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

Let’s pick up with the ILS chapter and go straight into the DME pairing, because that’s the first new fact I want you to lock in. A DME that is frequency paired with an ILS supplements or replaces the range information you’d normally get from markers or locators. So think of it this way: the DME isn’t just a standalone distance tool — when it’s paired with an ILS channel, it becomes part of the approach picture, giving you range to the runway. And here’s the critical detail: the DME ranges are zero referenced to the ILS runway threshold. That means when the DME reads zero, you are at the threshold, not at the DME transmitter site. That’s a big deal for your mental picture on approach. Now, the protection volume. The DME is protected only within the ILS localizer service area, and that protection extends up to 25,000 feet. So if you’re flying within that localizer service area and below 25,000 feet, the DME signal is guaranteed to be there. But 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 those cases, the DME coverage is increased beyond that normal limit. And here’s the warning I want you to remember: using the DME outside the stated limits may give rise to errors. So don’t assume the DME is valid just because you can hear it — if you’re outside the protected volume, the range information can be wrong. Now let’s move to ILS identification, because this is where a lot of pilots get confused. Separate identification is unnecessary for the 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 comes along with 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 typically hear something like “I-AA” or “I-BB” — that leading “I” tells you it’s an ILS. Now, the identification is automatically suppressed if the ILS becomes unserviceable or is withdrawn. So if the system goes down, the Morse ident stops. But here’s a subtle and important case. 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 those conditions, no attempt should be made to use the ILS, because completely erroneous indications may be received. So a continuous tone instead of the Morse ident is your red flag — the system is not for navigation. 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 — so you’ll hear the normal Morse ident. But ATC will warn all users of this fact, and no attempt should be made to use the glide path. So even though the ident is normal, the glide path is not available, and you must not fly it. 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. So the minimum is two, and a third is optional depending on the site. Here’s a specific rule: 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 if you’re flying the back course, the marker ident must be different enough that you can’t confuse it with the front course markers. Now, the radiation patterns. The radiation patterns for ILS marker beacons are vertical, and they appear lens shaped or bone shaped in plan view. So if you look down from above, the pattern looks like a lens or a bone. The signal is only received if the aircraft is flying within that fan — it is not a directional aid. That’s a key point: the marker doesn’t tell you left or right, it just tells you you’re passing through that vertical fan of energy. Reception is indicated by synchronous aural identifiers and lights. Let me walk you through the three markers and their characteristics, because you need these numbers cold. The Outer Marker, or OM, has a cockpit light that is BLUE. Its ident is 2 dashes per second, modulating at 400 Hz, with a low pitch. Its touchdown range is 6.5 to 11.1 kilometers, which is 3.5 to 6 nautical miles. So the outer marker is your first fix on the approach, typically 4 to 7 miles from touchdown. The Middle Marker, or MM, has an AMBER cockpit light. Its ident is alternate dots and dashes at 3 per second — so you hear dot-dash-dot-dash. It modulates at 1300 Hz, with a medium pitch. Its touchdown range is 1050 meters plus or minus 150 meters, which is 3500 feet plus or minus 500 feet. So the middle marker is much closer in, right around the decision point. The Inner Marker, or IM, has a WHITE cockpit light. Its ident is 6 dots per second, modulating at 3000 Hz, with a high pitch. Its touchdown range is 75 to 450 meters, which is 250 to 1500 feet. The inner marker is only used on Category II and III approaches, right at the threshold. Now, one more type of marker: Z markers. Z markers have cylindrical vertical radiation patterns. They are used to mark airway reporting points, or they can be co-located with an NDB — that’s a Non-Directional Beacon. Here’s the reason: due to the cone of silence directly above an NDB, either Z markers or fan-shaped markers provide an indication when the aircraft is overhead. So the NDB has a dead zone directly above it, and the Z marker fills that gap so you know when you’re overhead. Let me show you the radiation patterns so you can see the lens shape and the vertical profile. So to tie it together: the DME gives you range zeroed at the threshold, the ident tells you the system is healthy, and the markers give you discrete fixes along the approach path. Each marker has its own light color, its own Morse ident, its own modulating frequency, and its own pitch — and you need to recognize all of them instantly in the cockpit.