How does the Instrument Landing System (ILS) work?

Landing an aircraft in low-visibility conditions, while flying through thick fog layers and clouds that compromise the view on the runway. How do pilots do that? One part of the answer is the so called “instrument landing system (ILS)”. This system enables the flight deck crew to locate the runway, even in bad weather conditions. The instrument landing system makes use of a lateral and a vertical guidance, to lead the aircraft down to the runway. The localizer (LOC) serves as a horizontal guidance, while the glideslope acts as a vertical guidance.

Instrument Landing System – Localizer and Glideslope!

Instrument landing at Nuremberg, Germany, using the ILS!

The localizer antenna arrey is located exactly 300m behind the runway. This system transmits two signals to the left and the right of the runway in different frequencies. A third frequency can then be tuned in the flight deck. The first two frequencies, sent by the localizer, superimpose each other. The aircraft is on the ideal course to the runway, if the difference of those two localizer frequencies is equal to zero. The board computer now has the task to “find” this zero. The flight deck’s primary flight display (PFD) can then show any deviation from the ideal flight path.

The instrument landing system’s glideslope works in a very similar way. The glideslope employs a 90-hertz signal as a “too low” indicator and a 150-hertz signal as a “too high” indicator. The aircraft is on the ideal glidepath, if it flies right in the middle of those two signals. This path usually leads down to the runway in a 3° angle. At some airports, however, this angle can be as high as 5.5°, for instance at London City Airport.  Of course, the PFD is also able to visually show the glidepath to the pilots. The instrument landing system benefits the pilots in bad weather, since the autopilot of modern airplanes can use it to automatically land the aircraft!

MINIMUM – The Decision Height!

We have learned quite a bit about the instrument landing system. Now I want to explain the so called decision height to you. There is a time in every final approach, when the pilot has to decide wether to continue or abort the landing. This point in time is the decision height. This decision height is predtermined by some factors like the outside visibility. But more to that later on!



If the runway is not in sight when the pilots reach the decision height, or some other factor makes a safe landing impossible, the pilots aboirt the landing and initiate a go-around. Those factors could be, for instance, another aircraft not vacating the runway, or a grizzly bear enjoying the winter sun on the threshold. Joking apart: Let’s continue.

Reaching decision height at Cologne Bonn Airport, Germany!

CAT I, CAT II and CAT III!

Beside the decision height, there is another altitude relevant in an instrument landing system, namely the so called “decision altitude”. The classic barometric altimeter measures this altitude. Pilots only use this altitude in ILS-CAT I approaches. But what are those instrument landing system categories all about? CAT I, II and III are ILS categories and determine the decision height. Also, the ILS category indicates until what visibility conditions an approach is allowed.

This is quite relevant, since not all airports have the equipment for an instrument landing of the category II or III, due to the high cost of setting up and maintaining such a system.

• CAT I: At CAT I, the decision height lies 200ft above the ground.  CAT I requires a “runway visual range (RVR)” of more than 550m.  If the RVR is below 550m and the airport has no instrument landing system of the category II or III, the pilots have to abort the landing. Good, that there are alternate airports, right? 😉
• CAT II: Let’s go right down to the business. CAT II requires a RVR of at least 300m. The decision height lies between 100-200ft. If the visibility range is worse than 300m, an ILS landing of the category III is possible. For safety reasons, one mostly performs CAT III landings automatically with the autopilot’s help.
• CAT IIIa: CATIIIa requires a RVR of at least 175m and has a decision altitude of 0-100ft above the ground.
• CAT IIIb: Now the fog gets really thick! CATIII requires a RVR of at least 50m and a maximum of 175m. The decision height lies at 0-50ft.
• CAT IIIc: This category is not yet permitted. CATIIIc would, however, make landings with no visibility without any decision height possible.

Can a plane land automatically?

Here you can see both of the A320’s autopilots work!

Well, of course! As I said before, an automatic landing is even mandatory in some instances. For a successful automatic landing of the category one, a single autopilot equipped with a “fail-passive” system is sufficient. If this autopilot fails, the pilot flying still can take over control of the plane manually.

Instrument landings of higher categories get slightly more difficult. The require a “fail-operational” system, since the pilot cannot just take over the control of the plane easily in the case of an autopilot failure, due to the lack of visibility. In a fail-operational system, multiple autopilots work together and control each other, by calculating the control inputs independently from each other and by comparing the own data with the other autopilot’s data.

If one autopilot fails, the other can still fly the aircraft perfectly. The following video shall show a CATIII landing at Amsterdam airpor, with a visibility of just 125 metres. Of course, pilots cannot just lean back and enjoy some coffee, while the autopilots land. An “autoland” requires the crew’s attention and the pilots have to monitor the instruments constantly.

More aviation know-how!

I do hope that you have enjoyed this article about the instrument landing system. More aviation know-how will come every week on pilotstories. In the meantime, feel free to subscribe my email newsletter and to follow me on Facebok and Instagram.

Aaron