Instrument Landing System (ILS) — Page 163, Lesson 156

Right, let's pick this up. We've covered the ILS ground equipment and the signals themselves. Now I want to walk you through the operational side — the protection and the procedure that surrounds an ILS approach. First, the ILS Sensitive Area. This is a defined zone that extends beyond the critical area. Its purpose is to control the parking or movement of vehicles and aircraft so that we prevent the possibility of unacceptable interference to the ILS signal during low visibility operations. The key point here is that the dimensions of this area are not fixed — they depend upon the object creating the disturbance. So a large aircraft or a big vehicle will create a larger sensitive area than a small one. Next, holding points. During Category II and Category III operations — that's the very low visibility stuff — the protection of the ILS signals may dictate that pre-take-off holding points are more distant from the runway than the holding positions used in good weather. These holding positions will be appropriately marked, and they will display signs reading 'Category II/III Hold'. There may also be a bar of red stop lights at that position. So if you see that sign and those red lights, you hold short of that point, not the normal holding point. Now, weather. Snow and heavy rain attenuate the ILS signals. That means they weaken the signal, which reduces the range and degrades the accuracy of the system. So in poor weather, the system doesn't perform as well. Then there's FM Broadcasts. FM transmitters have wide bandwidths. It's possible for such stations transmitting on frequencies just below 108 MHz to produce frequencies that overspill into the radio navigation band — that's 108 to 117.975 MHz — thereby causing interference with the ILS signals. Since the late 1990s, FM suppression circuits have been mandatory in ILS receivers to counter this. Finally, let's look at the ILS Approach Chart. An Instrument Approach Chart for an ILS approach is shown in Figure 9.14. The instrument approach can be divided into three segments. The initial approach is the procedure up to the IAF — that's the initial approach fix. The intermediate segment is the procedure between the IAF and the FAF — the final approach fix. And the final approach is the procedure after the FAF. An aircraft should be at or above certain altitudes depending upon the sector from which it is approaching. These are known as sector safety altitudes, or SSA, and they are denoted in some form on the chart — circular in the top left on this one. And finally, landing minima relates to the pilot's decision height, the DH, and the RVR — that's runway visual range. Before commencing the approach, the pilot would normally be advised by ATC to check his landing minima. So to tie it together: the sensitive area and Category II/III holding points protect the signal integrity, weather and FM broadcasts are the threats to that signal, and the approach chart lays out the three segments, the sector safety altitudes, and the landing minima you must check before you start down.