Autopilot — Page 373, Lesson 451

Let's pick this up right where the autopilot's lateral modes get interesting. We've already covered the basics of heading select, so now I want to walk you through what happens when the autopilot stops flying a heading and starts flying a radio beam. First, one critical rule about the HDG SEL mode: it automatically disengages the moment the autopilot captures the selected radio course in either the VOR LOC or the APP modes. So if you're flying heading select and you intercept the localizer, the autopilot hands control over to the radio navigation — you don't have to do anything, it just switches. Now let's look at the VOR Localizer Tracking mode, which we call VOR LOC. This is actually two separate modes that share one switch. The VOR mode gives roll commands to capture and track the selected VOR course. The LOC mode gives roll commands to capture and track the selected localizer, but only along the inbound front course bearing. And here's a limitation you must remember: back-course tracking is not available. You cannot use this mode to track the localizer from the back side. When you press the VOR LOC switch, the autopilot decides which mode to enter based on what frequency is tuned. If a VOR frequency is tuned, it selects the VOR mode. If a localizer frequency is tuned, it selects the LOC mode. The switch itself illuminates, and the annunciation "VOR LOC armed" appears on the flight mode annunciator. Now, here's the key part about intercepting. The selected course can be intercepted while you're engaged in L NAV, HDG SEL, or CWS ROLL — that's Control Wheel Steering in the roll axis — and this requires an autopilot engaged in CMD, the command mode. The capture point is not fixed; it's variable, and it depends on two things: the intercept angle and the closure rate. That makes sense — if you're crossing the beam at a steep angle and closing fast, you'll capture it differently than if you're creeping in at a shallow angle. There's a hard limit on localizer capture: it occurs not later than ½ dot deviation. That's half a dot on the course deviation indicator — the autopilot will not let the localizer slip past half a dot before it captures. When you're within the course capture area, the annunciation changes from "armed" to "captured," and from that moment the roll commands track the VOR or localizer course. Now I want to talk about something that trips up a lot of pilots: tracking through the VOR "cone of confusion." The cone of confusion is an area directly overhead a VOR navigation beacon where the signals are unusable. So an aircraft transiting the VOR will receive no usable signals for a period of time, and that period depends on two factors: your ground speed and your altitude. Higher and faster means you're in the unusable zone longer. Here's the physics of what happens as you approach. The radials are converging — they're all coming together at the beacon — so the course deviation indicator becomes more sensitive the closer you get. 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 needle is swinging so wildly that the data is no longer reliable. At that point, the VOR signals are "cut off" by what we call the "over station sensing" circuits. This is the clever part: the roll channel automatically de-couples from the radio beam. It stops trying to follow the needle. Instead, it 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 essentially freezes the heading it was holding, corrected for drift, and flies you straight through the dead zone until you emerge on the other side with usable signals again. That's the whole story of the cone of confusion — the autopilot doesn't chase a useless needle; it holds a known-good heading until the beam becomes usable again.