Procedure (IAP). From a pilot perspective, these limits are defined as Full Scale Deflection (FSD) of the deviation indication on the ILS displays in the flight deck, since once the deviation in respect of either the LOC or GS reaches FSD, it becomes impossible to know the extent of the deviation.

Because of this, pilots navigating their aircraft onto an ILS, whether from below the GS or above, have always been expected, when acquiring an ILS GS, to cross-check their range from touchdown against their indicated altitude/height and confirm that their aircraft is on the promulgated IAP GS.

False Glideslope Acquisition

An issue with ILS is that secondary glide slopes appear above the primary one. This is caused by the radiation pattern of the antenna and the ground reflection of some of the transmitted energy. The false glide slopes appear at odd multiples of the true glide-slope angle (e.g. if the main GS is at 3°, then the secondary slopes will be at 9°, 15° and 21°). Capturing a false GS can lead to unexpected aircraft behavior, e.g. AP disconnect, unexpected engagement of GS acquisition mode, excessive pitch down or climb. This may ultimately result in an unstabilised approach and the need for a go around. The unexpected behaviour is often due to a combination of factors - the existance of false GS, improper aircraft automation settings (e.g. AP) and loss of crew situational awareness (e.g. being unaware that the aircraft level does not correspond to the distance to touchdown). An altitude vs distance check (to determine that the aircraft is below the GS upper boundary) and strict adherence to SOPs can mitigate the risk. 

Examples of occurrences that happened because of false GS acquisition include:

• A318, vicinity Toulon-Hyères France, 2019
• B738, vicinity Eindhoven Netherlands, 2013
• A343, vicinity Paris CDG France, 2012

More information on this subject, including technical details and crew recommended actions can be found in a dedicated article by Airbus.