A Confident Approach to the ILS

An Instrument Landing System (ILS) is the most commonly used guidance system for precision landings.  It enables pilots to conduct an instrument only approach to a runway in poor visibility conditions. You would find this equipment installed at nearly every major airport across the world. As such, it can also allow precise control and regulation of arriving traffic at the most heavily congested airports through timings and distance.

The ILS equipment provides signals to a receiver on board the aircraft, which in turn interprets these signals for the pilot’s flight instrumentation.  This information can be used by the pilot to complete a manual, or automatic landing (should the aircraft autopilot permit this).

The use of an ILS is a pivotal skill for a Virtual Airline Captain to master and a series of articles will be published with the aim to improve knowledge of systems and procedures, as well as build confidence in using an ILS.

Components of an ILS

We will begin the series of ILS articles by breaking the system down into its component parts to see how each operates and what it is designed to achieve.

A complete ILS system comprises of:

(a) A Localizer (LOC or LLZ) radio beam;

(b) A Glideslope (G/S) radio beam;

(c, d, e) Marker beacons; and

(f) Distance Measuring Equipment (DME).

 

The above diagram shows how all the components fit together.

Localizer

The localizer is a lateral direction radio beam situated at the far end of the runway. It provides lateral guidance to the pilot, assisting them in positioning the aircraft correctly on the extended runway centreline and then maintaining that position until touchdown (and beyond – see subsequent article on low visibility operations). The localizer can be offset from the runway centreline to allow a safe instrument approach if obstacles are present in the approach path, but a consequence of this is the reduced precision status of the procedure. Examples of these are in Innsbruck and Genoa.

Glideslope

The glideslope is a vertical direction radio beam originating from abeam the runway touchdown zone. It provides vertical guidance to the pilot, allowing them to position the aircraft on the correct vertical approach angle. These approach angles may vary, from 2.5° to 5.5°. The lower angles tend to be left over at airfields that have/used to have military operations. The higher angles are often used when terrain needs to be cleared on final approach (or obstacles such as at London City (5.5°),  or for noise sensitive areas, where a steeper glideslope keeps aircraft engines at lower power settings and at a higher altitude for more of the approach.

Distance Measuring Equipment (Marker Beacons)

Distance Measuring Equipment (DME) measures the range of the aircraft from the touchdown point by broadcasting radio beams and intercepting a response from the aircraft equipment. The on board equipment usually reads zero at the touchdown point and has a useful range out to approximately 40 nautical miles. Each runway usually has its own associated DME, but some airfields have one centrally located – this would obviously not read zero at the touchdown point.

Not all ILS’s have an associated DME. The distance to go to touchdown can be derived by timing (quite old fashioned now!), a VOR/TACAN or Marker beacons. The marker beacons operate at lower frequencies and typically denote outer, middle and inner positions. A pilot would cross check their altitude as they pass over each marker beacon to ensure the correct approach profile is being flown. The beacons also trigger lights and sounds in some cockpits as they’re passed over. These are blue for the outer marker with slow beeps, orange for the middle marker with beeps at a faster interval and lastly, white for the inner parker with rapid high pitch beeps.

In the next article, we’ll look at the ILS instrumentation in the aircraft and how to interpret the displays.