ATPL · Navigation · Oxford Atpl Book Instrumentation 6th Edition lesson — Page 373, Lesson 452

I want to walk you through the final piece of the VOR mode behaviour, and then we're moving into the lateral navigation modes of the autoflight system. Let's start with what happens when the aircraft is flying a VOR radial and passes overhead the station. When the aircraft gets close to the VOR station, the radio signals become unreliable — that's the cone of confusion directly above the beacon. The autopilot de-couples from the radio beam and controls the aircraft through the cone of confusion on the drift-corrected heading that existed at the moment the radio signals were 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 holds the heading, but it's a temporary, automatic function, not a pilot-selected heading mode. So the aircraft flies straight through the cone on the last good drift-corrected heading, then re-captures the outbound radial automatically. Now let's move on 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 the pilot manually flying each leg, the autopilot follows the stored waypoint sequence. Now we come to the modern system — Lateral Navigation, abbreviated L NAV. In the L NAV mode, the Flight Management Computer, the FMC, controls the AFDS roll to intercept and track the active FMC route. Let me unpack that. AFDS is the Autopilot Flight Director System — so the FMC is commanding the roll channel of the autopilot and flight director. The desired route is activated and modified through the FMC CDUs — the Control Display Units, which are the pilot's interface to the FMC. In addition to en route guidance, the active routes can include terminal procedures such as SIDs — Standard Instrument Departures — STARs — Standard Terminal Arrival Routes — and instrument approaches. Now, there are engagement criteria that must be met before you can use L NAV. There must be an active route in the FMC, capture criteria must be satisfied, and the L NAV switch must be pressed. So it's a three-part requirement. The L NAV capture criteria itself is divided into two categories. First, any aeroplane satisfies capture criteria when it is within 3 nautical miles of the active route segment. Second, outside of that 3 nautical mile band, 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 good regardless of your heading. Outside 3 NM, you need a shallow intercept angle of 90 degrees or less, and you must join the route before you reach the active waypoint. Let me show you the sensor inputs and the mode control panel so you can see how all this fits together. So to summarise the key points: the VOR mode holds the drift-corrected heading through the cone of confusion, then reverts to VOR Mode — not Heading Mode. The INS or IRS couples to the autopilot for sequential waypoint steering on older aircraft. And L NAV uses the FMC to control AFDS roll, with strict engagement and capture criteria — active route, capture criteria satisfied, L NAV switch pressed, and the 3 NM versus 90-degree intercept rule.