ILS approach explained simply: how aircraft find the runway safely in fog

Dense autumn fog, visibility at the fence barely reaching the next taxiway – and yet at regular intervals you hear engines approaching the runway. What makes it possible is a system that has been refined for almost a hundred years: the instrument landing system, ILS for short. If you understand how an ILS approach works, you can not only judge foggy days better but also work out at what height an aircraft passes over your own spot. This guide explains the localizer, glide slope and categories without jargon and shows what you can make of it at the spot.

ILS approach: the short answer

An ILS approach is a precision approach in which two radio signals from the ground guide the aircraft laterally and vertically to the runway. The localizer keeps the aircraft on the extended runway centerline, the glide slope (also called glide path) on a steadily descending path – usually at a 3-degree slope. In the cockpit both signals appear as a deviation display: if the markers are centered, the aircraft is exactly on course. Depending on the equipment of airport, aircraft and crew, you can fly this way down to a few dozen feet above the ground or even to touchdown without visibility.

Important: only the combination of localizer and glide slope counts as a precision approach. If the glide slope fails, what remains is a localizer-only approach with higher weather minimums.

The localizer: am I on the runway centerline?

The localizer’s antenna array is usually located beyond the end of the runway, opposite the approach side. If you have ever seen a long, flat wall of antennas across the runway centerline at an airport, that was most likely a localizer.

The principle is surprisingly simple: the transmitter emits two signals modulated with different tones – 90 Hz and 150 Hz. On one side of the centerline one tone dominates, on the other side the other. Exactly on the centerline both are equally strong. The receiver in the aircraft measures the difference in modulation depth and turns it into a display of “too far left” or “too far right”.

A few key facts you will also find again on approach charts:

• Frequency: Localizers work in the VHF aeronautical band between 108.10 and 111.95 MHz, on the channels with an odd tenths digit. The even channels in this range are reserved for VOR beacons.
• Identification: Every localizer transmits a Morse identifier, which generally begins with “I” for ILS.
• Range: Under ICAO specifications the localizer is typically usable up to about 25 nautical miles in the narrow sector around the centerline, and over shorter distances farther out to the sides.

The glide slope: am I at the right height?

The glide slope transmitter stands beside the runway, a little way past the threshold in the touchdown zone. It works on the same 90/150 Hz principle, just vertically: above the path one tone dominates, below it the other. Its UHF channel around 330 MHz is permanently paired with the localizer frequency. So the crew selects just one frequency, and the glide slope comes along automatically.

The standard angle is 3 degrees. For CAT II and III approaches, steeper angles are not provided for. Deviations exist where terrain or noise abatement demand it. The best-known example is London City Airport in the Docklands: it uses 5.5 degrees, which requires special approval of the crews and suitable aircraft types.

A technical detail pilots have to know: besides the real glide slope, physics produces false glide slopes at multiples of the angle, at around 9 degrees for 3 degrees. That is why the glide slope is intercepted from below and the height is cross-checked at fixed distances.

Markers and DME: how far is it still?

Localizer and glide slope say nothing about distance. Classically, marker beacons on 75 MHz, which radiate straight upward, did that. When the aircraft flies over a marker, a signal sounds in the cockpit:

Marker	Typical position	Tone	Meaning
Outer Marker (OM)	usually around 4 to 7 NM before the threshold	low, dashes	Start of final approach, height check
Middle Marker (MM)	around 1 km before the threshold	medium, dot-dash	Near the CAT I decision height
Inner Marker (IM)	just before the threshold	high, fast dots	no longer in use in Germany

Today markers are increasingly replaced by DME (Distance Measuring Equipment), which continuously shows the distance in the cockpit. A DME associated with the ILS usually transmits the same identifier as the localizer.

How an ILS approach works step by step

This is how a typical approach goes – whether it is an Airbus A320neo from Mallorca or a Boeing 777-300ER from Asia.

How the runway is named and why 08R becomes 26L at the other end is explained in the article Runway designations at airports. Which runway direction is approached at all is usually decided by the wind – more on this in the guide Understanding runway direction in use.

CAT I, CAT II, CAT III: the categories in poor visibility

An ILS approach’s category determines how low an aircraft may descend without sight of the runway (the decision height) and how far the runway visual range (RVR) must reach at minimum. The following values are the usual guideline figures; which minimums actually apply depends on the airport, the procedure, the aircraft and the airline’s approval.

