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

Let’s pick up right where the ILS ground equipment left off and move into the operational side of the system. I want to walk you through the protection areas, the environmental threats, and then how we actually read an ILS approach chart. First, the ILS Sensitive Area. This is a defined zone that extends beyond the critical area. The critical area, as you’ll recall, is the zone closest to the antennas where nothing may enter during an approach. The sensitive area is larger, and here the rule is different: parking or movement of vehicles and aircraft is controlled — not absolutely forbidden, but managed — to prevent the possibility of unacceptable interference to the ILS signal during low visibility ILS operations. The key point is that the dimensions of this sensitive area are not fixed; they depend upon the object creating the disturbance. A large aircraft parked nearby will cast a bigger shadow than a small vehicle, so the protected zone grows accordingly. Next, holding points. During Category II and III operations — that is, very low visibility approaches — the protection of ILS signals may dictate that pre-take-off holding points are placed more distant from the runway than the holding positions used in good weather. These holding positions will be appropriately marked and will display signs reading ‘Category II/III Hold’; there may also be a bar of red stop lights. So if you see that sign and those red lights, you know you are holding further back specifically to keep your aircraft’s mass from corrupting the localizer or glide path signals. Now, the environmental threats. Weather: snow and heavy rain attenuate the ILS signals. Attenuation means the signal loses energy as it passes through the precipitation, which reduces the range and degrades the accuracy. So in a snowstorm, the usable coverage shrinks and the beam you’re tracking becomes less precise. Then FM broadcasts. FM transmitters have wide bandwidths. It is possible for such stations transmitting on frequencies just below 108 MHz to produce frequencies that overspill into the radio navigation band, which runs from 108 to 117.975 MHz. That overspill causes interference with the ILS signals. Since the late 1990s, FM suppression circuits have been mandatory in ILS receivers — so the receiver itself is designed to reject that out-of-band energy. Now let’s turn to 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: - Initial approach — the procedure up to the IAF, the initial approach fix. - Intermediate — the procedure between the IAF and the FAF, the final approach fix. - Final approach — the procedure after the FAF. So the IAF is where you join the procedure, the FAF is where you begin the final descent toward the runway, and everything after that is the final approach segment. 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, abbreviated SSA, and they are denoted in some form on the chart — on this particular chart, they appear as a circular notation in the top left. Finally, landing minima relates to the pilot’s decision height, or DH, and the RVR, the runway visual range. Before commencing the approach, the pilot would normally be advised by ATC to check his landing minima. So you verify that the reported RVR and your decision height meet the published minima for that approach before you commit. That’s the operational picture: the protected areas on the ground, the weather and FM threats to the signal, and how the approach chart lays out the segments, the sector safety altitudes, and the landing minima you must check before you start down.