Area Navigation Systems (RNAV) — Page 270, Lesson 266

I want to walk you through the heart of the Flight Management System — the FMC, the Flight Management Computer. This is the brain of the whole RNAV operation, and I want you to see exactly what it contains and why that matters for you as a professional pilot. Let's start with the two databases that live inside the FMC, because everything the system does depends on them. The first is the performance database. This holds all the parameters of the aircraft's performance — think of it as the aircraft's own capabilities, its limits and its behaviour. And critically, it also contains the company's cost index strategy. That's the airline's economic plan — how the aircraft should be flown to balance fuel against time, according to company policy. So the performance database isn't just physics; it's the airline's business logic baked into the machine. The second database is the navigation database. This one holds the aeronautical information for the planned area of operations of the aircraft. And I want you to note that phrase carefully — planned area of operations. This isn't a global map of everything; it's the data relevant to where this aircraft will actually fly. Let me walk you through what it comprises, because there are five components and you need each one. First, aerodrome details — positions, elevations, runways and their lengths. So every airport the aircraft might use, with its exact location, its height above sea level, and the physical dimensions of its runways. Second, navigation facilities — and here's a list you must remember: location, altitude, frequency, identification, and DOC. DOC stands for Declared Operational Coverage — that's the published area within which a navaid is guaranteed to provide service. So the database tells you where the facility is, how high it is, what frequency it transmits on, its identifier, and the extent of its reliable coverage. Third, airways routes, including reporting points. These are the structured routes through the airspace, and the defined points along them where you report position. Fourth, SIDs and STARs and runway approaches. SID is Standard Instrument Departure, STAR is Standard Terminal Arrival Route — the published departure and arrival procedures — plus the approach procedures for each runway. And fifth, company routes. These are the pre-defined routes that the airline itself has established for its operations. Now, here's a critical operational detail about the navigation data. It is updated every 28 days, and the FMC contains the current and next 28 days database. So at any moment, the machine holds two full cycles of data. And this 28-day period coincides with the ICAO navigation data cycle — that's the international standard cycle that all navigation data follows. This is why you'll see database effective dates in the cockpit; you must always check that your current database is valid for the day you're flying. One more thing — the data may be customized for the specific airline operations. So the same FMC hardware, in different airlines, can carry different data tailored to how that airline operates. Now let me bring this to life with the system architecture. Look at the FMS schematic — the Flight Management System. The FMC does the computations, and it commands two key functions: L NAV and V NAV. L NAV is Lateral Navigation — steering the aircraft along the horizontal flight path. V NAV is Vertical Navigation — managing the vertical profile, the climbs and descents. These commands go to the A/T — the Autothrottle — and the AFDS, the Autopilot Flight Director System. And here's the key concept I want you to grasp: there are two modes of operation. In independent operation, the IRSs — the Inertial Reference Systems — and the pilots feed the FMC, and the FMC computes. But in integrated FMS operation, the condition is that L NAV and V NAV are engaged. When both are engaged, you have the full integrated system — the FMC is actively commanding the autopilot and autothrottle in both the lateral and vertical planes. That's the difference between the FMC just calculating, and the FMC actually flying the aircraft. And you'll see the CDUs — the Control Display Units — with their full keyboards and page selection keys. That's your interface with the FMC. The page title at the top, the data lines, the page select keys — that's how you talk to this computer. So to tie it together: the FMC holds a performance database and a navigation database. The navigation database is updated on the 28-day ICAO cycle and holds two cycles at once. And the FMC, when L NAV and V NAV are engaged, commands the autopilot and autothrottle to fly the aircraft along both the lateral and vertical paths. That's the integrated FMS operation.