Category	Decision height	Runway visual range (RVR)	What it means
CAT I	200 ft	from about 550 m	Standard approach, can be flown by hand
CAT II	100 to 200 ft	from about 300 m	Additional equipment and training required
CAT IIIA	under 100 ft	from about 200 m	In practice mostly autoland
CAT IIIB	under 50 ft	well under 200 m	Autoland with guidance during rollout
CAT IIIC	none	none	Theoretical, not common in scheduled operations

For an airport to offer CAT II or III, a good transmitter alone is not enough. Required are, among other things, an independent backup power supply, additional monitoring of the signals, suitable lighting on the runway and a flight crew and aircraft approved for this level. For approaches without visibility in the CAT III range, an automatic landing is generally required: the autopilot follows the localizer and glide slope down to the ground, flares and holds the centerline.

You find the weather data that decides the category in airports’ weather reports. How to read visibility, RVR and cloud base in them is shown in the article Reading METAR and TAF.

Why foggy days are so slow at the airport

ILS signals are sensitive. Anything large and metallic can reflect and distort them – a taxiing Airbus A380 in front of the localizer antenna just as much as a vehicle next to the glide slope transmitter. That is why there are protected areas (critical and sensitive areas) around the antennas, which have to stay clear in poor visibility.

For you as an observer this has visible consequences when an airport switches to low visibility procedures:

• Larger spacing on approach: The aircraft ahead must have left the protected areas before the next one enters the critical phase. The landing rate drops.
• Holding points set back: Departing aircraft wait at special CAT II/III holding points, which lie farther from the runway than in good weather.
• Delays and diversions: Because fewer aircraft per hour can land, holding patterns, delays and occasional diversions occur.

In good weather, by the way, separation depends mainly on the wake turbulence of the aircraft ahead – why a small jet behind a widebody needs more spacing is explained in the article Wake turbulence and separation.

What the approach path means for your spot

This is where it gets really practical for spotters. Because the final approach lies on a straight line at a fixed angle, you can roughly calculate the overflight height at any point on the extended runway centerline. That helps with choosing location and focal length: a spot two kilometers before the threshold delivers completely different perspectives from one right at the runway end.

A few examples from our spot profiles:

• The Zeppelinheim viewing platform at Frankfurt Airport lies south of the approach axis to south runway 25L – with a west wind the landings float past you over the last meters before touchdown.
• At the Vienna spotters’ hill landings on 16/34 pass by at the side, at the Hamburg “Affenfelsen” (Monkey Rock) you stand at the taxiway in front of runway 15/33 – two examples of how much the position relative to the runway centerline matters for the shot.
• At the Munich Airport visitors’ hill (Besucherhügel) you look from a raised position across the grounds to the two east-west runways and see the traffic more as an overview.
• The extreme is Maho Beach on Sint Maarten, where aircraft on final approach come in very low over the beach.

You can work out the approximate value for your own spot here:

Which lens fits which height is covered in more depth in the guide The right lens for plane spotting. And so that the underside of the fuselage on approach does not sink into shadow, take a look at the sun position planner article.

Spotting and following the ILS at the airport

With a bit of practice you spot the ILS installations on every airport visit:

• Localizer antenna: a long row of identical antennas across the runway centerline, behind the far end of the runway.
• Glide slope transmitter: a mast with antennas beside the runway, usually level with the touchdown zone, often with a small equipment shed next to it.
• PAPI: four lights beside the runway. From the cockpit the crew sees two white and two red when the approach angle is right; more white means too high, more red too low.

You see the approaches on flight tracker maps too: many miles before the airport, the aircraft thread onto a perfectly straight line in extension of the runway. At big airports like Frankfurt or Munich, this straight final approach often begins far out. How to read live data and what information ADS-B provides is covered in the guide Flight tracking with ADS-B. Whether and under what conditions you may listen in on aviation radio is clarified in the article Listening to aviation radio: the legal situation.

A look back and ahead

The idea is old: in the US, development of radio-based landing aids began in the late 1920s, and in 1929 the first landing by instruments alone succeeded there. In Germany the Lorenz beam system went into operation in Berlin-Tempelhof in 1932 – at first without vertical guidance. In 1938 a Boeing 247 landed in Pittsburgh in a snowstorm with ILS alone. After ICAO was founded, the ILS was established as the first international standard for precision approaches and published in Annex 10 in 1950.

Today the ILS faces competition from space: satellite-based procedures with ground-based augmentation (GBAS) are likewise standardized as a precision approach system and can in theory reach down to CAT III. In everyday operation, though, the ILS remains the workhorse for bad weather at the big airports.

Conclusion

At its core the ILS is two guidance beams: one for lateral position, one for height. Together they form an invisible 3-degree path on which aircraft find the runway even in dense fog, down to just short of it or even onto it. The categories CAT I to III determine how much visibility is needed for that, and explain why foggy days run slower at the airport. For you as a spotter, the fixed angle makes the approach predictable – height, perspective and focal length can be planned in advance.