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The ILS approach explained

How the Instrument Landing System guides an aircraft down to the runway — and how you fly arrivals onto it in the simulator.

The Instrument Landing System (ILS) is the most common precision approach in the world. It lets an aircraft find the runway and descend to it safely even when the crew cannot see the ground until the last few hundred feet. As an approach controller, most of your arrivals will finish on an ILS, so understanding how it works is central to the job.

Two beams: localizer and glideslope

An ILS is made of two radio beams. The localizer is a beam aligned with the runway centreline that tells the aircraft whether it is left or right of the extended centreline — it provides lateral guidance. The glideslope (or glide path) is an angled beam, normally 3°, that tells the aircraft whether it is high or low on the correct descent path — it provides vertical guidance. Where the two beams cross is the exact path down to the touchdown zone.

How the controller sets it up

You don't fly the ILS for the pilot — you deliver the aircraft to it. Using radar vectors you turn the arrival onto a heading that will intercept the localizer at a shallow angle (usually 30° or less), a comfortable distance from the runway, and at an appropriate altitude just below the glideslope. Once the aircraft is established, you clear it for the approach; it captures the localizer, then intercepts the glideslope from below and follows it down. This is exactly how it works in the ATC Radar Simulator: vector the aircraft toward the extended centreline, issue the approach clearance, and watch it capture and track the beams automatically.

Categories and minima

ILS approaches come in categories — CAT I, II and III — that define how low the aircraft may descend before the crew must see the runway (the decision height, DH) and how much runway visual range (RVR) they need. CAT II and III go progressively lower, down to near-zero visibility auto-lands, and require more capable aircraft, crews and ground equipment. The ICAO definitions:

The minima an airline actually uses are approved for each aircraft type, crew and runway, and are often higher than these. Runways in the simulator are marked with their approach type so you know which finals are precision ILS and which are RNP.

When there is no ILS

Not every runway has an ILS. Many now use RNP or RNAV (GNSS) approaches, which use satellite navigation and onboard computers to fly a very similar final path without a ground localizer or glideslope. From the controller's point of view the technique is much the same: build a well-spaced stream, turn each aircraft onto final, and clear it for the approach.

Spacing on final

The real skill is the sequence. Aircraft must arrive on final evenly spaced — at least 3 NM apart, and more behind a heavier aircraft because of wake turbulence. You manage this with headings (lengthening or shortening each aircraft's path) and speed control, so that when they roll onto the ILS the gaps are already right and the runway can accept a steady flow.

Quick answers

What is an ILS approach?

A precision instrument approach in which two radio beams from the ground — the localizer for left and right, the glideslope for up and down — guide the aircraft along the exact path to the runway, so it can land when the crew cannot see the ground until late.

What is the glideslope angle of an ILS?

Normally 3°, which works out at roughly 300 ft of descent for every nautical mile. A few airports need steeper paths for terrain or noise — London City's is 5.5°.

What is the difference between CAT I, CAT II and CAT III?

How low the aircraft may go before the crew must see the runway, and how little visibility they may land in — see the table above. CAT I stops at 200 ft; CAT III allows automatic landings in fog.

Can I practise ILS approaches online?

Yes — as the controller. In the free ATC Radar Simulator you vector arrivals onto the localizer, clear them for the ILS and watch them capture the beams, at real airports with real ILS runways.

Ready to try it? Open the simulator, pick an airport with ILS runways, and turn your first arrival onto final. New to the wider picture? Start with the guide to approach radar control.