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

Let’s pick up right where the FMS schematic left off, because now we’re looking at the actual heart of the system — the FMC, the Flight Management Computer. I want you to picture the FMC as the brain of the whole FMS. It does the computations, and it holds two very specific databases. The first is the performance database. This contains all the parameters of the aircraft’s performance — things like engine thrust, fuel flow, speeds, and climb rates. But it also holds something very important: the company’s cost index strategy. That cost index is a number the airline sets that tells the FMC how to balance speed against fuel economy — whether to fly fast and burn more fuel, or fly slower and save fuel. So the performance database is essentially the aircraft’s own performance envelope, tuned to how your airline wants to operate. The second database is the navigation database. This one contains aeronautical information for the planned area of operations of the aircraft. And here’s where I want you to be precise, because this is a list you’ll need to know cold. The navigation database comprises: - Aerodrome details — positions, elevations, runways, and runway lengths. - Navigation facilities — and for each one, its location, altitude, frequency, identification, and DOC. DOC stands for Designated Operational Coverage — that’s the area within which the facility is guaranteed to provide a reliable signal. - Airways routes, including reporting points. - SIDs and STARs — that’s Standard Instrument Departures and Standard Terminal Arrival Routes — plus runway approaches. - And finally, company routes — the specific routes your airline has pre-programmed. Now, here’s a critical operational detail. The navigation data is updated every 28 days, and the FMC contains both the current and the next 28 days of database. This coincides with the ICAO navigation data cycle — ICAO being the International Civil Aviation Organization. So the FMC always has the current data and the upcoming data loaded, ready for the cycle change. And that data can be customized for the specific airline’s operations — so one airline’s FMC database won’t necessarily match another’s. Now, let me tie this back to the big picture. Look at the FMS schematic we have here. You’ve got the CDUs — the Control Display Units — which are the pilot interfaces. You’ve got the IRSs — the Inertial Reference Systems — which provide position and attitude data. You’ve got the A/T — autothrottle — and the AFDS, the Autopilot Flight Director System, which receive the commands. And in the middle, the FMC does the computations. When the FMC is driving the lateral navigation — L NAV — and the vertical navigation — V NAV — both engaged, you have what’s called integrated FMS operation. If one of those isn’t engaged, you’re in independent operation — the FMC is still computing, but it’s not commanding both axes. So the key takeaway here: the FMC is the computational core, and its two databases — performance and navigation — are what make it able to plan and execute a flight. The navigation database is refreshed on the 28-day ICAO cycle, and it holds everything from aerodromes to airways to SIDs and STARs. That’s the foundation you’ll build on as we go deeper into RNAV.