Autopilot — Page 373, Lesson 451

I want to walk you through the VOR and localizer tracking modes of the autopilot, and then we'll get into a really important operational concept called the "cone of confusion." Let's start with a quick note on the HDG SEL mode, because it sets up the logic for everything else. The heading select mode automatically disengages — it switches itself off — the moment the autopilot captures the selected radio course in either the VOR LOC mode or the APP mode. So think of it this way: you're flying on a heading, the autopilot is steering to a compass bearing, and the moment it locks onto the radio beam, it hands control over to the radio tracking. That's the handoff. Now, the VOR Localizer Tracking mode. There are actually two distinct modes bundled under that VOR LOC switch, and the autopilot decides which one to use based on what frequency you've tuned. The VOR mode gives roll commands to capture and track the selected VOR course. That's a VOR radial you've dialled in. The LOC mode gives roll commands to capture and track the selected localizer — that's the ILS localizer — but here's the critical limitation: it tracks along the inbound front course bearing only. Back-course tracking is not available. So if you're on the back side of the localizer, this mode will not track it. That's a hard limitation you need to remember. How do you select it? You press the VOR LOC switch. If a VOR frequency is tuned, pressing it selects VOR mode. If a localizer frequency is tuned, pressing it selects LOC mode. When you press it, the switch illuminates — the light comes on in the button — and the annunciation "VOR LOC armed" is displayed. That's your cue that the mode is standing by, ready to capture. Now, the intercept logic. The selected course can be intercepted while you're engaged in L NAV, HDG SEL, or CWS ROLL — that's control wheel steering in roll — and this all requires an autopilot engaged in CMD, command mode. So the autopilot has to be actively flying. Here's the key point about the capture point: it is variable. It's not a fixed distance. It depends on two things — the intercept angle and the closure rate. The steeper your intercept angle and the faster you're closing on the beam, the earlier the autopilot will start its capture manoeuvre. There's a hard limit though: localizer capture occurs not later than ½ dot deviation. That's half a dot on the course deviation indicator. So no matter what the intercept geometry is doing, the autopilot will not let the needle get past half a dot before it starts capturing. When you're within the course capture area, the annunciation changes. It goes from "armed" to "captured." And from that moment, the roll commands track the VOR or localizer course. The autopilot is now steering along the beam. Now let's get into the cone of confusion, because this is where the system has to handle a real-world problem. The cone of confusion is an area directly overhead a VOR navigation beacon where the signals are unusable. Think about the geometry — the VOR transmits radials that spread out from the station. Directly above the station, those radials converge to a point, and the signals become ambiguous. So an aircraft transiting the VOR — flying over the top of it — will receive no usable signals for a period of time. And that period depends on two things: your ground speed and your altitude. Higher altitude means a bigger cone, because the radials have spread further apart by the time they reach you. Faster ground speed means you transit it quicker. Here's what happens as you approach. The radials are converging — they're getting closer together as you near the station — and the course deviation indicator becomes more sensitive. The needle starts swinging more aggressively for the same lateral displacement. At some point, before you even enter the cone of confusion, the information from the selected inbound radial becomes unusable, purely because of that convergence. The signals are too compressed to be meaningful. At that point, the VOR signals are "cut off" by what's called the "over station sensing" circuits. This is the clever part. The roll channel automatically de-couples from the radio beam — it lets go of the beam — and controls the aircraft through the cone of confusion on the drift-corrected heading that existed at the moment the signals were cut off. So the autopilot remembers the heading it was holding, corrected for drift, and flies straight through the cone on that heading. It's not chasing a needle that's gone wild. It's holding a steady course until it emerges on the other side, where the signals become usable again and it can re-capture the radial. That's the full picture of VOR LOC tracking and the cone of confusion. The takeaway is the logic: the autopilot knows when the beam is unusable, it cuts off the radio input, and it flies a remembered drift-corrected heading through the dead zone.