ATPL · Navigation · Instrumentation Atpl Ground Training Series… lesson — Page 373, Lesson 452

We're in the middle of the autopilot modes, and I want to pick up with a specific behaviour you'll see on older systems. When the autopilot is coupled to a VOR and the aircraft enters the cone of confusion — that's the area directly above the VOR station where the radio signals become unreliable — the autopilot de-couples from the radio beam. It then controls the aircraft through the cone on the drift-corrected heading that existed at the moment the radio signals de-coupled. In other words, the autopilot goes into Heading Hold for a set period, and after that set period it reverts back to the VOR Mode. Now, this is a subtle but important distinction: the autopilot does not go into Heading Mode — it goes into Heading Hold. Heading Hold is a temporary state, a holding pattern of sorts, whereas Heading Mode is a selected, deliberate mode. So the system is designed to ride out the cone of confusion and then automatically re-engage the VOR tracking once the signals are reliable again. Now let's move to the Inertial Navigation System, or INS, and the Inertial Referencing System, IRS. On older aircraft that don't have a Flight Management System, you may be able to couple the INS or IRS to the autopilot. This allows the aircraft to be steered sequentially through a series of waypoints that have been loaded into the INS or IRS before flight. So instead of tracking a radio beam, you're following a pre-programmed path defined by waypoints. Then we come to Lateral Navigation, or L NAV. This is a mode where the Flight Management Computer, the FMC, controls the AFDS roll — that's the Autopilot Flight Director System's roll channel — to intercept and track the active FMC route. The desired route is activated and modified through the FMC CDUs, the Control Display Units. In addition to en route guidance, the active routes can include terminal procedures such as SIDs, STARs, and instrument approaches. SIDs are Standard Instrument Departures, STARs are Standard Terminal Arrival Routes. Now, there are engagement criteria that must be met to use L NAV. First, there must be an active route in the FMC. Second, capture criteria must be satisfied. And third, the L NAV switch must be pressed. So it's not just a matter of selecting the mode — you need an active route loaded, the capture conditions met, and the switch physically engaged. The L NAV capture criteria are divided into two categories. First, any aeroplane satisfies capture criteria when it's within 3 nautical miles of the active route segment. Second, outside of 3 nautical miles, the aeroplane must be on an intercept course of 90 degrees or less, and it must intercept the active route segment before the active waypoint. So inside 3 NM, you're close enough to just capture. Outside 3 NM, you need to be converging at an angle no greater than 90 degrees, and you need to reach the route segment before you pass the waypoint. That's the discipline of L NAV capture.