Instrument Landing System

In this article, we are going to see what this famous approach, known to all of us as the "ILS", is about....
Despite beeing in use for more than 40 years, it is still considered to be the most accurate landing system. It consists of two directions : one lateral (localizer) and the other one vertical (glideslope):
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The localizer enables the airplanes to get aligned with the runway extended centerline. In most cockpits, it is depicted with a diamond moving accross a lateral line. Once the diamond gets in the middle of the line, it means we have successfully hit the centerline so we must maintain our final heading, also known as the course of the runway











The glideslope, also installed a few meters below the threshold, runs on UHF frequencies, contrary to the localizer, which is on VHF frequencies. In combination with the localizer, the pilot is also informed about the glidepath, whether he is above or below the desired line, that means if he comes too high or too low. Of course, we can rely on the VASI lights ( the four lights near the runway touchown zone) but from a certain distance and longer we have no way of knowning, especially if there 's a thunderstorm during a night approach etc. The glideslope goes in pair with the localizer, so there 's only one frequencie in the specific runway which combines these two in order for the approach to be executed.



Marker beacons: here we have three markers, the outter, the middle and the inner one. In the panel, there is a blue color when crossing the O.M, an orange color for the M.M and a white for the I.M. They are automatically updated once we tune the frequencies. They are used mostly for reasons of distance as well as chronometring affairs.
But in most ILS approaches, there are not markers, so we have the Distance Measuring Equipment which measures our slant distance from the runway in nautical miles. In this Diagram, we can see the ILS and an aircraft on its glidepath, encentered with the line, coming for the markers (about 4nm DME)



ILS Categories : 1, 2 , 3a, 3b , 3c . Amongst these , only 3a, 3b, 3c allows the autopilot to set up for the Autoland figure.






Here we have the ILS approach chart of Venizelos International Airport of Greece in Athens, for the runway 03R. we can distinguish that our glidepath starts from 9 nm DME at 3200 ft AGL. Professional pilots study these charts carefully so as to know how to react in every possible case, such as going around.