Instrument Landing System (ILS) — Page 147, Lesson 137

Let's pick up with the DME side of the ILS, because that's where the range information really gets sharpened. I want to walk you through how a DME pairs with an ILS channel. A DME that is frequency paired with an ILS supplements or replaces the range information from markers or locators. So instead of relying only on the marker beacons for your distance checks, the DME gives you continuous range. The key point here is that the DME ranges are zero referenced to the ILS runway threshold. That means when the DME reads zero, you are exactly at the threshold of the runway, not at the DME antenna site. This is a deliberate design so your distance readout matches the runway position. Now, the protection and coverage limits. The DME is protected only within the ILS localizer service area up to 25,000 feet. That's the altitude ceiling for guaranteed service. When necessary and notified, the DME is also used for published SIDs and STARs — that's Standard Instrument Departures and Standard Terminal Arrival Routes. In such cases the DME coverage is increased beyond that 25,000-foot limit. But here's the caution: the use of a DME outside the stated limits may give rise to errors. So if you're flying outside that protected volume and you're not on a published SID or STAR, don't trust the DME reading as gospel. Now let's move to ILS identification. Separate identification is unnecessary for ILS localizer and glide path transmissions, because the localizer and glide path frequencies are paired. When you select the localizer VHF frequency, that automatically energizes the glide path receiver circuits. So you don't tune the glide path separately — it's slaved to the localizer selection. The Ident on the localizer transmission is a 2 or 3 letter Morse signal at 7 groups per minute. The first letter is usually "I". So you'll hear something like "I-ABC" in Morse, repeated at that rate. Here's a critical safety feature: the identification is automatically suppressed if the ILS becomes unserviceable or is withdrawn. So if you stop hearing the ident, that's a red flag. When an ILS is undergoing maintenance, or is radiating for test purposes only, the identification coding will either be removed completely or replaced by a continuous tone. Under these conditions, no attempt should be made to use the ILS, because completely erroneous indications may be received. That's a hard rule — no ident, no approach. There's one more scenario. In some instances, because of an unserviceable glide path, the ILS may be radiating for localizer approaches only. In that case, the identification coding will still be radiating — you'll hear the ident — but ATC will warn all users of this fact, and no attempt should be made to use the glide path. So the ident tells you the transmitter is alive, but it doesn't tell you the glide path is healthy. You rely on ATC for that warning. Now let's talk about marker beacons. Two markers are required for each installation, and a third may be added if considered necessary at a particular site. When a marker is used in conjunction with the back course of a localizer, it should have an identification signal that is clearly distinguishable from the front course markers. So you can tell by the sound which side of the runway you're on. Let me show you the radiation patterns. The radiation patterns for ILS marker beacons are vertical and appear lens shaped or bone shaped in plan view. Figures 9.2 and 9.3 show the horizontal and vertical profiles. The signal is only received if the aircraft is flying within the fan — it is not a directional aid. So the marker doesn't tell you left or right; it just tells you you're crossing that specific point along the approach path. Reception is indicated by synchronous aural identifiers and lights. Let me walk you through the three markers and their characteristics. The Outer Marker, or OM, has a BLUE cockpit light. Its ident is 2 dashes per second, modulating frequency 400 Hz, pitch LOW, and it's located at a touchdown range of 6.5 to 11.1 kilometers, which is 3.5 to 6 nautical miles. The Middle Marker, MM, has an AMBER light. Its ident is alternate dots and dashes at 3 per second, modulating frequency 1300 Hz, pitch MEDIUM, and it's at 1050 meters plus or minus 150 meters, which is 3500 feet plus or minus 500 feet. The Inner Marker, IM, has a WHITE light. Its ident is 6 dots per second, modulating frequency 3000 Hz, pitch HIGH, and it's at 75 to 450 meters, which is 250 to 1500 feet. That's the one right at the threshold area. Now, Z markers. These have cylindrical vertical radiation patterns. They're used to mark airway reporting points or co-located with an NDB — that's a Non-Directional Beacon. Because there's a cone of silence directly above an NDB, either Z markers or fan-shaped markers provide an indication when the aircraft is overhead. So the Z marker fills that gap where the NDB signal goes quiet directly above the antenna. Let me show you the marker beacon radiation patterns and the outer and middle marker profiles. So to tie it together: the DME gives you continuous range zero-referenced to the threshold, the ident tells you the transmitter is healthy, and the markers give you discrete position fixes with distinct lights and sounds. Each piece has its own limits and failure modes, and you need to respect all of them.